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Network Working Group P Karn Internet Draft Qualcomm W A Simpson DayDreamer expires in six months June 1996 The Photuris Session Key Management Protocol draft-simpson-photuris-11.txt | Status of this Memo Distribution of this memo is unlimited. This document is an Internet-Draft. Internet Drafts are working doc- uments of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute work- ing documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months, and may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as refer- ence material, or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the internet-drafts Shadow Directories on: ftp.is.co.za (Africa) nic.nordu.net (Europe) ds.internic.net (US East Coast) ftp.isi.edu (US West Coast) munnari.oz.au (Pacific Rim) Abstract Photuris is an experimental session-key management protocol intended for use with the IP Security Protocols (AH and ESP). Applicability Photuris is intended for Internet nodes that frequently access or are accessed by a large and unpredicatable number of other nodes. It features defense against resource clogging, perfect forward secrecy, and (optional) privacy protection of the exchange parties. Karn & Simpson expires in six months [Page i] DRAFT Photuris June 1996 Photuris is primarily used for creating virtual private networks, establishing sessions for mobile users and networks operating over bandwidth-limited links, and short-lived sessions between numerous clients and servers. Photuris is extensible. A wide variety of authentication, compres- sion, encryption, identification, and other operational types are supported. Photuris is independent of any particular party identification method or certificate format. Support for symmetric key party identifica- tion is required to be implemented, and asymmetric key party identi- fication is optionally supported by extensions. Karn & Simpson expires in six months [Page ii] DRAFT Photuris June 1996 1. Introduction The ultimate goal of Internet Security is to facilitate direct IP connectivity between sensitive hosts and users across the Internet. Users will rely on Internet Security to protect the confidentiality of the traffic they send across the Internet and depend on it to block unauthorized external access to their internal hosts and net- works. Users must have confidence in every Internet Security component, including key management. Without this confidence, users may erect barriers that impede legitimate use of the Internet, or forego the Internet entirely. Internet Security does not place any significance on easily forged IP Source addresses. It relies instead on proof of possession of secret knowledge: that is, a cryptographic key. However, secure manual distribution and maintainance of these keys is often cumbersome and problematic. User distribution often leads to long-lived keys, with concommitant opportunity for compromise of the keys. A fundamental role of this key management protocol is to verify the values exchanged, while ensuring that the resulting key is not known by another party. It has been shown [DOW92] that key exchange must be coupled to authentication. Each party requires assurance that an exchanged key is not shared with an imposter. Protecting sensitive data on the Internet against compromise -- either by interception or by unauthorized access -- is necessary, but not sufficient. The computing resources themselves must also be pro- tected against malicious attack or sabotage. With these criteria in mind, Photuris [Firefly] is designed: A. for frequent exchange of limited lifetime individual session-keys, with a minimum of configuration and effort. B. for associating security parameters with these session-keys. C. to support the use of a variety of authentication methods, and facilitate the exchange of many identification types. D. to thwart certain types of denial of service attacks on host resources. Karn & Simpson expires in six months [Page 1] DRAFT Photuris June 1996 E. to provide these services with minimal network activity, balanced with computational efficiency. This document is primarily intended for implementing the Photuris protocol. It is not intended to detail service and application interface definitions, although it does mention some basic policy areas as required for the proper implementation and operation of the protocol mechanisms. 1.1. Terminology exchange-value The publically distributable value used to calculate a shared-secret. As used in this document, refers to a Diffie-Hellman exchange, not the public part of a public/private key-pair. private-key A value that is kept secret, and is part of an asym- metric public/private key-pair. public-key A publically distributable value that is part of an asymmetric public/private key-pair. secret-key A symmetric key that is not publically dis- tributable. As used in this document, this is dis- tinguished from an asymmetric public/private key- pair. An example is a user password. Security Association A collection of parameters describing the security relationship between two nodes. These parameters include the identities of the parties, the transform (including algorithm and algorithm mode), the key(s) (such as a session-key, secret-key, or appropriate public/private key-pair), and possibly other infor- mation such as sensitivity labelling. For further details, see [RFC-1825]. Security Parameters Index (SPI) A number that indicates the Security Association. The number is relative to the IP Destination, which is the SPI Owner. session-key A key that is independently derived from a shared- secret by the parties, and used for keying one direction of traffic. This key is changed fre- quently. Karn & Simpson expires in six months [Page 2] DRAFT Photuris June 1996 shared-secret As used in this document, the calculated result of the Photuris exchange. SPI Owner The party that corresponds to the IP Destination; the receiver of the datagram. SPI User The party that corresponds to the IP Source; the sender of the datagram. transform A cryptographic manipulation of a particular set of data. As used in this document, refers to certain well-specified methods (which are defined else- where). For example, AH-MD5 [RFC-1828] transforms an IP datagram into a cryptographic hash, and ESP- DES-CBC [RFC-1829] transforms plaintext to cipher- text and back again. Implementors will find details of cryptographic hashing (such as MD5), encryption algorithms and modes (such as DES), digital signa- tures (such as DSS), and other algorithms in [Schneier95]. 1.2. Protocol Overview The Photuris protocol consists of several simple phases: 1. A "Cookie" Exchange guards against simple flooding attacks sent with bogus IP Sources or UDP Ports. Each party passes a "cookie" to the other. In addition, supported exchange-schemes are offered by the Respon- der for calculating a shared-secret. 2. A Value Exchange establishes a shared-secret between the parties. Each party passes an Exchange-Value to the other. These values are used to establish a shared-secret between the parties. The Responder remains stateless until a shared-secret has been cre- ated. In addition, supported attributes are offered by each party for use in establishing new Security Associations. 3. An Identification Exchange identifies the parties to each other, and verifies the integrity of values sent in phases 1 and 2. In addition, the shared-secret provides a basis to generate sepa- rate Security Association session-keys in each direction, which are in turn used for conventional authentication or encryption. Karn & Simpson expires in six months [Page 3] DRAFT Photuris June 1996 Additional security attributes are also exchanged as needed. This exchange may also be encrypted for party privacy protection using an exchange session-key based on the shared-secret. This protects the identities of the parties, hides the security parame- ter values, and improves security for the exchange protocol and security transforms. 4. Additional messages may be exchanged to periodically change the session-keys, and to establish new or revised security parameters. These exchanges may also be encrypted for party privacy protection in the same fashion as above. The sequence of message types and their purposes are summarized in the diagram below. The first three phases (cookie, exchange, and identification) must be carried out in their entirety before any security association can be used. Initiator Responder ========= ========= Cookie_Request -> <- Cookie_Response offer schemes Value_Request -> pick scheme offer value offer attributes <- Value_Response offer value offer attributes [generate shared-secret from exchanged values] Karn & Simpson expires in six months [Page 4] DRAFT Photuris June 1996 Identity_Request -> make SPI pick SPI attribute(s) identify self authenticate (make protection key) (encrypt message) <- Identity_Response make SPI pick SPI attribute(s) identify self authenticate (make protection key) (encrypt message) [make SPI session-keys in each direction] SPI User SPI Owner ======== ========= SPI_Needed -> list SPI attribute(s) make integrity key authenticate (encrypt message) <- SPI_Update make SPI pick SPI attribute(s) make SPI session-key(s) make integrity key authenticate (encrypt message) Either party may initiate an exchange at any time. For example, the Initiator need not be a "caller" in a telephony link. The Initiator is responsible for recovering from all message losses by retransmission. A Photuris exchange between two parties results in a pair of SPI val- ues (one in each direction). Each SPI is used in creating separate session-key(s) in each direction. When both parties initiate Photuris exchanges concurrently, or one party initiates more than one Photuris exchange, the Initiator Cook- ies (and UDP Ports) keep the exchanges separate. This results in more than one initial SPI for each Destination. Karn & Simpson expires in six months [Page 5] DRAFT Photuris June 1996 To create multiple Security Associations with different parameters, the parties may also send SPI_Updates. There is no requirement that all such outstanding SPIs be used. The SPI User (sender) selects an appropriate SPI for each datagram trans- mission. 1.3. Clogging Defense To grant access to authorized users regardless of location, it must be possible to cheaply detect and discard bogus datagrams. Other- wise, an attacker intent on sabotage might rapidly send datagrams to exhaust the host's CPU or memory resources. Using Internet Security authentication facilities, when a datagram does not pass an authentication check, it can be discarded without further processing. This is easily done with manual (null) key man- agement between trusted hosts at relatively little cost, given the speed of cryptographic hashing functions compared to public-key algo- rithms. Unfortunately, such a trusted host will have only a fixed number of keys available. The keys will tend to have long lifetimes. This carries significant security risks. Automatic key management is necessary to generate keys between par- ties without prior arrangement. But, there is a potential Achilles heel in the key management protocol. Because of their use of CPU-intensive operations such as modular exponentiation, key management schemes based on public-key cryptogra- phy are vulnerable to resource clogging attacks. Although a complete defense against such attacks is impossible, Photuris features make them much more difficult. 1.3.1. Cookie Exchange Photuris exchanges a pair of "cookies" based on the IP node addresses before initiating any extensive computational operations. This cookie exchange provides a weak form of message origin authentication and verifies the presence of network communications between the par- ties, thwarting the saboteur from using random IP Source addresses. The simple validation of these cookies uses the same level of resources as other Internet Security authentication mechanisms. This forces the attacker to: Karn & Simpson expires in six months [Page 6] DRAFT Photuris June 1996 1) use its own valid IP address, or 2) gain access to a physical transmission link and appropriate those addresses, or 3) subvert Internet routing for the same purpose. The first option allows the target to detect and filter out such attacks, and significantly increases the likelihood of identifying the attacker. The latter two options are much more difficult than merely sending large numbers of datagrams with randomly chosen IP Source addresses from an arbitrary point on the Internet. The cookie exchange does not protect against an observer that can copy a valid cookie, or an interceptor that can modify or substitute another cookie. These attacks are mitigated somewhat with time- variant cookies. 1.3.2. State Limitation There is a small amount of state associated with the Photuris exchange itself. This includes the Cookies, Exchange-Values, and the computed shared-secret. During the initial Cookie Exchange, the Responder does not maintain any state for the exchange. This prevents memory resource exhaustion from a simple flooding attack. Later exchange phases require saving of state to perform the key establishment calculations and identity verification. An attacker that is willing to expose itself to a larger window of detection can waste substantial resources by repeating the steps of the Photuris process without using the results. The Responder combines time-variant cookies with a counter to limit the number of multiple concurrent Photuris exchanges with the same Internet nodes. Initiators will not be issued additional cookies by the Responder until their previous exchanges have concluded or expired. This combination also prevents an attack by hoarding valid cookies, and then flooding the Responder with a large number of con- current exchanges. Karn & Simpson expires in six months [Page 7] DRAFT Photuris June 1996 1.3.3. State Precomputation Prior to accepting Cookie_Requests, the Responder can precompute its Exchange-Value. Successive requests from multiple Initiators will not require additional computation until the Identification Exchange. Once Photuris exchange state has been established between nodes, repetitive exchanges can use many of the same previously computed values. This prevents an attacker with more CPU power from easily exhausting the target. 1.3.4. State Expiration During a Photuris exchange, the Responder Exchange TimeOut limits the wait for the exchange to complete. This includes the packet round trips, and the time for completing the Identification Exchange calcu- lations. The time is bounded by both the maximum amount of calcula- tion delay expected for the processing power of an unknown peer, and the minimum user expectation for results (default 60 seconds). In addition, all retained exchange state of both parties has an asso- ciated Exchange LifeTime, and is subject to periodic expiration. This depends on the physical and logistical security of the machine, and is typically in the range of 10 minutes to one day (default 30 minutes). When an Exchange-Value expires (or is replaced by a newer value), all related exchange state is purged. The periodic expiration and purge of exchange state reduces the risk of compromise of keys and secrets, and is an important consideration in attaining Perfect Forward Secrecy. If an attacker has succeeded in overwhelming a target, the target will eventually recover its resources as the expired state is purged. Implementation Notes: These Exchange LifeTimes and TimeOuts are implementation dependent and are not disclosed in any Photuris message. The paranoid oper- ator will have a fairly short Exchange LifeTime, but it MUST NOT be less than twice the Exchange TimeOut. To prevent synchronization between Photuris exchanges, the imple- mentation SHOULD randomly vary each Exchange LifeTime within twice the range of seconds that are required to calculate a new Exchange-Value. For example, if the Responder uses a base Exchange LifeTime of 30 minutes, and takes 10 seconds to calculate Karn & Simpson expires in six months [Page 8] DRAFT Photuris June 1996 the new Exchange-Value, the equation might be (in milliseconds): 1800000 + random(20000) The exchange-scheme, Exchange-Values, and resulting shared-secret MAY be cached in short-term storage for the Exchange LifeTime. When repetitive Photuris exchanges occur between the same parties, and the Exchange-Values are discovered to be unchanged, the pre- computed shared-secret can be used to rapidly generate new ses- sion-keys. 1.4. Perfect Forward Secrecy Many security breaches in cryptographic systems have been facilitated by designs that generate traffic-encrypting keys (or their equiva- lents) well before they are needed, and then keep them around longer than necessary. This creates many opportunities for compromise, especially by insiders. A carefully designed public-key system can avoid this problem. The rule is to avoid using any long-lived keys (such as a RSA public- private key-pair) to encrypt session-keys or actual traffic. Such keys should be used solely for identification (entity authentication) purposes. All keys for traffic encryption should be randomly generated immedi- ately before use, and then destroyed immediately after use, so that they cannot be recovered. The keys should not be based on the values of any previous keys, or any other long-lived stored information. The Photuris exchange messages can provide perfect forward secrecy, as defined by Diffie [Diffie90]. When the calculated shared-secret is eventually destroyed, it is unrecoverable. Theft of the private/secret key used to sign the exchanges would allow the thief to impersonate the party in future conversations, but it would not decode any past traffic that might have been recorded. 1.5. Security Parameters Photuris key management is used to determine a number of parameters for each Security Association between the communicating parties. This includes the particular authentication and/or encryption trans- forms, and the key(s) used to authenticate, encrypt or decrypt the payload. Karn & Simpson expires in six months [Page 9] DRAFT Photuris June 1996 The key management implementation usually maintains a table or list containing the several parameters for each concurrent Security Asso- ciation. The implementation needs to access that security parameter table to determine how to process each datagram. To indicate a par- ticular table entry, a Security Parameters Index (SPI) is used. The SPI is assigned by the entity controlling the IP Destination: the SPI Owner (the receiver). The parties use the combination of SPI and IP Destination to distinguish the correct association. Each SPI has an associated LifeTime, specified by the SPI owner (receiver). This SPI LifeTime is usually related to the speed of the link (typically 30 seconds to 30 minutes). The SPI can also be deleted by the SPI Owner using the SPI_Update. Once the SPI has expired or been deleted, the parties cease using the SPI. Implementation Notes: The method used for SPI assignment is implementation dependent. However, selection of a cryptographically random value can help prevent attacks that depend on a predicatable sequence of values. To prevent synchronization between Photuris exchanges, the imple- mentation SHOULD randomly vary each SPI LifeTime by a few seconds. To prevent resurrection of deleted or expired SPIs, implementa- tions SHOULD remember those SPIs, but mark them as unusable until the Photuris exchange shared-secret used to create them also expires and purges the associated state. When more than one unexpired SPI is available for the same func- tion, a common implementation technique is to select the SPI with the greatest remaining LifeTime. However, selecting randomly among a large number of SPIs may provide some defense against traffic analysis. When an implementation detects an incoming SPI that has recently expired, but the associated state has not yet been purged, the implementation MAY accept the SPI. The length of time allowed is highly dependent on clock drift and variable packet round trip time, and is therefore implementation dependent. Karn & Simpson expires in six months [Page 10] DRAFT Photuris June 1996 1.6. LifeTimes The Photuris exchange results in two kinds of state, each with sepa- rate LifeTimes. 1) The Exchange LifeTime of the small amount of state associated with the Photuris exchange itself. This state may be viewed as between Internet nodes. 2) The SPI LifeTimes of the multiple Security Associations that are established. This state may be viewed as between users and nodes. The SPI LifeTimes may be shorter or longer than the Exchange Life- Time. These LifeTimes are not required to be related to each other. When an Exchange-Value expires (or is replaced by a newer value), any unexpired derived SPIs are not affected. This is important to allow traffic to continue without interruption during new Photuris exchanges. 1.7. Identification Every party requires its own Identification. When the Photuris exchange is node to node, such as single user personal computers or unattended firewalls used in virtual private networks, the nodes themselves may be viewed as the users. When required for secure multi-user environments, the Iden- tity_Messages can be used to provide separate limited authentication from each user of one node when communicating with another common node. To provide bi-directional user-oriented keying, the parties can initiate multiple concurrent Photuris exchanges. These may pro- vide separate user Identification from the Initiator to the Responder in each direction. Each secure multi-user operating system MUST be capable of separately maintaining multiple Identification Exchange SPI values for each Value Exchange calculated shared-secret. It is the responsibility of the Source to internally segregate the shared-secret and different session-keys provided per Destination, and select an appropriate SPI for each datagram transmission. Karn & Simpson expires in six months [Page 11] DRAFT Photuris June 1996 Design Notes: Successful use of user-oriented keying requires a significant level of operating system support. Use of multi-user segregated exchanges likely requires added functionality in the transport API of the implementation operating system. Such mechanisms are out- side the scope of this document. It has been suggested that the Photuris exchange could also be established between particular application or transport processes associated with a user of a node. Such mechanisms are emphati- cally outside the scope of this document. 1.8. Multicast Support Key management is more difficult in a multicast environment. Senders to a multicast group may share common a Security Parameters Index, if all communications are using the same security configura- tion parameters. In this case, the receiver only knows that the mes- sage came from a node knowing the SPI for the group, and cannot authenticate which member of the group sent the datagram. Multicast groups may also use a separate SPI value for each Source. If each sender is keyed separately and asymmetric algorithms are used, data origin authentication is also provided. A given Destination is not necessarily in control of the selection process. In the case of multicast groups, a single node or coop- erating subset of the multicast group may work on behalf of the entire group to set up a Security Association. It is anticipated that Photuris would be used first to establish a distribution SPI and session-key, and that another orthogonal key distribution mechanism will use that SPI to send the group keys. This is a matter for future research. Such mechanisms are outside the scope of this document. Karn & Simpson expires in six months [Page 12] DRAFT Photuris June 1996 2. Protocol Details The Initiator begins a Photuris exchange under several circumstances: - The Initiator has a datagram that it wishes to send with privacy, and has no current Photuris exchange state with the IP Destina- tion. This datagram is discarded, and a Cookie_Request is sent instead. - The Initiator has received the ICMP message [RFC-1812] Destination Unreachable: Communication Administratively Prohibited (Type 3, Code 13), and has no current Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-xxxx] Security Failures: Bad SPI (Type 40, Code 0), that matches current Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-xxxx] Security Failures: Need Authentication (Type 40, Code 4), and has no cur- rent Photuris exchange state with the ICMP Source. - The Initiator has received the ICMP message [RFC-xxxx] Security Failures: Need Authorization (Type 40, Code 5), that matches cur- rent Photuris exchange state with the ICMP Source. When the event is an ICMP message, special care MUST be taken that the ICMP message actually includes information that matches a previ- ously sent IP datagram. Otherwise, this could provide an opportunity for a clogging attack, by stimulating a new Photuris Exchange. 2.1. UDP All Photuris messages use the User Datagram Protocol header [RFC-768]. The Initiator sends to UDP Destination Port 468. When replying to the Initiator, the Responder swaps the IP Source and Destination, and the UDP Source and Destination Ports. The UDP checksum MUST be correctly calculated when sent. When a mes- sage is received with an incorrect UDP checksum, it is silently dis- carded. Karn & Simpson expires in six months [Page 13] DRAFT Photuris June 1996 Implementation Note: It is expected that installation of Photuris will ensure that UDP checksum calculations are enabled for the computer operating sys- tem and later disabling by operators is prevented. 2.2. Header Format All of the messages have a format similar to the following, as trans- mitted left to right in network order (most significant to least sig- nificant): +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. Responder-Cookie 16 octets. Type one octet. Each message type has a unique value. Initial values are assigned as follows: 0 Cookie_Request 1 Cookie_Response 2 Value_Request 3 Value_Response 4 Identity_Request 5 Secret_Response 6 Secret_Request 7 Identity_Response 8 SPI_Needed 9 SPI_Update 10 Bad_Cookie 11 Resource_Limit 12 Verification_Failure 13 (reserved) Karn & Simpson expires in six months [Page 14] DRAFT Photuris June 1996 Further details and differences are elaborated in the individual mes- sages. Design Note: The fixed size of the cookies was chosen for convenience, based on the output of commonly available cryptographic hashing functions. It is anticipated that this size is likely to be more than suffi- cient to protect against very high bit-rate flooding attacks. 2.3. Variable Precision Numbers +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Size | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Size two, four, or eight octets. The number of signifi- cant bits used in the Value field. Always transmit- ted most significant octet first. When the Size is zero, no Value field is present; there are no significant bits. This means "missing" or "null". It should not be confused with the value zero, which includes an indication of the number of significant bits. When the most significant octet is in the range 0 through 254 (0xfe), the field is two octets. Both octets are used to indicate the size of the Value field, which ranges from 1 to 65,279 significant bits (in 1 to 8,160 octets). When the most significant octet is 255 (0xff), the field is four octets. The remaining three octets are added to 65,280 to indicate the size of the Value field, which is limited to 16,776,959 signifi- cant bits (in 2,097,120 octets). When the most significant two octets are 65,535 (0xffff), the field is eight octets. The remaining six octets are added to 16,776,960 to indicate the size of the Value field. This is vastly too long for these UDP messages, but is included for com- pleteness. Karn & Simpson expires in six months [Page 15] DRAFT Photuris June 1996 Value Zero or more octets. Always transmitted most sig- nificant octet first. The bits used are right justified within octet boundaries; that is, any unused bits are in the most significant octet. Unused bits are zero filled. Shortened forms SHOULD NOT be used when the Value includes a number of leading zero significant bits. The Size SHOULD indicate the cor- rect number of significant bits. Design Notes: Some of the message fields require a value that may vary in the number of bits. These bits may not make up an integral number of octets. The numbers are assumed to be unsigned. The emphasis on significant bits was based on concerns that cryp- tographic lengths and strengths be readily determined. This is in contrast to the usual concern that each number have only one unique (shortest) representation. When processing datagrams containing variable size values, the length must be checked against the overall datagram length. An invalid size (too long or short) that causes a poorly coded receiver to abort could be used as a denial of service attack. 2.4. Exchange Schemes +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Scheme | Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Value ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Scheme two octets. A unique value indicating the exchange- scheme. See the "Exchange Scheme List". Size two octets, ranging from 0 to 65,279. See "Variable | Precision Number". Value Zero or more octets. See "Variable Precision Num- | ber". Karn & Simpson expires in six months [Page 16] DRAFT Photuris June 1996 Selection among several different exchange-schemes is needed to enable experimental and proprietary extensions without affecting the basic protocol. The target of the exchange (Responder) specifies a list of the schemes supported, and the Initiator chooses one that it also supports. The scheme list includes alternative algorithms and distinguishing parameters. These are mixed in the same list for simplicity. The implementation can easily distinguish between the separate uses of each supported scheme. These uses are indicated in the "Exchange Scheme List". Design Notes: Although exchange-schemes offer great flexibility, only a few well-chosen algorithms and parameters are specified. This pro- vides opportunity for intensive review by the cryptographic commu- nity, reduces implementation complexity, and improves potential for interoperability. Only one exchange-scheme (#2) is required to be supported, and SHOULD be present in every Offered-Schemes list. 2.5. Attributes +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Value(s) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type one octet. A unique value indicating the kind of attribute. See the "Attribute List" for details. When the Type is zero (padding), no Length field is present (always zero). Length one octet. The size of the Value(s) field in octets. When the Length is zero, no Value(s) field is pre- sent. Value(s) Zero or more octets. See the "Attribute List" for details. Selection among several different security parameter attributes is needed to enable future implementation changes without affecting the Karn & Simpson expires in six months [Page 17] DRAFT Photuris June 1996 basic protocol. Each party (the sender) offers a list of the attributes supported and its peer (the receiver) selects from that list when making its incoming Security Associations. The attribute list includes authentication, compression, encryption, identification, and other operational types available for exchange between the parties. These are mixed in the same number space for simplicity. The implementation can easily distinguish between the separate uses of each supported attribute. See the "Attribute List" for details. The Length MUST NOT be assumed to be constant for a particular Type. The same Type MAY be present in a list of attributes with varying Lengths. Design Notes: Although attributes offer great flexibility, only a few well- chosen algorithms are specified. This provides opportunity for intensive review by the cryptographic community, reduces implemen- tation complexity, and improves potential for interoperability. The authentication, compression, encryption and identification mechanisms chosen, as well as the encapsulation modes (if any), need not be the same in both directions. When processing datagrams containing variable length values, the length must be checked against the overall datagram length. An invalid length (too long or short) that causes a poorly coded receiver to abort could be used as a denial of service attack. 2.5.1. Authentication Authentication decisions are in the SPI Owner (receiver) direction. Only the receiver can determine that arriving traffic is authentic. Its need for authentication is indicated by choosing authentication attributes, and/or authenticated encryption attributes, when creating each SPI. It enforces authentication through the simple expedient of dropping all datagrams with missing or invalid authentication, and sending an appropriate ICMP Security Failures message [RFC-xxxx], such as Need Authentication (Type 40, Code 4) or Need Authorization (Type 40, Code 5). Support is required for the "MD5-KDP" and "Simple MD5-DP Verifica- tion" Attributes, and they SHOULD be present in every Offered- Attributes list. Karn & Simpson expires in six months [Page 18] DRAFT Photuris June 1996 If the potential SPI Owner (receiver) has not created any authentica- tion SPIs although Photuris exchange state has been established, but it sends ICMP Security Failures messages, the prospective SPI User (sender) is unable to provide authentication for its datagrams. When this situation occurs, the prospective SPI User SHOULD log the occu- rance, and notify an operator as appropriate. Design Notes: This feature is particularly important for deployment and scaling. It cannot be expected that the prospective SPI User will be omni- scient about the upgrade status and policy of potential receivers. Instead, the datagram receiver indicates its authentication needs. The coupling of the ICMP message with the Cookie Exchange provides additional defense against clogging, at the expense of another round trip. 2.5.2. Encapsulation Encapsulation decisions are in the SPI User (sender) direction. Only the sender can determine whether each datagram needs privacy protec- tion. It uses an encryption SPI created by the SPI Owner (receiver), in addition to an authentication SPI (as needed). Since SPI creation is by the receiver, but privacy (and potentially other) decisions are made in the sending direction, a message is needed to stimulate the SPI creation. When the prospective SPI User (sender) needs privacy protection for a datagram and Photuris exchange state has been established, but has no current privacy encapsulation SPI from the potential SPI Owner (receiver), an SPI_Needed message is sent by the prospective SPI User, listing pri- vacy attributes that both parties have previously offered. The orig- inal datagram is discarded. Support is required for the "DES-CBC" Attribute, and it SHOULD be present in every Offered-Attributes list. Where encryption is pro- hibited in a particular environment, the "DES-CBC" Attribute MAY be omitted. If either party has not offered any encryption attributes, the prospective SPI User (sender) is unable to provide privacy for its datagrams. When this situation occurs, the prospective SPI User SHOULD log the occurance, and notify an operator as appropriate. Karn & Simpson expires in six months [Page 19] DRAFT Photuris June 1996 Implementation Notes: Typically, an encryption method is chosen for the primary attribute of the initial SPI in each direction. If integrity is needed, and there is no existing separate SPI that offers authentication, it is recommended that an authentication method be included as a secondary attribute in the initial SPI. When both authentication and encryption attributes are used for the same SPI, care must be exercised that there is no interaction between the algorithms that might reveal some portion of the ses- sion-key(s). There is no known interaction between MD5 and DES- CBC. 3. Cookie Exchange Initiator Responder ========= ========= Cookie_Request -> <- Cookie_Response offer schemes 3.0.1. Send Cookie_Request The Initiator initializes local state, and generates a "cookie". The Initiator-Cookie MUST be different in each new Cookie_Request between the same parties. See "Cookie Generation" for details. By default, the Responder-Cookie and Counter are set to zero. If the new Cookie_Request is in response to a message from a previous exchange in which this party was the Responder, the Responder-Cookie is set to the previous Initiator-Cookie, and the Counter is set to zero. Otherwise, the IP Destination for the Responder is examined. If any previous exchange between the peer IP nodes has not expired, the Responder-Cookie is set to the most recent Responder-Cookie, and the request Counter is set to the corresponding Counter. The Initiator also starts a retransmission timer. If no valid Cookie_Response arrives within the time limit, the same Cookie_Request is retransmitted for the remaining number of Retrans- missions. The Initiator-Cookie value MUST be the same in each such Karn & Simpson expires in six months [Page 20] DRAFT Photuris June 1996 retransmission to the same IP Destination and UDP Port. When Retransmissions have been exceeded, if a Bad_Cookie message has been received during the exchange, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request. 3.0.2. Receive Cookie_Request On receipt of a Cookie_Request, the Responder determines whether there are sufficient resources to begin another Photuris exchange. - When too many SPI values are already in use for this particular peer, or too many concurrent exchanges are in progress, or some other resource limit is reached, a Resource_Limit message is sent. - When any previous exchange initiated by this particular peer has not exceeded the Exchange TimeOut, and the Responder-Cookie does not specify one of these previous exchanges, a Resource_Limit mes- sage is sent. Otherwise, the Responder returns a Cookie_Response. Note that the Responder creates no additional state at this time. 3.0.3. Send Cookie_Response The IP Source for the Initiator is examined. If any previous exchange between the peer IP nodes has not expired, the response Counter is set to the most recent exchange Counter plus one (allowing for out of order retransmissions). Otherwise, the response Counter is set to the request Counter plus one. If the new value is zero (modulo 256), the value is set to one. The Responder generates a cookie. The Responder-Cookie value in each successive response SHOULD be different. See "Cookie Generation" for details. The exchange-schemes available between the peers are listed in the Offered-Schemes. Karn & Simpson expires in six months [Page 21] DRAFT Photuris June 1996 3.0.4. Receive Cookie_Response The Initiator validates the Initiator-Cookie, and the Offered- Schemes. - Whenever an invalid/expired Initiator-Cookie is detected, the mes- sage is silently discarded. - Whenever the variable length Offered-Schemes do not match the UDP Length, or all Offered-Schemes are obviously defective and/or insufficient for the purposes intended, the message is silently discarded; the implementation SHOULD log the occurance, and notify an operator as appropriate. - Once a valid message has been received, later Cookie_Responses with matching Initiator-Cookies are also silently discarded, until a new Cookie_Request is sent. When the message is valid, an exchange-scheme is chosen from the list of Offered-Schemes. This Scheme-Choice may affect the next Photuris message sent. By default, the next Photuris message is a Value_Request. Design Notes: Having the scheme chosen by the Initiator allows the greatest pro- tocol flexibility, and follows the requirement that no state be kept by the Responder until the shared-secret is calculated. Unfortunately, this allows the weakest scheme to be chosen by an attacker. This is no worse than the alternative: to have the Responder choose from weak schemes offered by the attacker. Various proposals for extensions utilize the Scheme-Choice to indicate a different message sequence. Such mechanisms are out- side the scope of this document. Karn & Simpson expires in six months [Page 22] DRAFT Photuris June 1996 3.1. Cookie_Request +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Counter | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. A randomized value that identifies the exchange. The value MUST NOT be zero. See "Cookie Generation" for details. Responder-Cookie 16 octets. Identifies a specific previous exchange. Copied from a previous Cookie_Response. When zero, no previous exchange is specified. When non-zero, and the Counter is zero, contains the Initiator-Cookie of a previous exchange. The speci- fied party is requested to be the Responder in this exchange, to retain previous party pairings. When non-zero, and the Counter is also non-zero, contains the Responder-Cookie of a previous exchange. The specified party is requested to be the Responder in this exchange, to retain previous party pairings. Also, can be used for bidirectional User, Transport, and Process oriented keying. Such mechanisms are outside the scope of this document. Type 0 Counter one octet. Indicates the number of the current exchange. Copied from a previous Cookie_Response. When zero, no previous Responder is specified. Karn & Simpson expires in six months [Page 23] DRAFT Photuris June 1996 3.2. Cookie_Response +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Counter | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Offered-Schemes ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. Copied from the Cookie_Request. Responder-Cookie 16 octets. A randomized value that identifies the exchange. The value MUST NOT be zero. See "Cookie Generation" for details. Type 1 Counter one octet. Indicates the number of the current exchange. Must be greater than zero. Reserved two octets. For future use; MUST be set to zero when transmitted, and MUST be ignored when received. Offered-Schemes A list of one or more exchange-schemes supported by the Responder, beginning with most preferred. Each scheme is four or more octets (see "Exchange Scheme List"). Only one of each kind of scheme may be offered. The end of the list is indicated by the UDP Length. Karn & Simpson expires in six months [Page 24] DRAFT Photuris June 1996 3.3. Cookie Generation The exact technique by which a Photuris party generates a cookie is implementation dependent. The method chosen must satisfy some basic requirements: 1. The cookie MUST depend on the specific parties. This prevents an attacker from obtaining a cookie using a real IP address and UDP port, and then using it to swamp the victim with requests from randomly chosen IP addresses or ports. 2. It MUST NOT be possible for anyone other than the issuing entity to generate cookies that will be accepted by that entity. This implies that the issuing entity will use local secret information in the generation and subsequent verification of a cookie. It must not be possible to deduce this secret information from any particular cookie. 3. The cookie generation and verification methods MUST be fast to thwart attacks intended to sabotage CPU resources. A recommended technique is to use a cryptographic hashing function (such as MD5). An incoming cookie can be verified at any time by regenerating it locally from values contained in the incoming datagram and the local secret random value. 3.3.1. Initiator Cookie The Initiator secret value that affects its cookie SHOULD change for each new Photuris exchange, and is thereafter internally cached on a per Responder basis. This provides improved synchronization and pro- tection against replay attacks. An alternative is to cache the cookie instead of the secret value. Incoming cookies can be compared directly without the computational cost of regeneration. It is recommended that the cookie be calculated over the secret value, the IP Source and Destination addresses, and the UDP Source and Destination ports. Karn & Simpson expires in six months [Page 25] DRAFT Photuris June 1996 3.3.2. Responder Cookie The Responder secret value that affects its cookies MAY remain the same for many different Initiators. However, this secret SHOULD be changed periodically to limit the time for use of its cookies (typi- cally each 60 seconds), and MUST be changed whenever any precalcu- lated Responder Exchange-Value is changed. The Responder-Cookie SHOULD include the Counter from the Cookie_Response. This provides improved synchronization and protec- tion against replay attacks. It is recommended that the cookie be calculated over the secret value, the IP Source and Destination addresses, its own UDP Destina- tion port, the Counter, and the Initiator-Cookie. On receipt of a Value_Request, the Responder regenerates its cookie for validation. The cookie is not cached per Initiator to avoid sav- ing state during the initial Cookie Exchange. Once the Value_Response is sent, both Initiator and Responder cookies are cached to identify the exchange. 4. Value Exchange Initiator Responder ========= ========= Value_Request -> pick scheme offer value offer attributes <- Value_Response offer value offer attributes [generate shared-secret from exchanged values] 4.0.1. Send Value_Request The Initiator generates an appropriate Exchange-Value for the Scheme- Choice. This Exchange-Value may be precalculated and used for multi- ple Responders. The IP Destination for the Responder is examined, and the attributes available between the parties are listed in the Offered-Attributes. Karn & Simpson expires in six months [Page 26] DRAFT Photuris June 1996 The Initiator also starts a retransmission timer. If no valid Value_Response arrives within the time limit, the same Value_Request is retransmitted for the remaining number of Retransmissions. When Retransmissions have been exceeded, if a Bad_Cookie message has been received during the exchange, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request. 4.0.2. Receive Value_Request The Responder validates the Responder-Cookie, the Counter, the Scheme-Choice, the Exchange-Value, and the Offered-Attributes. - Whenever an invalid/expired Responder-Cookie is detected, a Bad_Cookie message is sent. - Whenever an invalid Scheme-Choice is detected, or the Exchange- Value is obviously defective, or the variable length Offered- Attributes do not match the UDP Length, the message is silently discarded; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the Responder sets its Exchange timer to the Exchange TimeOut, and returns a Value_Response. The Responder keeps a copy of the incoming Value_Request cookie pair, and its Value_Response. If a duplicate Value_Request is received, it merely resends its previous Value_Response, and takes no further action. 4.0.3. Send Value_Response The Responder generates an appropriate Exchange-Value for the Scheme- Choice. This Exchange-Value may be precalculated and used for multi- ple Initiators. The IP Source for the Initiator is examined, and the attributes available between the parties are listed in the Offered-Attributes. Implementation Notes: At this time, the Responder begins calculation of the shared- secret. Calculation of the shared-secret is executed in parallel to minimize delay. This may take a substantial amount of time. The implementor Karn & Simpson expires in six months [Page 27] DRAFT Photuris June 1996 should ensure that retransmission is not blocked by this calcula- tion. This is not usually a problem, as retransmission timeouts typically exceed calculation time. 4.0.4. Receive Value_Response The Initiator validates the pair of Cookies, the Exchange-Value, and the Offered-Attributes. - Whenever an invalid/expired cookie is detected, the message is silently discarded. - Whenever the Exchange-Value is obviously defective, or the vari- able length Offered-Attributes do not match the UDP Length, the message is silently discarded; the implementation SHOULD log the occurance, and notify an operator as appropriate. - Once a valid message has been received, later Value_Responses with both matching cookies are also silently discarded, until a new Cookie_Request is sent. When the message is valid, the Initiator begins its parallel computa- tion of the shared-secret. When the Initiator completes computation, it sends an Iden- tity_Request to the Responder. Karn & Simpson expires in six months [Page 28] DRAFT Photuris June 1996 4.1. Value_Request +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Counter | Scheme-Choice | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Exchange-Value ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Initiator-Offered-Attributes ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Initiator-Cookie 16 octets. Copied from the Cookie_Response. Responder-Cookie 16 octets. Copied from the Cookie_Response. Type 2 Counter one octet. Copied from the Cookie_Response. Scheme-Choice two octets. A value selected by the Initiator from the list of Offered-Schemes in the Cookie_Response. Only the Scheme is specified; the size and value(s) are implicit. Initiator-Exchange-Value variable precision number. Provided by the Initia- tor for calculating a shared-secret between the par- ties. The Value format is indicated by the Scheme- Choice. The field may be any integral number of octets in length, as indicated by its Size field. It does not require any particular alignment. The 32-bit align- ment shown is for convenience in the illustration. Initiator-Offered-Attributes A list of Security Parameter attributes supported by Karn & Simpson expires in six months [Page 29] DRAFT Photuris June 1996 the Initiator. The contents and usage of this list are further described in "Offered Attributes List". The end of the list is indicated by the UDP Length. 4.2. Value_Response +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Exchange-Value ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Responder-Offered-Attributes ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Initiator-Cookie 16 octets. Copied from the Value_Request. Responder-Cookie 16 octets. Copied from the Value_Request. Type 3 Reserved Three octets. For future use; MUST be set to zero when transmitted, and MUST be ignored when received. Responder-Exchange-Value variable precision number. Provided by the Respon- der for calculating a shared-secret between the par- ties. The Value format is indicated by the current Scheme-Choice as indicated by the Value_Request. The field may be any integral number of octets in length, as indicated by its Size field. It does not require any particular alignment. The 32-bit align- ment shown is for convenience in the illustration. Karn & Simpson expires in six months [Page 30] DRAFT Photuris June 1996 Responder-Offered-Attributes A list of Security Parameter attributes supported by the Responder. The contents and usage of this list are further described in "Offered Attributes List". The end of the list is indicated by the UDP Length. 4.3. Offered Attribute List This list includes those attributes supported by the party that are available to the other party. The attribute formats are specified in the "Attribute List", where mandatory attributes are also specified. The list is composed of three sections: Identification-Attributes, Authentication-Attributes, and Encapsulation-Attributes. Within each section, the attributes are listed from most to least preferable. The first section of the list includes methods of identification. An Identity-Choice is selected from this list. The second section of the list begins with "AH-Attributes" (#1). It includes methods of authentication, and other operational types. The third section of the list begins with "ESP-Attributes" (#2). It includes methods of compression, encryption, and other operational types. Attribute-Choices are selected from the latter two sections of the list. Implementation Notes: Since the offer is made by the prospective SPI User (sender), order of preference likely reflects the capabilities and engineer- ing tradeoffs of a particular implementation. However, the critical processing bottlenecks are frequently in the receiver. The SPI Owner (receiver) may express its needs by choosing a less preferable attribute. The order may also be affected by operational policy and requested services for an application. Such considerations are outside the scope of this document. Karn & Simpson expires in six months [Page 31] DRAFT Photuris June 1996 5. Identification Exchange Initiator Responder ========= ========= Identity_Request -> make SPI pick SPI attribute(s) identify self authenticate (make protection key) (encrypt message) <- Identity_Response make SPI pick SPI attribute(s) identify self authenticate (make protection key) (encrypt message) [make SPI session-keys in each direction] The exchange of messages is ordered, although the formats and mean- ings of the messages are identical in each direction. The messages are easily distinguished by the parties themselves, by examining the Type and Identification fields. Implementation Notes: The amount of time for the calculation may be dependent on the value of particular bits in secret values used in generating the shared-secret or identity verification. To prevent analysis of these secret bits by recording the time for calculation, sending of the Identity_Messages SHOULD be delayed until the time expected for the longest calculation. This will be different for different processor speeds, different algorithms, and different length vari- ables. Therefore, the method for estimating time is implementa- tion dependent. Any authenticated and/or encrypted user datagrams received before the completion of identity verification can be placed on a queue pending completion of this step. If verification succeeds, the queue is processed as though the datagrams had arrived subsequent to the verification. If verification fails, the queue is dis- carded. Karn & Simpson expires in six months [Page 32] DRAFT Photuris June 1996 5.0.1. Send Identity_Request The Initiator chooses an appropriate Identification, an SPI and SPI LifeTime, a set of Attributes for the SPI, calculates the Verifica- tion, and optionally encrypts the message for party privacy protec- tion (when a Privacy-Method is indicated by the Scheme-Choice). The Initiator also starts a retransmission timer. If no valid Iden- tity_Response arrives within the time limit, its previous Iden- tity_Request is retransmitted for the remaining number of Retransmis- sions. When Retransmissions have been exceeded, if a Bad_Cookie message has been received during the exchange, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request. 5.0.2. Receive Identity_Request The Responder validates the pair of Cookies, the Identification, the Verification, and the Attribute-Choices. - Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- sage is sent. - Whenever an invalid Identification is detected, or the message verification fails, a Verification_Failure message is sent. - Whenever the variable length Attribute-Choices do not match the UDP Length, or the attributes are not a subset of those in the Offered-Attributes, the message is silently discarded. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the Responder sets its Exchange timer to the Exchange LifeTime (if this has not already been done for a previ- ous exchange). When its parallel computation of the shared-secret is complete, the Responder returns an Identity_Response. The Responder keeps a copy of the incoming Identity_Request values, and its Identity_Response. If a duplicate Identity_Request is received, it merely resends its previous Identity_Response, and takes no further action. Karn & Simpson expires in six months [Page 33] DRAFT Photuris June 1996 5.0.3. Send Identity_Response The Responder chooses an appropriate Identification, an SPI and SPI LifeTime, a set of Attributes for the SPI, calculates the Verifica- tion, and optionally encrypts the message for party privacy protec- tion (when a Privacy-Method is indicated by the Scheme-Choice). The Responder calculates the SPI session-keys in both directions. The Responder sets its Update timer to half the value of its SPI LifeTime. If no new Photuris exchange occurs within the time limit, and the Exchange timer has not expired, an SPI_Update is sent to cre- ate another SPI. At this time, the Responder begins the authentication and/or encryp- tion of user datagrams. 5.0.4. Receive Identity_Response The Initiator validates the pair of Cookies, the Identification, the Verification, and the Attribute-Choices. - Whenever an invalid/expired cookie is detected, the message is silently discarded. - Whenever an invalid Identification is detected, or the message verification fails, a Verification_Failure message is sent. - Whenever the variable length Attribute-Choices do not match the UDP Length, or the attributes are not a subset of those in the Offered-Attributes, the message is silently discarded. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. - Once a valid message has been received, later Identity_Responses with both matching cookies are also silently discarded, until a new Cookie_Request is sent. When the message is valid, the Initiator sets its Exchange timer to the Exchange LifeTime (if this has not already been done for a previ- ous exchange). The Initiator calculates the SPI session-keys in both directions. The Initiator sets its Update timer to half the value of its SPI Karn & Simpson expires in six months [Page 34] DRAFT Photuris June 1996 LifeTime. If no new Photuris exchange occurs within the time limit, and the Exchange timer has not expired, an SPI_Update is sent to cre- ate another SPI. At this time, the Initiator begins the authentication and/or encryp- tion of user datagrams. 5.1. Identity_Messages +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | LifeTime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security-Parameter-Index | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | Identity-Choice | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Identification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute-Choices ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | PadLength | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. Copied from the Value_Request. Responder-Cookie 16 octets. Copied from the Value_Request. Type 4 (Request) or 7 (Response) LifeTime three octets. The number of seconds remaining before the indicated SPI expires. Must be greater than zero. Karn & Simpson expires in six months [Page 35] DRAFT Photuris June 1996 Security-Parameter-Index four octets. The SPI to be used for incoming commu- nications. When zero, indicates that no SPI is created in this direction. Identity-Choice An identity attribute is selected from the list of Offered-Attributes sent by the peer, and is used to calculate the Verification. The field may be any integral number of octets in length, as indicated by its Length field. It does not require any particular alignment. The 16-bit alignment shown is for convenience in the illustra- tion. Identification variable precision number, or alternative format indicated by the Identity-Choice. See the "Attribute List" for details. The field may be any integral number of octets in length. It does not require any particular align- ment. The 32-bit alignment shown is for convenience in the illustration. Verification variable precision number, or alternative format indicated by the Identity-Choice. The calculation of the value is described in "Identity Verifica- tion". The field may be any integral number of octets in length. It does not require any particular align- ment. The 32-bit alignment shown is for convenience in the illustration. Attribute-Choices Zero or more octets. A list of attributes for this (non-zero) SPI, selected from the list of Offered- Attributes supported by the peer. The contents and usage of this list are further described in "Attribute Choices List". The end of the list is indicated by the UDP Length after remov- | ing the PadLength and Padding fields (UDP Length - | PadLength - 1). Karn & Simpson expires in six months [Page 36] DRAFT Photuris June 1996 Padding Zero or more octets. Prior to (optional) encryp- tion, it is filled to align the PadLength field at a | boundary appropriate to the Privacy-Method indicated by the current Scheme-Choice. The padding values begin with the value 0, and count up to the number | of padding octets (zero relative). For example, if | the PadLength is 5, the padding values are 0, 1, 2, 3, 4. After (optional) decryption, if the padding octets | are not the correct values for the PadLength, then verification fails. PadLength one octet. The size of the Padding field in octets | (not including the PadLength field). The value typ- ically ranges from 0 to 7, but may be up to 255 to permit hiding of the actual data length. This field is always present, even when no Padding is required. The portion of the message after the SPI MAY be encrypted for party privacy protection. Such mechanisms are outside the scope of this document. The fields following the SPI are opaque. That is, the values are set prior to (optional) encryption, and examined only after (optional) decryption. 5.2. Attribute Choices List This list specifies the attributes of a Security Association. The attribute formats are specified in the "Attribute List". The list is composed of one or two sections: Authentication- Attributes, and/or Encapsulation-Attributes. When sending from the SPI User to the SPI Owner, the attributes are processed in the order listed. For example, "ESP-Attributes", "DES-CBC", "AH-Attributes", "MD5-KDP", would result in ESP with encryption, and then AH authentication of the ESP payload. Karn & Simpson expires in six months [Page 37] DRAFT Photuris June 1996 The SPI Owner will naturally process the datagram in the reverse order. This ordering also affects the order of key generation. Both SPI Owner and SPI User generate the keys in the order listed. Implementation Notes: When choices are made from the list of Offered-Attributes, it is not required that any Security Association include every kind of offered attribute in any single SPI, or that a separate SPI be created for every offered attribute. Some analysts have recommended that the AH should always be out- side the ESP. This is a matter for future research. Some kinds of attributes may be included more than once in a sin- gle SPI. The set of allowable combinations of attributes are dependent on implementation and operational policy. Such consid- erations are outside the scope of this document. 5.3. Shared-Secret The shared-secret is used in a number of calculations. Regardless of the internal representation of the shared-secret, when used in calcu- lations it is in the same form as the Value part of a Variable Preci- sion Number: - most significant octet first. - bits used are right justified within octet boundaries. - any unused bits are in the most significant octet. - unused bits are zero filled. 5.4. Identity Verification This message is authenticated using the Identity-Choice. The Verifi- cation value is calculated prior to (optional) encryption, and veri- fied after (optional) decryption. The Identity-Choice authentication function is supplied with two input values: - the computed shared-secret. - the data to be verified (as a concatenated sequence of octets). Karn & Simpson expires in six months [Page 38] DRAFT Photuris June 1996 The resulting output value is stored in the Verification field. The Identity-Choice authentication function is calculated over the following concatenated data values: + the Initiator Cookie, + the Responder Cookie, + the Responder Offered-Schemes, + the SPI Owner Exchange-Value, + the SPI Owner Offered-Attributes, + the SPI Owner Identification, + the SPI Owner secret-key, + the SPI User Exchange-Value, + the SPI User Offered-Attributes, + the SPI User Identification (when known), + the SPI User secret-key (when known), + the message Type, LifeTime and SPI fields, + the Attribute-Choices following the Verification field, + the Padding (if any), + the PadLength. | Note that the order of the Exchange-Value and Offered-Attribute fields is different in each direction. The Identification and SPI fields are also likely to be different in each direction. Note also that the SPI User Identification and secret-key will be omitted in the Identity_Request. If the verification fails, the users are notified, and a Verifica- tion_Failure message is sent, without adding any Security Associa- tions. On success, normal operation begins with the authentication and/or encryption of user datagrams. Implementation Notes: This is separate from any authentication method specified for Security Associations. The exact details of the Identification and secret-keys that are included in the Verification calculation are dependent on the Identity-Choice, as described in the "Attribute List". Each party may wish to keep their own trusted databases, such as the Pretty Good Privacy (PGP) web of trust, and accept only those identities found there. Failure to find the Identification in either an internal or external database results in the same Veri- fication_Failure message as failure of the verification computa- tion. Karn & Simpson expires in six months [Page 39] DRAFT Photuris June 1996 The hash of the Exchange-Value includes both the Size and Value fields. The hash of the Offered-Attributes and Attribute-Choices includes the Type, Length and Value fields. 5.5. Session-Key Computation Each Security Association SPI has one or more session-keys. These keys are generated based on the attributes of the Security Associa- tion. See the "Attribute List" for details. The Attribute-Choice specified key generation cryptographic hash is used to create an SPI session-key for that particular attribute. This hash is calculated over the following concatenated values: + the Initiator Cookie, + the Responder Cookie, + the SPI Owner secret-key, + the SPI User secret-key, + the message Verification field, + the computed shared-secret. Since the message Verification field is likely to be different in each direction, and the order of the secret-keys is different in each direction, the resulting session-key will usually be different in each direction. When a larger number of keying-bits are needed than are available from the specified cryptographic hash, these keying-bits are gener- ated by duplicating the trailing shared-secret, and recalculating the hash. That is, the first hash will have one trailing copy of the shared-secret, the second hash will have two trailing copies of the shared-secret, and so forth. Implementation Notes: Inclusion of the Verification field (dependent on the SPI), together with the party secret-keys, allows reuse of the same Exchange-Values and resulting shared-secret among several parties and multiple users of the same node without generating the same session-keys. The exact details of the Verification field and secret-keys that are included in the session-key calculation are dependent on the Identity-Choices, as described in the "Attribute List". To avoid keeping the secret-keys in memory after the initial veri- fication, it is often possible to precompute the hash of the Karn & Simpson expires in six months [Page 40] DRAFT Photuris June 1996 initial octets of the concatenated data values for each direction. When both authentication and encryption attributes are used for the same SPI, there may be multiple session-keys associated with the same SPI. These session-keys are generated in the order of the Attribute-Choices list. 6. SPI Messages SPI User SPI Owner ======== ========= SPI_Needed -> list SPI attribute(s) make integrity key authenticate (encrypt message) <- SPI_Update make SPI pick SPI attribute(s) make SPI session-key(s) make integrity key authenticate (encrypt message) The exchange of messages is not related to the Initiator and Respon- der. Instead, either party may send one of these messages at any time. The messages are easily distinguished by the parties. 6.0.1. Send SPI_Needed At any time after completion of the Identification Exchange, either party can send an SPI_Needed. This message is sent when a prospec- tive SPI User needs particular attributes for a datagram (such as privacy protection), and no current SPI has those attributes. The prospective SPI User selects from the intersection of attributes that both parties have previously offered, calculates the Verifica- tion, and optionally encrypts the message for party privacy protec- tion (when a Privacy-Method is indicated by the Scheme-Choice). Karn & Simpson expires in six months [Page 41] DRAFT Photuris June 1996 6.0.2. Receive SPI_Needed The potential SPI Owner validates the pair of Cookies, the Verifica- tion, and the Attributes-Needed. - Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- sage is sent. - Whenever the message verification fails, a Verification_Failure message is sent. - Whenever the variable length Attributes-Needed do not match the UDP Length, or the attributes are not a subset of those in the Offered-Attributes, the message is silently discarded. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, the party SHOULD send an SPI_Update that includes the necessary attributes. 6.0.3. Send SPI_Update At any time after completion of the Identification Exchange, either party can send an SPI_Update. This message has effect in only one direction, from the SPI Owner to the SPI User. The SPI Owner chooses an SPI and SPI LifeTime, a set of Attributes for the SPI, calculates the Verification, and optionally encrypts the message for party privacy protection (when a Privacy-Method is indi- cated by the Scheme-Choice). 6.0.4. Receive SPI_Update The prospective SPI User validates the pair of Cookies, the Verifica- tion, and the Attributes-Needed. - Whenever an invalid/expired cookie is detected, a Bad_Cookie mes- sage is sent. - Whenever the message verification fails, a Verification_Failure message is sent. - Whenever the variable length Attribute-Choices do not match the UDP Length, or the attributes are not a subset of those in the Karn & Simpson expires in six months [Page 42] DRAFT Photuris June 1996 Offered-Attributes, the message is silently discarded. - Whenever such a problem is detected, the Security Association is not established; the implementation SHOULD log the occurance, and notify an operator as appropriate. When the message is valid, further actions are dependent on the value of the SPI LifeTime field, as described later. 6.1. SPI_Needed +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attributes-Needed ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | PadLength | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. Copied from the Value_Request. Responder-Cookie 16 octets. Copied from the Value_Request. Type 8 Reserved seven octets. For future use; MUST be set to zero when transmitted, and MUST be ignored when received. Verification variable precision number, or other format indicated by the Scheme-Choice. The calculation of the value is described in "Validity Verification". Karn & Simpson expires in six months [Page 43] DRAFT Photuris June 1996 The field may be any integral number of octets in length. It does not require any particular align- ment. The 32-bit alignment shown is for convenience in the illustration. Attributes-Needed Four or more octets. A list of two or more attributes, selected from the list of Offered- Attributes supported by the peer. The contents and usage of this list are as previ- ously described in "Attribute Choices List". The end of the list is indicated by the UDP Length after | removing the PadLength and Padding fields (UDP | Length - PadLength - 1). Padding Zero or more octets. Prior to (optional) encryp- tion, it is filled to align the PadLength field at a | boundary appropriate to the Privacy-Method indicated by the current Scheme-Choice. The padding values begin with the value 0, and count up to the number | of padding octets (zero relative). For example, if | the PadLength is 5, the padding values are 0, 1, 2, 3, 4. After (optional) decryption, if the padding octets | are not the correct values for the PadLength, then verification fails. PadLength one octet. The size of the Padding field in octets | (not including the PadLength field). The value typ- ically ranges from 0 to 7, but may be up to 255 to permit hiding of the actual data length. This field is always present, even when no Padding is required. The portion of the message after the SPI MAY be encrypted for party privacy protection, in the same fashion specified for Iden- tity_Messages. The fields following the SPI are opaque. That is, the values are set prior to (optional) encryption, and examined only after (optional) decryption. Karn & Simpson expires in six months [Page 44] DRAFT Photuris June 1996 6.2. SPI_Update +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Initiator-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Responder-Cookie ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | LifeTime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Security-Parameter-Index | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | | ~ Verification ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Attribute-Choices ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... Padding | PadLength | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Initiator-Cookie 16 octets. Copied from the Value_Request. Responder-Cookie 16 octets. Copied from the Value_Request. Type 9 LifeTime three octets. The number of seconds remaining before the indicated SPI expires. The value zero indicates deletion of the indicated SPI. Security-Parameter-Index four octets. The SPI to be used for incoming commu- nications. This may be a new SPI value (for creation), or an existing SPI value (for deletion). The value zero indicates all old SPIs for this IP Destination (used for deletion). Verification variable precision number, or other format indicated by the Scheme-Choice. The calculation of the value is described in "Validity Verification". Karn & Simpson expires in six months [Page 45] DRAFT Photuris June 1996 The field may be any integral number of octets in length. It does not require any particular align- ment. The 32-bit alignment shown is for convenience in the illustration. Attribute-Choices Four or more octets. A list of two or more attributes for this SPI, selected from the list of Offered-Attributes supported by the peer. The contents and usage of this list are as previ- ously described in "Attribute Choices List". The end of the list is indicated by the UDP Length after | removing the PadLength and Padding fields (UDP | Length - PadLength - 1). Padding Zero or more octets. Prior to (optional) encryp- tion, it is filled to align the PadLength field at a | boundary appropriate to the Privacy-Method indicated by the current Scheme-Choice. The padding values begin with the value 0, and count up to the number | of padding octets (zero relative). For example, if | the PadLength is 5, the padding values are 0, 1, 2, 3, 4. After (optional) decryption, if the padding octets | are not the correct values for the PadLength, then verification fails. PadLength one octet. The size of the Padding field in octets | (not including the PadLength field). The value typ- ically ranges from 0 to 7, but may be up to 255 to permit hiding of the actual data length. This field is always present, even when no Padding is required. The portion of the message after the SPI MAY be encrypted for party privacy protection, in the same fashion specified for Iden- tity_Messages. The fields following the SPI are opaque. That is, the values are set prior to (optional) encryption, and examined only after (optional) decryption. Karn & Simpson expires in six months [Page 46] DRAFT Photuris June 1996 6.2.1. Creation When the SPI LifeTime is greater than zero, the SPI_Update can be used to create a new Security Association. Frequently, this message is used to create replacement SPIs as the LifeTime of an earlier SPI approaches expiration. In addition, this message allows more rapid SPI creation for high bandwidth applications. The messages flow in the opposite direction from the primary traffic flow. The new session-keys are calculated in the same fashion as the Iden- tity_Messages. Since the SPI value is always different than any pre- vious SPI during the Exchange LifeTime of the shared-secret, the resulting session-keys will necessarily be different from all others used in the same direction. When the peer finds that too many SPI values are already in use for this party, or some other resource limit is reached, a Resource_Limit message is sent. No retransmission timer is necessary. Success is indicated by the peer use of the new SPI. Should all creation attempts fail, eventually the peer will find that all existing SPIs have expired, and will begin the Photuris exchange again by sending a new Cookie_Request. When appropriate, this Cookie_Request MAY include a Responder-Cookie to retain previous party pairings. 6.2.2. Deletion When the SPI LifeTime is zero, the SPI_Update can be used to delete existing Security Associations. This is especially useful when the application that needed them terminates, to prevent another applica- tion from replaying the datagrams. No retransmission timer is necessary. This message is advisory, to reduce the number of ICMP Security Failures messages. Should any deletion attempts fail, the peer will learn that the deleted SPIs are invalid through the normal ICMP Security Failures messages, and will initiate a Photuris exchange by sending a new Cookie_Request. Karn & Simpson expires in six months [Page 47] DRAFT Photuris June 1996 6.2.3. Modification The SPI_Update cannot be used to modify existing Security Associa- tions, such as lengthen an existing SPI LifeTime, resurrect an expired SPI, or add/remove an Attribute-Choice. On receipt, such an otherwise valid message is silently discarded. 6.2.4. Validity Verification This message is authenticated using the Validity-Method indicated by the current Scheme-Choice (see "Exchange Scheme List"). The Verifi- cation value is calculated prior to (optional) encryption, and veri- fied after (optional) decryption. The Validity-Method authentication function is supplied with two input values: - the computed shared-secret, - the data to be verified (as a concatenated sequence of octets). The resulting output value is stored in the Verification field. The Validity-Method authentication function is calculated over the following concatenated data values: + the Initiator Cookie, + the Responder Cookie, + the SPI Owner Identity Verification, + the SPI User Identity Verification, + the message Type, LifeTime and SPI fields, + the Attribute-Choices following the Verification field, + the Padding (if any), + the PadLength. | Note that the order of the Identity Verification fields (from the Identity_Messages) is different in each direction. If the verification fails, the users are notified, and a Verifica- tion_Failure message is sent, without adding or deleting any Security Associations. On success, normal operation begins with the authenti- cation and/or encryption of user datagrams. Karn & Simpson expires in six months [Page 48] DRAFT Photuris June 1996 Implementation Notes: This is separate from any authentication method specified for Security Associations. The hash of the Identity Verification includes both the Size and Value fields. The hash of the Attribute-Choices includes the Type, Length and Value fields. 7. Error Messages Issued in response to Photuris state loss or other problems. The message has effect in only one direction. No retransmission timer is necessary. These messages are not encrypted for party privacy protection. The receiver checks the Cookies for validity. Special care MUST be taken that the Cookie pair in the Error Message actually match a pair currently in use, and that the protocol is currently in a state where such an Error Message might be expected. Otherwise, these messages could provide an opportunity for a denial of service attack. Invalid messages are silently discarded. 7.1. Bad_Cookie For the format of the message, see "Header Format". There are no additional fields. Initiator-Cookie 16 octets. Copied from the offending message. Responder-Cookie 16 octets. Copied from the offending message. Type 10 This error message is sent when a Value_Request, Identity_Request, SPI_Needed, or SPI_Update is received, and the receiver's Cookie is invalid or the associated Exchange-Value has expired. During the Photuris exchange, when this error message is received, it has no immediate effect on the operation of the protocol phases. When Retransmissions have been exceeded, if this error message has been received, the Initiator SHOULD begin the Photuris exchange again by sending a new Cookie_Request. After the Photuris exchange has completed, when this error message is Karn & Simpson expires in six months [Page 49] DRAFT Photuris June 1996 received in response to an SPI_Needed or SPI_Update, the party SHOULD initiate a Photuris exchange by sending a new Cookie_Request. However, existing SPIs are not deleted. They expire normally, and are purged sometime later. Design Notes: This message will occur normally at any time after the Cookie_Response, whenever the Responder dynamically changes its local secret for cookie generation and the secret for generating its Exchange-Value, or either party expires its exchange state. On the other hand, an observer could attempt to use this message for denial of service by copying the valid cookies and sending it faster than the round-trip of the valid exchange peer. Therefore, the protocol gracefully recovers during the Value and Identification Exchanges by using the Retransmission TimeOut to give sufficient time for a valid exchange reply to arrive. It recovers during the SPI Messages by using cached prior exchange values to eliminate the intensive calculations of a new Photuris exchange. 7.2. Resource_Limit For the format of the message, see "Header Format". There are no additional fields. Initiator-Cookie 16 octets. Copied from the offending message. Responder-Cookie 16 octets. Copied from the offending message. Type 11 This error message is sent when a Cookie_Request or SPI_Update is received, and too many SPI values are already in use for that peer, or some other Photuris resource is unavailable. During the Photuris exchange, when this error message is received in response to a Cookie_Request, the implementation SHOULD double the retransmission timeout for sending another Cookie_Request. After the Photuris exchange has completed, when this error message is received in response to an SPI_Update, the implementation SHOULD NOT send another SPI_Update until it has deleted an existing SPI, or waited for a cached SPI entry to expire. Karn & Simpson expires in six months [Page 50] DRAFT Photuris June 1996 Design Notes: This message will occur normally instead of a Cookie_Response, during such events as server recovery after a power failure. It | also regulates overly aggressive SPI creation. Again, an observer could attempt to use this message for denial of service by copying the valid cookies and sending it faster than the round-trip of the valid exchange peer. Therefore, the protocol gracefully recovers during the Cookie Exchange by using the Retransmission TimeOut to give sufficient time for a valid exchange reply to arrive. It recovers during the SPI Messages by the normal SPI expiration process. 7.3. Verification_Failure For the format of the message, see "Header Format". There are no additional fields. Initiator-Cookie 16 octets. Copied from the offending message. Responder-Cookie 16 octets. Copied from the offending message. Type 12 This error message is sent when an Identity_Message, SPI_Needed or SPI_Update is received, and verification fails. When this error message is received, the implementation SHOULD log the occurance, and notify an operator as appropriate. However, receipt has no effect on the operation of the protocol. Design Notes: This message will not occur normally. The message will notify an | operator when the Identification used is not valid, or an inter- | cepter has been sending faked exchange messages that failed final | authentication. Again, an observer could attempt to use this message for denial of service by copying the valid cookies and sending it faster than the round-trip of the valid exchange peer. Therefore, the protocol gracefully recovers during the Identifica- tion Exchange by using the Retransmission TimeOut to give suffi- cient time for a valid exchange reply to arrive. It recovers Karn & Simpson expires in six months [Page 51] DRAFT Photuris June 1996 during the SPI Messages by using cached prior exchange values to eliminate the intensive calculations of a new Photuris exchange. 8. Public Value Exchanges Photuris is based in principle on public-key cryptography, specif- ically Diffie-Hellman key exchange. Exchange of D-H Exchange- Values based on private/secret values results in a mutual shared- secret between the parties. This shared-secret can be used on its own, or to generate a series of session-keys for authentication and encryption of subsequent traffic. Widespread deployment and use of an Internet Security protocol is possible without public-key cryptography. For example, Kerberos [RFC-1510] can generate host-pair keys for use in Internet Secu- rity, much as it now generates session-keys for use by encrypted telnet and other "kerberized" applications. The Kerberos model has some widely recognized drawbacks. Foremost is the requirement for a highly available on-line Key Distribution Center (KDC), with a database containing every principal's secret- key. This carries significant security risks. Public-key cryptography enables decentralization. Entities gener- ate session-keys without real-time communication with any other party. This draft assumes familiarity with the Diffie-Hellman public-key algorithm. A good description can be found in [Schneier95]. 8.1. Modular Exponentiation Groups The original Diffie-Hellman technique [DH76] specified modular exponentiation. An Exchange-Value is generated using a generator (g), raised to a private/secret exponent (x), modulo a prime (p). (g**x) mod p When these public-values are exchanged between parties, the par- ties can calculate a shared-secret value between themselves. (g**xy) mod p The security depends on the relative difficulty of calculating discrete logarithms, compared to the ease of exponentiation in the same finite field. The prime modulus MUST be sufficiently large Karn & Simpson expires in six months [Page 52] DRAFT Photuris June 1996 to prevent calculation of its discrete logs within the lifetime of the protected data. When a strong prime modulus and generator pair are well chosen, the difficulty of a discrete log attack is maximized. By choosing the pairs in advance, analysis of the pair characteristics is pos- sible. This analysis can promote confidence in the security of the implementations. The generator (g) and modulus (p) are established by the Scheme- Choice (see "Exchange Scheme List" for details). They are offered in the Cookie_Response, and one pair is chosen in the Value_Request. The exponents (x) and (y) are kept secret by the parties. Only | the public-value result of the modular exponentiation with (x) or (y) is sent as the Initiator and Responder Exchange-Value. | These public-values are represented in single Variable Precision | Numbers. The Size of these Exchange-Values will match the Size of | the modulus (p). 8.2. Moduli Selection Each implementation proposes one or more moduli in its Offered- Schemes. Every implementation MUST support up to 4096-bit moduli. For any particular Photuris node, these moduli need not change for significant periods of time; likely days or weeks. A background process can periodically generate new moduli. 8.2.1. Strong Primes Ideally, each prime modulus (p) should have the property that both p and (p-1)/2 are prime. This provides the strongest defense against factoring. Discovery of strong primes is extremely computationally intensive, and practically impossible for commercially available processors to find in a reasonable interactive time. Complete verification can take hours or days. Karn & Simpson expires in six months [Page 53] DRAFT Photuris June 1996 8.2.2. Prime-Order Subgroups An alternative is the use of a large subgroup where q is a prime factor of (p-1). This technique is described in [OW96], and based on [Schnorr91]. Discovery of prime-order subgroups is less computationally inten- sive than verification of strong primes. The computational cost of finding such a prime (p) with a prime divisor (q) is only a little more than finding any random prime. 8.2.3. Unstructured Primes A random unstructured prime (p), where (p-1) may have small prime factors, is subject to a Pohlig-Hellman attack. Strong primes and prime-order subgroups prevent this attack. Discovery of random primes is the bulk of the computational pro- cessing of the previously described primes. Therefore, they SHOULD be used instead of unstructured primes. 8.2.4. Non-Primes Technically, the modulus is not required to be prime. Any suffi- ciently large modulus would be useful, and provide a minimal amount of security. To improve security, a potential modulus should be sieved to reject those with small prime factors (less than 1,000,000). However, the security of non-prime moduli is considered insuffi- cient for data of any long-term value. These SHOULD NOT be used, except in the most ephemeral cases -- such as cellular telephones, and other low computational power devices. 8.2.5. Bootstrap Moduli Each implementation is likely to use a fixed modulus during its bootstrap, until it can generate another modulus in the back- ground. As the bootstrap modulus will be widely distributed, and reused whenever the machine reinitializes, it SHOULD be a strong prime to provide the greatest long-term protection. Karn & Simpson expires in six months [Page 54] DRAFT Photuris June 1996 8.2.6. Learning Moduli As Photuris exchanges are initiated, new moduli will be learned from the Responder Offered-Schemes. The Initiator MAY cache these moduli for its own use. Before offering any learned modulus, the implementation MUST per- form at least one iteration of probable primality verification. In this fashion, many processors will perform verification in par- allel as moduli are passed around. When primality verification failures are found, the failed moduli SHOULD be retained for some (implementation dependent) period of time, to avoid relearning and retesting after subsequent exchanges. 8.3. Generator Selection The generator (g) should be chosen such that the secret exponents will generate all possible public-values, evenly distributed throughout the range of the modulus (p), without cycling through a smaller subset. Such a generator is called a "primitive root" (which is trivial to find when p is strong). Only one generator (2) is required to be supported. Implementation Notes: One useful technique is to select the generator, and then limit the modulus selection sieve to primes with that generator. 2 when p (mod 24) = 11. 3 when p (mod 12) = 5. 5 when p (mod 10) = 3 or 7. The required generator (2) improves efficiency in multiplica- tion performance. It is usable even when it is not a primitive root, as it still covers half of the space of possible residues. 8.4. Exponent Selection Each implementation generates a separate random secret exponent for each different modulus. Then, a D-H Exchange-Value is calcu- lated for the given modulus, generator, and exponent. Karn & Simpson expires in six months [Page 55] DRAFT Photuris June 1996 The exponent 0 will generate the public value 1, and exponent 1 will generate the public value g mod p. These exponents do not qualify as secret. Although the same exponent and Exchange-Value may be used with several parties whenever the same modulus and generator are used, the exponent SHOULD be changed at random intervals. A background process can periodically destroy the old values, generate a new random secret exponent, and recalculate the Exchange-Value. This limits the exposure of both the secret exponent and shared-secret, protecting earlier communications, as past secrets are not kept for possible discovery by a future intrusion. Also, the secret exponent currently in use is less likely to be anticipated, as the element of random timing is introduced. Since these operations involve several time-consuming modular exponentiations, moving them to the "background" substantially improves the apparent execution speed of the Photuris protocol. It also reduces CPU loading sufficiently to allow a single pub- lic/private key-pair to be used in several closely spaced Photuris executions, when creating Security Associations with several dif- ferent nodes over a short period of time. Consideration should also be given to the speed versus security tradeoffs of modular exponentiation. While an exponent may be used that is shorter than the modulus, the cryptologic literature is indeterminate as to the minimum proportionate size. This spec- ification recommends that the exponent length be at least twice the desired cryptographic strength of the longest session-key needed by the strongest offered-attribute. Implementation Notes: The size of the exponent is entirely implementation dependent, is unknown to the other party, and can be easily changed. A single modular exponentiation on a 486-66DX2 processor using RSAREF and Borland C under MS-DOS took 20 seconds with a 1024-bit prime modulus and a 1024-bit random exponent. This dropped to about 1 to 1.5 seconds when the random exponent was shortened to 128 bits, with the same 1024-bit modulus. Other precomputation suggestions are described in [BGMW93] and [Rooij94]. Karn & Simpson expires in six months [Page 56] DRAFT Photuris June 1996 9. Exchange Scheme List Initial values are assigned as follows: (0) Reserved. (1) Reserved. (2) Implementation Required. Any modulus (p) with a recommended generator (g) of 2. The modulus is contained in the Exchange Scheme Value field in the list of Offered-Schemes. The "Identification Exchange" and "SPI Messages" Privacy- Method is "not protected". The "SPI Messages" Validity-Method is "MD5-DP". (3) Exchange-Schemes 3 to 255 are intended for future well-known published schemes. (256) Exchange-Schemes 256 to 32767 are intended for vendor- specific unpublished schemes. Implementors wishing a number MUST request the number from the authors. (32768) Exchange-Schemes 32768 to 65535 are available for cooperat- ing parties to indicate private schemes, regardless of ven- dor implementation. These numbers are not reserved, and are subject to duplication. Other criteria, such as the IP Source and Destination of the Cookie_Request, are used to differentiate the particular Exchange-Schemes available. Karn & Simpson expires in six months [Page 57] DRAFT Photuris June 1996 10. Validity Methods 10.1. MD5-DP As described in "Validity Verification", the MD5 [RFC-1321] hash is calculated over the concatenation of MD5( key, data, datafill, key, md5fill ) The leading key is not padded to any particular alignment. The datafill uses the same pad-with-length technique defined for md5fill. The length includes the leading key and data. The resulting Verification field is a 128-bit Variable Precision | Number (18 octets including Size). 11. Attribute List Implementors wishing a number MUST request the number from the authors. Initial values are assigned as follows: Use Type - 0* padding - 1* AH-Attributes - 2* ESP-Attributes I 3* Simple MD5-DP Verification A 5* MD5-KDP E 8* DES-CBC X 255 Organizational A AH Attribute-Choice E ESP Attribute-Choice I Identity-Choice X dependent on list location * feature must be supported (mandatory) Other attributes are specified in companion documents. Karn & Simpson expires in six months [Page 58] DRAFT Photuris June 1996 11.1. Padding +-+-+-+-+-+-+-+-+ | Type | +-+-+-+-+-+-+-+-+ Type 0 Each attribute may have value fields that are multiple octets. To facilitate processing efficiency, these fields are aligned on integral modulo 8 octet (64-bit) boundaries. Padding is accomplished by insertion of 1 to 7 Type 0 padding octets before the attribute that needs alignment. No padding is used after the final attribute in a list. 11.2. AH-Attributes +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 1 Length 0 When a list of Attributes is specified, this Attribute begins the section of the list which applies to the Authentication Header (AH). Karn & Simpson expires in six months [Page 59] DRAFT Photuris June 1996 11.3. ESP-Attributes +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 2 Length 0 When a list of Attributes is specified, this Attribute begins the section of the list which applies to the Encapsulating Security Payload (ESP). 11.4. Simple MD5-DP Verification +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 3 Length 0 When selected as an Identity-Choice, the immediately following Identification field contains an unstructured Variable Precision | Number. Valid Identifications and symmetric secret-keys are pre- configured by the parties. There is no required format or content for the Identification value. The value may be a number or string of any kind. Typically, the Identification is a user name, a Fully Qualified Domain Name, or an email address which contains a user name and a domain name. Examples include: user node.site. user@node.site. rcmd@node.site. "Mundane Name" <user@node.site> | There is no requirement that the domain name match any of the par- ticular IP addresses in use by the parties. Karn & Simpson expires in six months [Page 60] DRAFT Photuris June 1996 The authentication symmetric secret-key (as specified) is selected based on the contents of the Identification field. All implemen- tations must support at least 62 octets. The selected symmetric secret-key SHOULD provide at least 64-bits of cryptographic strength. As described in "Identity Verification", the MD5 [RFC-1321] hash is calculated over the concatenation of: MD5( key, data, datafill, key, md5fill ) The leading key is not padded to any particular alignment. The datafill uses the same pad-with-length technique defined for md5fill. The length includes the leading key and data. The resulting Verification field is a 128-bit Variable Precision | Number (18 octets including Size). For identity verification and session-key calculation, the authen- tication symmetric secret-key is also used as the calculation secret-key. 11.5. MD5-KDP +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 5 Length 0 May be selected as an AH Attribute-Choice, pursuant to [RFC-1828] et sequitur. The selected Exchange Scheme SHOULD provide at least 64-bits of cryptographic strength. MD5 [RFC-1321] is used as the key generation cryptographic hash for generating the SPI session-key, as described in "Session-Key Computation". The most significant 496-bits (62 octets) of the generated hashes are used for the key. The remaining least significant 16-bits (2 octets) of the last hash are discarded. When combined with other uses of key genera- tion for the same SPI, the next such attribute will begin with a new hash. Karn & Simpson expires in six months [Page 61] DRAFT Photuris June 1996 Profile: When negotiated with Photuris, the transform differs slightly from [RFC-1828]. The form of the authenticated message is: MD5( key, keyfill, datagram, datafill, key, md5fill ) The additional datafill protects against the attack described in [PO96]. This is also filled to the next 512-bit boundary, using the same pad-with-length technique defined for MD5. The length includes the leading key and data. 11.6. DES-CBC +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 8 Length 0 May be selected as an ESP Attribute-Choice, pursuant to [RFC-1829] et sequitur. The selected Exchange Scheme SHOULD provide at least 56-bits of cryptographic strength. MD5 [RFC-1321] is used as the key generation cryptographic hash for generating the SPI session-key, as described in "Session-Key Computation". The most significant 64-bits (8 octets) of the gen- | erated hash are used for the key. The least significant bit of each key octet is ignored (or set to parity when the implementa- tion requires). If the key matches any of the weak, semi-weak or possibly weak keys [Schneier95, pages 280-282], that key is discarded; the next 64-bits of the generated hash are used instead, recursively. The remaining octets of the last hash are discarded. When com- bined with other uses of key generation for the same SPI, the next such attribute will begin with a new hash. Karn & Simpson expires in six months [Page 62] DRAFT Photuris June 1996 Profile: When negotiated with Photuris, the transform differs slightly from [RFC-1829]. The IV is always 32-bits. The 64-bit IV is generated from the 32-bit SPI field followed by (concatenated with) the 32-bit IV field. The bit-wise com- plement of the 32-bit IV value is XOR'd with the first 32-bits (SPI). The padding values begin with the value 0, and count up to the | number of padding octets (zero relative). For example, if the plaintext length is 41, the padding values are 0, 1, 2, 3, 4, | and the following PadLength is 5. After decryption, if the padding octets are not the correct | values for the PadLength, then the payload is discarded, and a "Decryption Failed" error is indicated, as described in [RFC- xxxx]. 11.7. Organizational +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | OUI +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ... | Kind | Value(s) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 255 Length >= 4 When the Length is four, no Value(s) field is present. OUI three octets. The vendor's Organizationally Unique Identifier, assigned by IEEE 802 (see [RFC-1700] for contact details). The bits within the octet are in canonical order, and the most significant octet is transmitted first. Kind one octet. Indicates a sub-type for the OUI. There is no standardization for this field. Each OUI implements its own values. Karn & Simpson expires in six months [Page 63] DRAFT Photuris June 1996 Value(s) Zero or more octets. The details are implementa- tion specific. Some implementors might not need nor want to publish their propri- | etary algorithms and attributes. This OUI mechanism is available to specify these without encumbering the authors with proprietary number requests. Karn & Simpson expires in six months [Page 64] DRAFT Photuris June 1996 A. Automaton An example automaton is provided to illustrate the operation of the protocol. It is incomplete and non-deterministic; many of the | Good/Bad semantic decisions are policy-based or too difficult to | represent in tabular form. Where conflicts appear between this example and the text, the text takes precedence. The finite-state automaton is defined by events, actions and state transitions. Events include reception of external commands such as expiration of a timer, and reception of datagrams from a peer. Actions include the starting of timers and transmission of data- grams to the peer. Events DU13 = Communication Administratively Prohibited SF0 = Bad SPI SF4 = Need Authentication SF5 = Need Authorization WP = Want Privacy RCQ+ = Receive Cookie_Request (Good) RCQ- = Receive Cookie_Request (Bad) RCR+ = Receive Cookie_Response (Good) RCR- = Receive Cookie_Response (Bad) RVQ+ = Receive Value_Request (Good) RVQ- = Receive Value_Request (Bad) RVR+ = Receive Value_Response (Good) RVR- = Receive Value_Response (Bad) RIQ+ = Receive Identity_Request (Good) RIQ- = Receive Identity_Request (Bad) RIR+ = Receive Identity_Response (Good) RIR- = Receive Identity_Response (Bad) RUN+ = Receive SPI_Needed (Good) RUN- = Receive SPI_Needed (Bad) RUM+ = Receive SPI_Update (Good) RUM- = Receive SPI_Update (Bad) RBC = Receive Bad Cookie RRL = Receive Resource Limit RVF = Receive Verification Failure TO+ = Timeout with counter > 0 TO- = Timeout with counter expired Karn & Simpson expires in six months [Page 65] DRAFT Photuris June 1996 UTO = Update TimeOut XTO = Exchange TimeOut Actions scq = Send Cookie_Request scr = Send Cookie_Response svq = Send Value_Request svr = Send Value_Response siq = Send Identity_Request sir = Send Identity_Response sum = Send SPI_Update se* = Send error message (see text) sbc = Send Bad Cookie srl = Send Resource Limit svf = Send Verification Failure brto = Backoff Retransmission TimeOut | buto = Backoff Update TimeOut rto = Set Retransmission TimeOut | uto = Set Update TimeOut xto = Set Exchange TimeOut log = log operator message A.1. State Transition Table States are indicated horizontally, and events are read vertically. State transitions and actions are represented in the form action/new-state. Multiple actions are separated by commas, and may continue on succeeding lines as space requires; multiple actions may be implemented in any convenient order. The state may be followed by a letter, which indicates an explanatory footnote. The dash ('-') indicates an illegal transition. Karn & Simpson expires in six months [Page 66] DRAFT Photuris June 1996 Initiator | 0 1 2 3 4 | Initial Cookie CookieBad Value ValueBad ------+-------------------------------------------------- DU13 |rto,scq/1 rto,scq/1 rto,scq/1 3 4 | SF0 |rto,scq/1 1 2 3 4 | SF4 |rto,scq/1 1 2 3 4 | SF5 |rto,scq/1 1 2 3 4 | WP |rto,scq/1 1 2 3 4 | | RCR+ | - rto,svq/3 rto,svq/3 3 4 | RCR- | 0 1 2 3 4 RVR+ | - - - rto,siq/5 rto,siq/5 | RVR- | 0 1 2 3 4 RIR+ | - - - - - RIR- | 0 1 2 3 4 | RUN+ | - - - - - RUN- | sbc/0 sbc/1 sbc/2 sbc/3 sbc/4 RUM+ | - - - - - RUM- | sbc/0 sbc/1 sbc/2 sbc/3 sbc/4 | RBC | - 2 2 4 4 RRL | - brto/1 brto/2 3 4 | RVF | - - - - - | TO+ | - scq/1 scq/2 svq/3 svq/4 TO- | - 0 scq/1 0 scq/1 UTO | - - - - - XTO | - 0 0 0 0 Karn & Simpson expires in six months [Page 67] DRAFT Photuris June 1996 Initiator | 5 6 8 |Identity IdentityBad Update ------+----------------------------- DU13 | 5 6 8 SF0 | 5 6 rto,scq/1 | SF4 | 5 6 rto,scq/1 | SF5 | 5 6 rto,scq/1 | WP | 5 6 sun/8 | RCR+ | 5 6 8 RCR- | 5 6 8 RVR+ | 5 6 8 RVR- | 5 6 8 RIR+ | uto/8 uto/8 8 RIR- | svf/5 svf/6 8 | RUN+ | - - sum/8 RUN- | sbc/5 sbc/6 se*/8 RUM+ | - - 8 RUM- | sbc/5 sbc/6 se*/8 | RBC | 6 6 rto,scq/1 | RRL | 5 6 buto/8 RVF | log/5 log/6 log/8 | TO+ | sim/5 sim/6 - TO- | 0 scq/1 - UTO | - - sum/8 XTO | 0 0 0 Karn & Simpson expires in six months [Page 68] DRAFT Photuris June 1996 Responder | 0 7 8 | Initial Ready Update ------+----------------------------- WP | - 7 sun/8 | RCQ+ | scr/0 scr/7 scr/8 RCQ- | srl/0 srl/7 srl/8 RVQ+ |xto,svr/7 svr/7 svr/8 RVQ- | sbc/0 sbc/7 sbc/8 RIQ+ | - uto,sir/8 sir/8 RIQ- | sbc/0 se*/7 se*/8 | RUN+ | - - sum/8 RUN- | sbc/0 sbc/7 se*/8 RUM+ | - - 8 RUM- | sbc/0 sbc/7 se*/8 | RBC | - 7 rto,scq/1 | RRL | - - buto/8 RVF | - - log/8 | UTO | - - sum/8 XTO | - 0 0 A.2. States Following is a more detailed description of each automaton state. The "Bad" version of a state is to indicate that the Bad_Cookie message has been received. A.2.1. Initial The Initial state is fictional, in that there is no state between the parties. A.2.2. Cookie In the Cookie state, the Initiator has sent a Cookie_Request, and is waiting for a Cookie_Response. Both the Restart and Exchange timers are running. Karn & Simpson expires in six months [Page 69] DRAFT Photuris June 1996 Note that the Responder has no Cookie state. A.2.3. Value In the Value state, the Initiator has sent its Exchange-Value, and is waiting for an Identity_Message. Both the Restart and Exchange timers are running. A.2.4. Identity In the Identity state, the Initiator has sent an Identity_Request, and is waiting for an Identity_Response in reply. Both the Restart and Exchange timers are running. A.2.5. Ready In the Ready state, the Responder has sent its Exchange-Value, and is waiting for an Identity_Message. The Exchange timer is run- ning. A.2.6. Update In the Update state, each party has concluded the Photuris exchange, and is unilaterally updating expiring SPIs until the Exchange LifeTime expires. Both the Update and Exchange timers are running. Karn & Simpson expires in six months [Page 70] DRAFT Photuris June 1996 B. Example Bootstrap Moduli During the initial bootstrap of the implementation, there may not be sufficient time to generate a new modulus before a security association is needed. These moduli are verified examples that may be used during this bootstrap period. (512-2) A 512-bit strong prime (p), expressed in hex: da58 3c16 d985 2289 d0e4 af75 6f4c ca92 dd4b e533 b804 fb0f ed94 ef9c 8a44 03ed 5746 50d3 6999 db29 d776 276b a2d3 d412 e218 f4dd 1e08 4cf6 d800 3e7c 4774 e833 The recommended generator (g) for this prime is 2. This prime modulus was randomly generated by a freely available program written by Phil Karn, verified using the mpz_probab_prime() function Miller-Rabin test in the Gnu Math Package (GMP) version 1.3.2; as well as independently developed test libraries by Rich Schroeppel (complete Elliptic Curve test). Currently estimated to provide 64 (pessimistic) bit-equivalents of cryptographic strength. Exponent lengths of 128 bits (or more) are recommended. Using current technology, calculation of the discrete loga- rithms is anticipated to take no more than a year. This is insufficient for long-term use. A modulus of this size is only used with transforms (such as DES) that already provide less protection than the estimated strength, for ephemeral authentication with short-lived ses- + sion-keys, and where rapid computation is of primary impor- tance. (1024-2) A 1024-bit strong prime (p), expressed in hex: Karn & Simpson expires in six months [Page 71] DRAFT Photuris June 1996 97f6 4261 cab5 05dd 2828 e13f 1d68 b6d3 dbd0 f313 047f 40e8 56da 58cb 13b8 a1bf 2b78 3a4c 6d59 d5f9 2afc 6cff 3d69 3f78 b23d 4f31 60a9 502e 3efa f7ab 5e1a d5a6 5e55 4313 828d a83b 9ff2 d941 dee9 5689 fada ea09 36ad df19 71fe 635b 20af 4703 6460 3c2d e059 f54b 650a d8fa 0cf7 0121 c747 99d7 5871 32be 9b99 9bb9 b787 e8ab The recommended generator (g) for this prime is 2. This prime modulus was randomly generated by a freely available program written by Phil Karn, verified using the mpz_probab_prime() function Miller-Rabin test in the Gnu Math Package (GMP) version 1.3.2; and also verified with GMP on other platforms by Wei Dai and Frank A Stevenson, as well as independently developed test libraries by Eric Young (Miller- Rabin test), and Rich Schroeppel (complete Elliptic Curve test). Currently estimated to provide 80 (pessimistic) through 98 (optimistic) bit-equivalents of cryptographic strength. Expo- nent lengths of 160 to 256 bits (or more) are recommended. Implementors are encouraged to generate their own bootstrap mod- uli, and to change bootstrap moduli in successive implementation releases. Operational Considerations The specification provides only a few configurable parameters, with defaults that should satisfy most situations. Retransmissions Default: 3. Initial Retransmission TimeOut (IRTO) Default: 10 seconds. Exchange TimeOut (ETO) Default: 60 seconds. Minimum: Retransmissions * IRTO. Exchange LifeTime (ELT) Default: 30 minutes. Minimum: 2 * ETO. Karn & Simpson expires in six months [Page 72] DRAFT Photuris June 1996 SPI LifeTime (SPILT) Default: 5 minutes. Minimum: 2 * ELT. In addition, each party configures local policy that determines what access (if any) is granted to the holder of a particular identity. For example, the party might allow anonymous FTP, but prohibit Telnet. Such considerations are outside the scope of this document. Security Considerations Photuris was based on currently available tools, by experienced network protocol designers with an interest in cryptography, rather than by cryptographers with an interest in network proto- cols. This specification is intended to be readily implementable without requiring an extensive background in cryptology. Therefore, only minimal background cryptologic discussion and rationale is included in this document. Although some review has been provided by the general cryptologic community, it is antici- pated that design decisions and tradeoffs will be thoroughly anal- ysed in subsequent dissertations and debated for many years to come. Cryptologic details are reserved for separate documents that may be more readily and timely updated with new analysis. Acknowledgements Thou shalt make no law restricting the size of integers that may be multiplied together, nor the number of times that an integer may be multiplied by itself, nor the modulus by which an integer may be reduced. [Prime Commandment] Phil Karn was principally responsible for the design of the proto- col phases, particularly the clogging defense, and provided much of the design rationale text. William Simpson designed the packet formats and attributes, and additional message types, editing and formatting. All such mis- takes are his responsibility. This protocol was later discovered to have many elements in common with the Station-To-Station authentication protocol [DOW92]. Angelos Keromytis suggested the cookie exchange rate limitation Karn & Simpson expires in six months [Page 73] DRAFT Photuris June 1996 counter, and developed the first complete independent implementa- tion. Paul C van Oorschot suggested signing both the public exponents and the shared-secret, to provide an authentication-only version of identity verification. Also, he provided text regarding mod- uli, generator, and exponent selection. Bart Preneel and Paul C van Oorschot in [PO96] suggested adding padding between the data and trailing key when hashing for authen- tication. Hilarie Orman suggested adding secret "nonces" to session-key gen- eration, and provided extensive review of the protocol details. Bill Sommerfeld suggested using the Cookie values on successive exchanges to provide bi-directional user-oriented keying, and | including the authentication secret-key in the session-key genera- | tion. Oliver Spatscheck developed a second independent implementation. International interoperability testing provided the impetus for many of the implementation notes herein. Randall Atkinson, Steven Bellovin, Wataru Hamada, James Hughes, + Brian LaMacchia, Cheryl Madson, Perry Metzger, Bob Quinn, Ron + Rivest, Rich Schroeppel, and Norman Shulman provided useful cri- | tiques of earlier versions of this document. References [BGMW93] E. Brickell, D. Gordon, K. McCurley, and D. Wilson, "Fast Exponentiation with Precomputation (Extended Abstract)", Advances in Cryptology -- EUROCRYPT '92, Lecture Notes in Computer Science, 658 (1993), Springer-Verlag, 200-207. Also U.S. Patent #5,299,262, E.F. Brickell, D.M. Gordon, K.S. McCurley, "Method for exponentiating in crypto- graphic systems", 29 Mar 1994. [Diffie90] Whitfield Diffie, "Authenticated Key Exchange and Secure Interactive Communication", Northern Telecom, Securicom '90, Paris France, 16 March 1990. [DH76] Diffie, W., and Hellman, H.E., "New Directions in Cryp- tography", IEEE Transactions on Information Theory, v Karn & Simpson expires in six months [Page 74] DRAFT Photuris June 1996 IT-22 n 6 pp 644-654, November 1976. [DOW92] Whitfield Diffie, Paul C van Oorshot, Michael J Wiener, "Authentication and Authenticated Key Exchanges", Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer Academic Publishers, 1992. [Firefly] "Photuris" is the latin name for the firefly. "Firefly" is in turn the name for the USA National Security Admin- istration's (classified) key exchange protocol for the STU-III secure telephone. Informed speculation has it that Firefly is based on very similar design principles. [OW96] Paul C van Oorshot, Michael J Weiner, "On Diffie-Hellman Key Agreement with Short Exponents", work in progress. [Prime Commandment] A derivation of an apocryphal quote from the usenet list sci.crypt. [PO96] Bart Preneel, Paul C van Oorshot, "...Two MACs", work in progress. [RFC-768] Postel, J., "User Datagram Protocol", STD 6, August 1980. [RFC-1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC-1321, MIT Laboratory for Computer Science, April 1992. [RFC-1510] Kohl, J., Neuman, B., "The Kerberos Network Authentica- tion Service (V5)", September 1993. [RFC-1700] Reynolds, J., and Postel, J., "Assigned Numbers", STD 2, USC/Information Sciences Institute, October 1994. [RFC-1812] Baker, F., Editor, "Requirements for IP Version 4 Routers", Cisco Systems, June 1995. [RFC-1825] Atkinson, R., "Security Architecture for the Internet Protocol", Naval Research Laboratory, July 1995. Karn & Simpson expires in six months [Page 75] DRAFT Photuris June 1996 [RFC-1828] Metzger, P., Simpson, W., "IP Authentication using Keyed MD5", July 1995. [RFC-1829] Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC Transform", July 1995. [RFC-xxxx] Karn, P., and Simpson, W., "ICMP Security Failures Mes- sages", draft-ietf-ipsec-icmp-fail-01.txt, work in progress. [Rooij94] P. de Rooij, "Efficient exponentiation using precomputa- tion and vector addition chains", EUROCRYPT '94, pp 403-415. [Schneier95] Schneier, B., "Applied Cryptography Second Edition", John Wiley & Sons, New York, NY, 1995. ISBN 0-471-12845-7. [Schnorr91] Schnorr, C.P., "Efficient signature generation by smart cards", Cryptology, v 4 pp 161-174, 1991. Karn & Simpson expires in six months [Page 76] DRAFT Photuris June 1996 Contacts - Comments about this document should be discussed on the pho- | turis@majordomo.soscorp.com mailing list. Questions about this document can also be directed to: | Phil Karn Qualcomm, Inc. 6455 Lusk Blvd. San Diego, California 92121-2779 karn@qualcomm.com karn@unix.ka9q.ampr.org (preferred) William Allen Simpson DayDreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071 wsimpson@UMich.edu wsimpson@GreenDragon.com (preferred) bsimpson@MorningStar.com Karn & Simpson expires in six months [Page 77]