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Network Working Group Randall Atkinson Internet Draft cisco Systems draft-ietf-ipsec-arch-sec-01.txt 10 November 1996 Expire in six months Security Architecture for the Internet Protocol STATUS OF THIS MEMO This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its working groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of 6 months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as "work in progress". This particular Internet Draft is a product of the IETF's IP Security (IPsec) working group. It is intended that a future version of this draft be submitted to the IESG for publication as a Draft Standard RFC. Comments about this draft may be sent to the author or to the IPsec WG mailing list <ipsec@tis.com>. 1. INTRODUCTION This memo describes the security protocols for IP version 4 (IPv4) and IP version 6 (IPv6) and the services that they provide. Each security protocol is specified in a separate document. This document also describes key management requirements for systems implementing these security protocols. This document is not an overall Security Architecture for the Internet; it addresses only IP-layer security. 1.1 Technical Definitions This section provides a few basic definitions that are applicable to this document. Other documents provide more definitions and background information. [VK83, HA94] Authentication (of Data Origin) Atkinson [Page 1] Internet Draft Security Architecture for IP 10 November 1996 A security property that ensures that the origin of received data is, in fact, the claimed sender. Usually bundled with integrity services, especially connectionless integrity. Integrity (Connectionless) A security property that ensures data is transmitted from source to destination without undetected alteration. If the order of transmitted data also is ensured, the service is termed connection-oriented integrity. The term anti-replay refers to a minimal `qform of connection-oriented integrity designed to detect and reject duplicated or very old data units. Confidentiality A security property that ensures that communicated data is disclosed to intended recipients, but is not disclosed to other, unauthorized parties. Traffic flow confidentiality extends this service to externally visible characteristics of communication, e.g., source and destination identifiers, message length, or frequency of communication. (See also traffic analysis, below.) Encryption A mechanism commonly used to provide confidentiality. Non-repudiation A security property that ensures that a participant in a communication cannot later deny having participated in the communication. This property may apply to either the sender or the recipient of communccated data, or both. SPI Acronym for "Security Parameters Index." The combination of an SPI and a destination address uniquely identifies a simplex security association (SA, see below). The SPI is carried in IPsec protocols to select the set of parameters bound to an SA. An SPI has only local significance, defined by the creator of the SA; thus an SPI is generally viewed as an opaque bit string. However, the creator of an SA may choose to interpret the bits in an SPI to facilitate local processing. Security Association (SA) A simplex (uni-directional) logical connection, created for security purposes. All traffic traversing an SA is subjected to the same security processing at the transmitter and receiver. In IPsec, an SA is a network layer abstraction enforced through the use of AH or ESP. Security Gateway A system that acts as the communications interface between untrusted external, networks and internal (trusted) hosts and subnetworks. Atkinson [Page 2] Internet Draft Security Architecture for IP 10 November 1996 The internal subnets and hosts served by a security gateway are presumed to be trusted by virtue of sharing a common, local, security administration. (See "Trusted Subnetwork" below.) In the IPsec context, a security gateway is a point at which AH and/or ESP is implemented in order to serve a set of internal hosts, providing security services for these hosts when they communicate with external hosts also employing IPsec (either directly or via another security gateway). Traffic Analysis The analysis of network traffic flow for the purpose of deducing information that is useful to an adversary. Examples of such information are frequency of transmission, the identities of the conversing parties, sizes of packets, flow Identifiers, etc. [Kent78] Trusted Subnetwork A subnetwork containing hosts and routers that trust each other not to engage in active or passive attacks and trust that the underlying communications channel (e.g., an Ethernet) isn't being attacked. 1.2 Requirements Terminology In this document, the words that are used to define the significance of each particular requirement are usually capitalised. These words are: - MUST This word or the adjective "REQUIRED" means that implementation of the item is an absolute requirement of the specification. - SHOULD This word or the adjective "RECOMMENDED" means that there might exist valid reasons in particular circumstances to not implement this item, but the full implications should be understood and the case carefully weighed before taking a different course. - MAY This word or the adjective "OPTIONAL" means that this item is truly optional to implement. One vendor might choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item. 1.3 Basic IPsec features Two specific headers used to provide security services in IPv4 and IPv6: the "IP Authentication Header (AH)" [Atk95a] and the "IP Encapsulating Security Payload (ESP)" [Atk95b] header. There are a number of ways in which these IP security mechanisms may be employed, in large part because AH and ESP may be used independently of one another, in Atkinson [Page 3] Internet Draft Security Architecture for IP 10 November 1996 combination with one another, or in an interated (nested) fashion. This section describes the basic features of both protocols and typical uses of these protocols. The next section defines the minimum header processing configurations that all compliant IPsec implementations must support. (Additional configurations may be supported at the discretion of the implementor.) Thus the configuration examples in this and the next section are not exhaustive. The IP Authentication Header (AH) can be used to provide connectionless integrity and data origin authentication for IP datagrams, and optionally to provide anti-replay integrity. This last, optional service may be selected when a security association is established. This header protects an entire IP datagram, including all immutable fields in the IP header. AH does not provide confidentiality; thus implementations of AH may be widely deployed, even in countries where controls on encryption would preclude deployment of technology that also offered confidentiality. This header should be used whenever the integrity and authenticity of the IP header, as well as the associted upper layer protocols, must be ensured. For example, AH can be used to bind a sensitivity label (e.g., IPSO [Kent91]) to an IP datagram. The Encapsulating Security Payload (ESP) can be used to provide confidentiality, data origin authentication, connectionless integrity, anti-replay integrity, and limited traffic flow confidentiality. The set of services provided depends on options selected at the time of security association establishment and the implementation placement. Confidentiality may be selected independent of all other services. Data origin authentication and connectionless integrity are joint services, independent of confidentiality. Anti-replay may be selected only if data origin authentication and connectionless integrity are selected, but is independent of confidentiality. Traffic flow confidentiality depends on confidentiality, and requires selection of tunnel mode (see below). Unlike AH, ESP provides security only for the protocols encapsulated by it, not the protocol that carries it. Perhaps the most obvious use of ESP is to provide security services for upper layer protocols such as TCP, without affording protection to the IP header that carries the these protocols. Because ESP is an encapsulation protocol, it may be employed recursively, to create nested security associations. For example, one ESP-protected SA might extend between a host and a security gateway, and a second, nested ESP-protected SA might extend from the same host through the security gateway to a host behind the gateway. Two modes of ESP are defined: transport mode and tunnel mode. In transport mode, ESP encapsulates any transport layer protocol defined for carriage in the TCP/IP suite, e.g., TCP, UDP. This is the simplest mode for use between a pair of hosts implementing ESP. In tunnel model, the protocol encapsulated by ESP is usually IP but could also be another Atkinson [Page 4] Internet Draft Security Architecture for IP 10 November 1996 network-layer protocol (e.g. IPX). Tunnel mode is always used by security gateways for all packets not originating on the gateway, to facilitate operation in multi-homed environments, especially in the face of possible fragmentation of ESP-protected packets. Tunnel mode can be used to create virtual private networks between sites protected by security gateways implementing ESP. Both AH and ESP can provide security services between a pair of communicating hosts, or between a pair of communicating security gateways or between a security gateway and a host. Depending on the choice of algorithms, AH and ESP also may support multicast communication, e.g., among a set of hosts or security gateways or combinations thereof. When a security gateway provides services for hosts on a trusted subnet, the security gateway is responsible for establishing and managing security associations on behalf of the trusted hosts it serves. The security gateway also is responsible for providing security services between the gateway itself and correspondant external systems (hosts or security gateways) through the implementation of AH and ESP. 1.4 Minimal Essential Support All IPsec-compliant implementations MUST support AH and MUST support ESP in both transport and tunnel modes. The requirement to support tunnel-mode is imposed to ensure interoperability between host and security gateway implementations of ESP. The requirement to support transport-mode ensures interoperability with other hosts using transport-mode and can permit some reduction in security overhead. A compliant host or security gateway implementation MUST be capable of creating and accepting security associations that employ either AH or ESP. A compliant host or security gateway SHOULD also be capable of creating pairs of AH and ESP security associations. A compliant host implementation also MUST also support a second (nested) ESP security association, in transport mode, above a tunnel mode ESP security association. The following sequences of combinations of AH and ESP, each represented by a separate security association, must be supported by an IPsec-compliant host: AH, ESP (tunnel), ESP(transport), AH-ESP(transport), AH-ESP(tunnel), ESP(tunnel)-AH, AH-ESP(tunnel)-ESP(transport), and ESP(tunnel)-ESP(transport). The following sequences of combinations of AH and ESP must be supported by a compliant IPsec security gateway: AH, ESP (tunnel), and AH-ESP(tunnel). Note that transport-mode ESP security associations may be employed across a security gateway, terminating at hosts behind the gateway. The gateway does not process these SAs; they are passed through transparently, and hence there is no required "support" in the gateway for Atkinson [Page 5] Internet Draft Security Architecture for IP 10 November 1996 these header combinations. A security gateway which receives a datagram containing a recognised sensitivity label, for example IPSO [Ken91], from a trusted host MUST take that label's value into consideration when creating/selecting an Security Association for use with AH between the gateway and the external destination. In such an environment, a gateway which receives a IP packet containing the IP Encapsulating Security Payload (ESP) should add appropriate authentication, including implicit (i.e. contained in the Security Association used) or explicit label information (e.g. IPSO), for the decrypted packet that it forwards to the trusted host that is the ultimate destination. The IP Authentication Header should always be used on packets containing explicit sensitivity labels to ensure end-to-end label integrity. In environments using security gateways, those gateways MUST perform address-based IP packet filtering on unauthenticated packets purporting to be from a system known to be using IP security. A gateway which receives a datagram containing a recognised sensitivity label from a trusted host should take that label's value into consideration when creating/selecting a Security Association for use with ESP between the gateway and the external destination. In such an environment, a gateway which receives a IP packet containing the ESP should appropriately label the decrypted packet that it forwards to the trusted host that is the ultimate destination. IP security should always be used on packets containing explicit sensitivity labels in a manner to ensure end-to-end label integrity. Routing headers for which integrity has not been cryptographically protected SHOULD be ignored by the receiver. If this rule is not strictly adhered to, then the system will be vulnerable to various kinds of attacks, including source routing attacks. [Bel89][CB94][CERT95] 1.5 Security Association Management The concept of a "Security Association" is fundamental to both the IP Encapsulating Security Payload and the IP Authentication Header. The combination of a given Security Parameter Index (SPI) and Destination Address uniquely identifies a particular "Security Association". An implementation of the Authentication Header or the Encapsulating Security Payload MUST support this concept of a Security Association. A single IPsec Security Association is a simplex (unidirectional) connection with which either AH or ESP (but not both) is employed. If both AH and ESP protection is to be applied to a traffic stream, then two (or more) security associations are created to control processing of the traffic stream. A compliant implementation of IPsec Security Association MUST Atkinson [Page 6] Internet Draft Security Architecture for IP 10 November 1996 support the following management parameters for each SA; other parameters MAY also be included at the discretion of the implementor: - Authentication algorithm and mode being used for AH or ESP. [REQUIRED for all implementations]. - Key(s) used with the above authentication algorithm [REQUIRED for all implementations]. - Encryption algorithm and mode used with ESP. [REQUIRED for ESP implementations] - Key(s) used with the above encryption algorithm. [REQUIRED for ESP implementations] - Presence/absence and size of a cryptographic synchronisation or initialisation vector field for the encryption algorithm. [REQUIRED for ESP implementations] - Authentication key crypto period (fixed future time or duration). [REQUIRED for all implementations]. - Encryption key crypto period (fixed future tie or duration) [REQUIRED for ESP implementations] - Lifetime of this Security Association [REQUIRED for all implementations] - Destination IP Address of the Security Association; this may be a wildcard address if more than one desitnation system shares the same Security Association (behind a security gateway) [REQUIRED for all implementations] - Source IP Address(es) of the Security Association; this might be a wildcard address if more than one sending system shares the same Security Association with the destination [REQUIRED for all implementations] - Proxy IP Address of the Security Association; this contains the IP address of the security gateway that provides security services on behalf of the source IP address when IPsec is not in use end-to-end from source to destination. [REQUIRED for Security Gateways; RECOMMENDED for all other implementations] - Replay Protection information, including whether it is in use, information on the window size for the sequence numbers, etc. [REQUIRED for all implementations] - Sensitivity level of the protected data (e.g., in IPSO terms) [REQUIRED for all systems claiming to provide multi-level security, RECOMMENDED for all other systems] - Next Protocol (from IP header) as an optional SA selector input for outbound traffic [RECOMMENDED for all implementations] - Source and/or Destination TCP/UDP Ports as an optional SA selector for outbound traffic [RECOMMENDED for all implementations] - Identity of the source of the Security Association [RECOMMENDED for all implementations] Atkinson [Page 7] Internet Draft Security Architecture for IP 10 November 1996 - Identity of the destination of the Security Association] [RECOMMENDED for all implementations] The way in which security associations are matched to offered (outbound) traffic varies based on whether IPsec is implemented in a host or a security gateway, and on the granularity of SA management selected. At a host, the inputs to SA selection are the userid of the sender, the Destination Address (perhaps including the next protocol field and source and/or destination port fields and source/destination identities) is used to select the appropriate Security Association for outbound traffic. For a multi-level secure host, the senstivity of the traffic, e.g., as expressed in a security label, also is an input to the SA selection. A security gateway generally does not have access to userid information and thus IPsec implementations in such devices are not required to select SAs for outbound traffic on that basis. Since a security gateway typically serves a number of hosts, the Source Address (perhaps including the next protocol field and source and/or destination port fields) is an input to the SA selection. The Destination Address address also is an input, and when in a multi-level secure network context a traffic sensitivity label is a REQUIRED additional input. Processing for received (inbound) IPsec traffic is much simpler. The receiving host uses the combination of the SPI and the Destination Address to distinguish the correct association. Hence, an IPsec implementation will always be able to use the SPI in combination with the Destination Address to determine the security association with which the traffic is associated. When a formerly valid Security Association is terminated, the destination system(s) SHOULD NOT immediately reuse that SPI and instead SHOULD let that SPI value become stale before reusing it for some other Security Association. As noted above, an IPsec Security Association is unidirectional. Hence, to secure typical, bi-directional communications between two hosts (or security gateways), two Security Associations (one in each direction) will be required. The Destination Address may be a unicast address, an IPv4 broadcast address, or a multicast group address. The receiver-orientation of the Security Association implies that, in the case of unicast traffic, the destination system will normally select the SPI value. By having the destination select the SPI value, there is no potential for manually configured Security Associations that conflict with automatically configured (e.g. via a key management protocol) Security Associations. For multicast traffic, there are multiple destination systems but a single destination multicast group, so some system or person will need to Atkinson [Page 8] Internet Draft Security Architecture for IP 10 November 1996 select SPIs on behalf of that multicast group and then communicate the information to all of the legitimate members of that multicast group via mechanisms not defined here. Multiple senders to a multicast group SHOULD use a single Security Association (and hence Security Parameter Index) for all traffic to that group when a symmetric cryptographic algorithm is in use. In that case, the receiver only knows that the message came from a system knowing the security association data for that multicast group. A receiver cannot generally authenticate which system sent the multicast traffic when symmetric algorithms (e.g. DES, IDEA) are in use. Multicast traffic SHOULD use a separate Security Association for each sender to the multicast group when an asymmetric cryptographic algorithm is in use. In this last case, the receiver can know the specific system that originated the message. 2. DESIGN OBJECTIVES This section describes some of the design objectives of this security architecture and its component mechanisms. The primary objective of this work is to ensure that IPv4 and IPv6 will have solid cryptographic security mechanisms available to users who desire security. These mechanisms are designed to avoid adverse impacts on Internet users who do not employ these security mechanisms for their traffic. These mechanisms are intended to be algorithm-independent so that the cryptographic algorithms can be altered without affecting the other parts of the implementation. These security mechanisms should be useful in enforcing a variety of security policies. Standard default algorithms (Keyed HMAC MD5, Keyed HMAC SHA, DES- CBC) are specified to ensure interoperability in the global Internet. The selected default algorithms are widely used in other contexts. 3. IP-LAYER SECURITY MECHANISMS There are two cryptographic security mechanisms for IP. The first is the Authentication Header which provides integrity and authentication without confidentiality. [Atk95a] The second is the Encapsulating Security Payload which always provides confidentiality, and usually provides integrity and authentication. [Atk95b] The two IP security mechanisms are normally used separately. When both confidentiality and authentication are needed, a combined ESP transform should be used instead of using AH with ESP. Atkinson [Page 9] Internet Draft Security Architecture for IP 10 November 1996 These IP-layer mechanisms do not provide complete security against all traffic analysis attacks, though the use of ESP between security gateways can provide partial traffic flow protection. However, there are several techniques outside the scope of this specification (e.g. bulk link encryption) that might be used to provide more comprehensive protection against traffic analysis. [VK83] 3.1 AUTHENTICATION HEADER The IP Authentication Header is designed to provide authentication, integrity, and replay protection to IP datagrams. [Atk95a] It does this by computing a cryptographic authentication function over the IP datagram and using one or more authentication keys in the computation. The authentication algorithm may be either symmetric or asymmetric. The sender computes the authentication data prior to sending the authenticated IP packet and places the authentication data inside the Authentication Data. Fragmentation occurs after the Authentication Header processing for outbound packets and reassembly occurs prior to Authentication Header processing for inbound packets. The receiver verifies the correctness of the authentication data upon reception. Certain fields of the outer IP header that may change in transit are zeroed for the authentication calculation. For IPv4, these fields are: Type of Service (TOS) Time To Live (TTL) Checksum Offset Flags Also, IPv4 options are zeroed for the authentication calculation, except for the IP Security Option BSO and ESO (RFC-1038, RFC-1108) and the undocumented non-standard CIPSO (IPv4 Option number 134) option, which are included and are not zeroed. For IPv6, the "IP Version", "Type of Service", "Flow Label", and "Hop Limit" fields of the outermost IPv6 header are zeroed for the authentication calculation. IPv6 options contain a bit that indicates whether the option might change during transit. Options that might change during transit are zeroed for the authentication calculation and all others are included in the authentication calculation. See the IPv6 specifications for more information. Non-repudiation is not normally provided with the Authentication Header. Confidentiality and traffic analysis protection are not Atkinson [Page 10] Internet Draft Security Architecture for IP 10 November 1996 provided by the Authentication Header. Replay Protection is normally provided by the Authentication Header, though a user might choose not to use it. Use of the Authentication Header will increase the IP protocol processing costs in participating systems and will also increase the communications latency. The increased latency is primarily due to the calculation of the authentication data by the sender and the calculation and comparison of the authentication data by each receiver for each IP datagram containing an Authentication Header (AH). The Authentication Header provides much stronger security than exists in most of the current Internet and should not affect exportability or significantly increase implementation cost. While the Authentication Header might be implemented by a security gateway on behalf of hosts on a trusted network behind that security gateway, this mode of operation is not encouraged. Instead, whenever possible the Authentication Header should be used from origin to final destination so that end-to-end protections are provided. All IPv6-capable nodes and all IPv4 systems claiming to implement the Authentication Header MUST implement the standards- track mandatory-to- implement AH transforms. As of this writing these are HMAC MD5 [OG96] and HMAC SHA [CG96], but implementers should consult the most recent edition of the "Internet Official Protocol Standards" [STD-1] for current guidance. An implementation MAY support other authentication algorithms in addition to the mandatory transforms. 3.2 ENCAPSULATING SECURITY PAYLOAD The IP Encapsulating Security Payload (ESP) is designed to provide integrity, authentication, and confidentiality to IP datagrams. [Atk96b] ESP can also provide replay protection when used with certain transforms. ESP encapsulates either an entire IP datagram or only the upper-layer protocol (e.g. TCP, UDP, ICMP) data inside the ESP, applies integrity and authentication protections, and encrypts the data. If tunnel-mode is in use, a new cleartext IP header is prepended to the now encrypted Encapsulating Security Payload. In tunnel-mode, the cleartext IP header is used to carry the protected data through the internetwork. 3.2.1 Description of the ESP Modes There are two modes within ESP. The first mode, which is known as Tunnel-mode, encapsulates an entire IP datagram within the ESP header. The second mode, which is known as Transport- Atkinson [Page 11] Internet Draft Security Architecture for IP 10 November 1996 mode, encapsulates an upper-layer protocol (for example UDP or TCP) inside ESP and then prepends a cleartext IP header. 3.2.2 Usage of ESP ESP works between hosts, between a host and a security gateway, or between security gateways. This support for security gateways permits trustworthy networks behind a security gateway to omit encryption and thereby avoid the performance and monetary costs of encryption, while still providing confidentiality for traffic transiting untrustworthy network segments. When both hosts directly implement ESP and there is no intervening security gateway, then they may use the Transport- mode (where only the upper layer protocol data (e.g. TCP or UDP) is encrypted and there is no encrypted IP header). This mode reduces both the bandwidth consumed and the protocol processing costs for users that don't need to keep the entire IP datagram confidential. ESP works with both unicast and multicast traffic. 3.2.3 Performance Impacts of ESP The encapsulating security approach used by ESP can noticeably impact network performance in participating systems, but use of ESP should not adversely impact gateways or other intermediate systems that are not participating in the particular ESP association. Protocol processing in participating systems will be more complex when encapsulating security is used, requiring both more time and more processing power. Use of encryption will also increase the communications latency. The increased latency is primarily due to the encryption and decryption required for each IP datagram containing an Encapsulating Security Payload. The precise cost of ESP will vary with the specifics of the implementation, including the encryption algorithm, key size, and other factors. Hardware implementations of the encryption algorithm are recommended when high throughput is desired. For interoperability throughout the worldwide Internet, all conforming implementations of the IP Encapsulating Security Payload MUST also implement the standard mandatory ESP transform. As of this writing, that is the Combined ESP transform with DES-CBC, HMAC MD5, and Replay Protection. [Hugh96] Implementers should consult the most recent "IAB Official Protocols" RFC for current information on the mandatory to implement ESP transform(s). Other confidentiality algorithms and modes may also be implemented in addition to this mandatory algorithm and mode. Export and use of encryption are regulated in some countries. [OTA94] Atkinson [Page 12] Internet Draft Security Architecture for IP 10 November 1996 3.3 COMBINING SECURITY MECHANISMS A node normally does not apply both ESP and AH to the same IP datagram. If confidentiality is not required, then AH should be used. If confidentiality is required, then ESP should be used. In some circumstances a security gateway might apply ESP (or AH) to a packet before forwarding that packet because a secure tunnel has been configured in that security gateway. Hence, IP packets containing both ESP and AH are not strictly prohibited. 3.4 OTHER SECURITY MECHANISMS Protection from traffic analysis is not provided by any of the security mechanisms described above. It is unclear whether meaningful protection from traffic analysis can be provided economically at the Internet Layer and it appears that few Internet users are concerned about traffic analysis. One traditional method for protection against traffic analysis is the use of bulk link encryption. Another technique is to send false traffic in order to increase the noise in the data provided by traffic analysis. Reference [VK83] discusses traffic analysis issues in more detail. 4. SECURITY ASSOCIATION MANAGEMENT The Security Management protocol that will be used with IP layer security is not specified in this document. However, because the security management protocol is coupled to AH and ESP only via the Security Parameters Index (SPI), we can meaningfully define AH and ESP without having to fully specify how security management is performed. We envision that several security management systems will be usable with these specifications, including manual key configuration. Support for key management methods where the key management data is carried in-line within the IP layer is not a design objective for these IP-layer security mechanisms. Instead these IP-layer security mechanisms will primarily use key management methods where the key management data will be carried by an upper layer protocol, such as UDP or TCP, on some specific port number or where the key management data will be distributed manually. This design permits clear decoupling of the key management mechanism from the other security mechanisms, and thereby permits one to substitute new and improved key management methods without having to modify the implementations of the other security mechanisms. This separation of mechanism is clearly wise given the long history of subtle flaws in published key management protocols. [NS78, NS81] What follows in this section is a brief discussion of a few alternative Atkinson [Page 13] Internet Draft Security Architecture for IP 10 November 1996 approaches to key management. Mutually consenting systems may additionally use other key management approaches by private prior agreement. 4.1 Manual Key Distribution The simplest form of key management is manual key management, where a person manually configures each system with its own key and also with the keys of other communicating systems. This is quite practical in small, static environments but does not scale. It is not a viable medium-term or long-term approach, but might be appropriate and useful in many environments in the near-term. For example, within a small LAN it is entirely practical to manually configure keys for each system. Within a single administrative domain it is practical to configure keys for each router so that the routing data can be protected and to reduce the risk of an intruder breaking into a router. Another case is where an organisation has an encrypting firewall between the internal network and the Internet at each of its sites and it connects two or more sites via the Internet. In this case, the security gateway might selectively encrypt traffic for other sites within the organisation using a manually configured key, while not encrypting traffic for other destinations. It also might be appropriate when only selected communications need to be secured. 4.2 Requirements for Key Management Protocols Widespread deployment and use of IP security requires an Internet-standard scalable key management protocol. This protocol should not be limited to supporting IP security. This protocol should be compatible with the IETF's DNS Security work and should include the ability to obtain bootstrapping information (e.g. keys, addresses) from the Secure DNS as a mandatory-to-implement feature. signed host keys to the Domain Name System [EK96] The DNS keys enable the originating party to authenticate key management messages with the other key management party using an asymmetric algorithm. A standards-track key management protocol for use with IP Security MUST provide the property of "Perfect Forward Secrecy" as a mandatory-to- implement feature. Further, any standards-track key management protocol MUST permit the secure negotiation or secure identification of all of the Security Association attributes [as defined above] to all parties of that Security Association. 4.4 Keying Approaches for IP There are several keying approaches for IP. The first approach, called host-oriented keying, has all users on host 1 share the same key for use on traffic destined for all users on host 2. The second Atkinson [Page 14] Internet Draft Security Architecture for IP 10 November 1996 approach, called user-oriented keying, lets user A on host 1 have one or more unique session keys for its traffic destined for host 2; such session keys are not shared with other users on host1. For example, user A's ftp session might use a different key than user A's telnet session. In systems claiming to provide multi-level security, user A will typically have at least one key per sensitivity level in use (e.g. one key for UNCLASSIFIED traffic, a second key for SECRET traffic, and a third key for TOP SECRET traffic). Similarly, with user-oriented keying one might use separate keys for information sent to a multicast group and control messages sent to the same multicast group. A third approach, called session-unique keying, has a single key being assigned to a given IP address, upper-layer protocol, and port number triple. In many cases, a single computer system will have at least two mutually suspicious users A and B that do not trust each other. When host-oriented keying is used and mutually suspicious users exist, it is sometimes possible for user A to determine the host-oriented key via well known methods, such as a Chosen Plaintext attack. Once user A has improperly obtained the key in use, user A can then either read user B's encrypted traffic or forge traffic from user B. When user- oriented keying is used, certain kinds of attack from one user onto another user's traffic are not possible. IP Security is intended to be able to provide Authentication, Integrity, and Confidentiality for applications operating on connected end systems when appropriate algorithms are in use. Integrity and Confidentiality can be provided by host-oriented keying when appropriate dynamic key management techniques and appropriate algorithms are in use. However, authentication of principals using applications on end-systems requires that processes running applications be able to request and use their own Security Associations. In this manner, applications can make use of key distribution facilities that provide authentication. Hence, support for session-unique keying MUST be present in all IP Security implementations, as is described in the "IPsec Key Management Requirements" section below. Support for other styles MAY also be implemented. 4.5 Multicast Key Distribution Multicast key distribution is an active research area in the published literature as of this writing. For multicast groups having relatively few members, manual key distribution or multiple use of existing unicast key distribution algorithms such as modified Diffie- Hellman appears feasible. For very large groups, new scalable techniques will be needed. In-line key management systems that rely Atkinson [Page 15] Internet Draft Security Architecture for IP 10 November 1996 on pre-distributed master keys and then have serious scaling issues that make them questionable for multicast traffic. 4.6 IPsec Key Management Requirements This section defines key management requirements for all IPv6 implementations and for those IPv4 implementations that implement the IP Authentication Header, the IP Encapsulating Security Payload, or both. It applies equally to the IP Authentication Header and the IP Encapsulating Security Payload. All such implementations MUST support manual configuration of Security Associations, including all of the attributes described in the Security Association definition section of this document, having variable SPI values within the non-reserved range. An implementation that only supports a fixed SPI value is NOT conforming or compliant. All IPv6 IP Security implementations MUST implement ISAKMP/Oakley. All IPv4 IP Security implementations SHOULD implement ISAKMP/Oakley. Other key management protocols MAY also be implemented. [Sch96] Implementations MAY also support other methods of configuring Security Associations. Given two endpoints, it MUST be possible to have more than one concurrent Security Association for communications between them. Implementations on multi-user hosts having at least discretionary access controls MUST support either user granularity (i.e. "user- oriented") Security Associations or session-unique Security Associations. All such implementations MUST permit the configuration of host- oriented keying. A device that encrypts or authenticates IP packets originated by other systems, for example a dedicated IP encryptor or an security gateway, cannot generally provide user-oriented keying for traffic originating on other systems. Such systems MUST support session- unique key selection based on source address, destination address, upper-layer protocol, source port (if any), and destination port (if any). Such systems MAY additionally implement support for user- oriented keying for traffic originating on other systems. The method by which keys are configured on a particular system is implementation-defined. A flat file containing security association identifiers and the security parameters, including the key(s), is an example of one possible method for manual key Atkinson [Page 16] Internet Draft Security Architecture for IP 10 November 1996 distribution. An IP Security implementation MUST take reasonable steps to protect the keys and other security association information from unauthorised examination or modification because all of the security lies in the keys. 5. USAGE This section describes the possible use of the security mechanisms provided by IP in several different environments and applications in order to give the implementer and user a better idea of how these mechanisms can be used to reduce security risks. 5.1 USE WITH FIREWALLS Firewalls are not uncommon in the current Internet. [CB94] While many dislike their presence because they restrict connectivity, they are unlikely to disappear in the near future. Both of these IP mechanisms can be used to increase the security provided by firewalls. Firewalls used with IP often need to be able to parse the headers and options to determine the transport protocol (e.g. UDP or TCP) in use and the port number for that protocol. Firewalls can be used with the Authentication Header regardless of whether that firewall is party to the appropriate Security Assocation. However, a firewall that is not party to the applicable Security Association will not normally be able to decrypt an encrypted upper-layer protocol to view the protocol or port number needed to perform per- packet filtering. Further, firewalls need to verify that the data (e.g. source, destination, transport protocol, port number) being used for access control decisions is correct and authentic. Hence, authentication might be performed not only within an organisation or campus but also end to end with remote systems across the Internet. This use of the Authentication Header with IP provides much more assurance that the data being used for access control decisions is authentic. Organisations with two or more sites that are interconnected using commercial IP service might wish to use a selectively encrypting firewall. If an encrypting firewall were placed between each site of a company and the commercial IP service provider, the firewall could provide an encrypted IP tunnel among all the company's sites. It could also encrypt traffic between the company and its suppliers, customers, and other affiliates. Traffic with the Network Information Center, with public Internet archives, or some other organisations might not be encrypted because of the unavailability of a standard key management protocol or as a deliberate choice to Atkinson [Page 17] Internet Draft Security Architecture for IP 10 November 1996 facilitate better communications, improved network performance, and increased connectivity. Such a practice could easily protect the company's sensitive traffic from eavesdropping and modification. Some organisations (e.g. governments) might wish to use a fully encrypting firewall to provide a Virtual Private Network (VPN) over commercial IP service. The difference between that and a bulk IP encryption device is that a fully encrypting firewall would provide filtering of the decrypted traffic as well as providing encryption of IP packets. A related scenario is to use encryption between a mobile computer and the security gateway or encrypting firewall of its home campus. 5.2 USE WITH IP MULTICAST In the past several years, the Multicast Backbone (MBONE) has grown rapidly. IETF meetings and other conferences are now regularly multicast with real-time audio, video, and whiteboards. Many people are now using teleconferencing applications based on IP Multicast in the Internet or in private internal networks. Others are using IP multicasting to support distributed simulation or other applications. Hence it is important that the security mechanisms in IP be suitable for use in an environment where multicast is the general case. The Security Parameters Indexes (SPIs) used in the IP security mechanisms are receiver-oriented, making them well suited for use in IP multicast. [Atk95a, Atk95b] Unfortunately, most currently published multicast key distribution protocols do not scale well. However, there is active research in this area. As an interim step, a multicast group could repeatedly use a secure unicast key distribution protocol to distribute the key to all members or the group could pre-arrange keys using manual key distribution. 5.3 USE TO PROVIDE QOS PROTECTION The recent IAB Security Workshop identified Quality of Service protection as an area of significant interest. [BCCH] The two IP security mechanisms are intended to provide good support for real- time services as well as multicasting. This section describes one possible approach to providing such protection. The Authentication Header might be used, with appropriate key management, to provide authentication of packets. This authentication is potentially important in packet classification within routers. The IPv6 Flow Identifier might act as a Low-Level Identifier (LLID). Used together, packet classification within routers becomes straightforward if the router is provided with the appropriate keying material. For performance reasons the routers Atkinson [Page 18] Internet Draft Security Architecture for IP 10 November 1996 might authenticate only every Nth packet rather than every packet, but this is still a significant improvement over capabilities in the current Internet. Quality of service provisioning is likely to also use the Flow ID in conjunction with a resource reservation protocol, such as RSVP. [ZDESZ93] Thus, the authenticated packet classification can be used to help ensure that each packet receives appropriate handling inside routers. 5.4 USE IN COMPARTMENTED OR MULTI-LEVEL NETWORKS A multi-level secure (MLS) network is one where a single network is used to communicate data at different sensitivity levels (e.g. Unclassified and Secret). [DoD85] [DoD87] Many governments have significant interest in MLS networking. [DIA] The IP security mechanisms have been designed to support MLS networking. MLS networking requires the use of strong Mandatory Access Controls (MAC), which ordinary users are incapable of controlling or violating. [BL73] This section pertains only to the use of these IP security mechanisms in MLS environments. The Authentication Header can be used to provide strong authentication among hosts in a single-level network. The Authentication Header can also be used to provide strong assurance for both mandatory access control decisions in multi-level networks and discretionary access control decisions in all kinds of networks. If explicit IP sensitivity labels (e.g. IPSO) [Ken91] are used and confidentiality is not considered necessary within the particular operational environment, the Authentication Header is used to provide authentication for the entire packet, including cryptographic binding of the sensitivity level to the IP header and user data. This is a significant improvement over labeled IPv4 networks where the label is trusted even though it is not trustworthy because there is no authentication or cryptographic binding of the label to the IP header and user data. IPv6 will normally use implicit sensitivity labels that are part of the Security Association but not transmitted with each packet instead of using explicit sensitivity labels. All explicit IP sensitivity labels MUST be authenticated using either ESP, AH, or both. The Encapsulating Security Payload can be combined with appropriate key policies to provide full multi-level secure networking. In this case each key must be used only at a single sensitivity level and compartment. For example, Key "A" might be used only for sensitive Unclassified packets, while Key "B" is used only for Secret/No-compartments traffic, and Key "C" is used only for Secret/Compartment-X traffic. The sensitivity level of the protected traffic MUST NOT dominate the sensitivity level of the Security Association used with that traffic. The sensitivity level of the Atkinson [Page 19] Internet Draft Security Architecture for IP 10 November 1996 Security Association MUST NOT dominate the sensitivity level of the key that belongs to that Security Association. The sensitivity level of the key SHOULD be the same as the sensitivity level of the Security Association. The Authentication Header can also have different keys for the same reasons, with the choice of key depending in part on the sensitivity level of the packet. Encryption is very useful and desirable even when all of the hosts are within a protected environment. The Internet-standard encryption algorithm could be used, in conjunction with appropriate key management, to provide strong Discretionary Access Controls (DAC) in conjunction with either implicit sensitivity labels or explicit sensitivity labels (such as IPSO provides for IPv4 [Ken91]). Some environments might consider the Internet-standard encryption algorithm sufficiently strong to provide Mandatory Access Controls (MAC). Full encryption should be used for all communications between multi-level computers or compartmented mode workstations even when the computing environment is considered to be protected. 6. SECURITY CONSIDERATIONS This entire draft discusses the Security Architecture for the Internet Protocol. It is not a general security architecture for the Internet, but is instead focused on the IP layer. Cryptographic transforms for ESP which use a block-chaining algorithm and lack a strong integrity mechanism are vulnerable to a cut-and-paste attack described by Bellovin and should not be used unless the Authentication Header is always present with packets using that ESP transform. [Bel95] If more than one sender shares a Security Association with a destination, then the receiving system can only authenticate that the packet was sent from one of those systems and cannot authenticate which of those systems sent it. Similarly, if the receiving system does not check that the Security Association used for a packet is valid for the claimed Source Address of the packet, then the receiving system cannot authenticate whether the packet's claimed Source Address is valid. For example, if senders "A" and "B" each have their own unique Security Association with destination "D" and "B" uses its valid Security Association with D but forges a Source Address of "A", then "D" will be fooled into believing the packet came from "A" unless "D" verifies that the claimed Source Address is party to the Security Association that was used. Users need to understand that the quality of the security provided by the mechanisms provided by these two IP security mechanisms depends completely on the strength of the implemented Atkinson [Page 20] Internet Draft Security Architecture for IP 10 November 1996 cryptographic algorithms, the strength of the key being used, the correct implementation of the cryptographic algorithms, the security of the key management protocol, and the correct implementation of IP and the several security mechanisms in all of the participating systems. The security of the implementation is in part related to the security of the operating system which embodies the security implementations. For example, if the operating system does not keep the private cryptologic keys (that is, all symmetric keys and the private asymmetric keys) confidential, then traffic using those keys will not be secure. If any of these is incorrect or insufficiently secure, little or no real security will be provided to the user. Because different users on the same system might not trust each other, each user or each session should usually be keyed separately. This will also tend to increase the work required to cryptanalyse the traffic since not all traffic will use the same key. Certain security properties (e.g. traffic analysis protection) are not provided by any of these mechanisms. One possible approach to traffic analysis protection is appropriate use of link encryption. [VK83] Users must carefully consider which security properties they require and take active steps to ensure that their needs are met by these or other mechanisms. Certain applications (e.g. electronic mail) probably need to have application-specific security mechanisms. Application-specific security mechanisms are out of the scope of this document. Users interested in electronic mail security should consult the RFCs describing the Internet's Privacy-Enhanced Mail system. Users concerned about other application-specific mechanisms should consult the online RFCs to see if suitable Internet Standard mechanisms exist. ACKNOWLEDGEMENTS Steve Kent provided detailed and helpful editorial input into this draft. His contributions improved the draft significantly. Many of the concepts here are derived from or were influenced by the US Government's SDNS security protocol specifications, the ISO/IEC's NLSP specification, or from the proposed swIPe security protocol. [SDNS, ISO, IB93, IBK93] The work done for SNMP Security and SNMPv2 Security influenced the choice of default cryptological algorithms and modes. [GM93] Steve Bellovin, Steve Deering, Phil Karn, Frank Kastenholz, Steve Kent, Perry Metzger, Dave Mihelcic, Hilarie Orman and Bill Simpson provided careful critiques of earlier versions of this draft. Atkinson [Page 21] Internet Draft Security Architecture for IP 10 November 1996 REFERENCES [Atk96a] Randall Atkinson, IP Authentication Header, RFC-xxxx, 4 June 1996. [Atk96b] Randall Atkinson, IP Encapsulating Security Payload, RFC-xxxx, 4 June 1996. [BCCH94] R. Braden, D. Clark, S. Crocker, & C. Huitema, "Report of IAB Workshop on Security in the Internet Architecture", RFC-1636, DDN Network Information Center, June 1994. [Bel89] Steven M. Bellovin, "Security Problems in the TCP/IP Protocol Suite", ACM Computer Communications Review, Vol. 19, No. 2, March 1989. [Bel95] Steven M. Bellovin, Presentation at IP Security Working Group Meeting, Proceedings of the 32nd Internet Engineering Task Force, March 1995, Internet Engineering Task Force, Danvers, MA. [BL73] Bell, D.E. & LaPadula, L.J., "Secure Computer Systems: Mathematical Foundations and Model", Technical Report M74-244, The MITRE Corporation, Bedford, MA, May 1973. [CERT95] CA-95:01 [CB94] William R. Cheswick & Steven M. Bellovin, Firewalls & Internet Security, Addison-Wesley, Reading, MA, 1994. [CG96] Shu-jen Chang & Rob Glenn, "HMAC-SHA IP Authentication with Replay Prevention", Internet Draft, 1 May 1996. [DIA] US Defense Intelligence Agency, "Compartmented Mode Workstation Specification", Technical Report DDS-2600-6243-87. [DoD85] US National Computer Security Center, "Department of Defense Trusted Computer System Evaluation Criteria", DoD 5200.28-STD, US Department of Defense, Ft. Meade, MD., December 1985. [DoD87] US National Computer Security Center, "Trusted Network Interpretation of the Trusted Computer System Evaluation Criteria", NCSC-TG-005, Version 1, US Department of Defense, Ft. Meade, MD., 31 July 1987. [DH76] W. Diffie & M. Hellman, "New Directions in Cryptography", IEEE Transactions on Information Theory, Vol. IT-22, No. 6, November 1976, pp. 644-654. [DH95] Steve Deering & Bob Hinden, Internet Protocol version 6 (IPv6) Atkinson [Page 22] Internet Draft Security Architecture for IP 10 November 1996 Specification, RFC-1883, December 1995. [EK96] D. Eastlake III & C. Kaufman, "Domain Name System Protocol Security Extensions", Internet Draft, 30 January 1996. [GM93] J. Galvin & K. McCloghrie, Security Protocols for version 2 of the Simple Network Management Protocol (SNMPv2), RFC-1446, DDN Network Information Center, April 1993. [HA94] N. Haller & R. Atkinson, "On Internet Authentication", RFC-1704, DDN Network Information Center, October 1994. [Hugh96] J. Hughes (Editor), "Combined DES-CBC, HMAC, and Replay Prevention Security Transform", Internet Draft, June 1996. [ISO] ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC DIS 11577, International Standards Organisation, Geneva, Switzerland, 29 November 1992. [IB93] John Ioannidis and Matt Blaze, "Architecture and Implementation of Network-layer Security Under Unix", Proceedings of USENIX Security Symposium, Santa Clara, CA, October 1993. [IBK93] John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network-Layer Security for IP", presentation at the Spring 1993 IETF Meeting, Columbus, Ohio. [Ken78] Steve Kent, ..., Proceedings of SIGCOMM '78, ACM SIGCOMM, 1978. [Ken91] Steve Kent, US DoD Security Options for the Internet Protocol, RFC-1108, DDN Network Information Center, November 1991. [Ken93] Steve Kent, Privacy Enhancement for Internet Electronic Mail: Part II: Certificate-Based Key Management, RFC-1422, DDN Network Information Center, 10 February 1993. [KB93] J. Kohl & B. Neuman, The Kerberos Network Authentication Service (V5), RFC-1510, DDN Network Information Center, 10 September 1993. [NS78] R.M. Needham & M.D. Schroeder, "Using Encryption for Authentication in Large Networks of Computers", Communications of the ACM, Vol. 21, No. 12, December 1978, pp. 993-999. [NS81] R.M. Needham & M.D. Schroeder, "Authentication Revisited", ACM Operating Systems Review, Vol. 21, No. 1., 1981. [OG96] Mike Oehler & Rob Glenn, "HMAC-MD5 IP Authentication with Replay Atkinson [Page 23] Internet Draft Security Architecture for IP 10 November 1996 Prevention", Internet Draft, 1 May 1996. [OTA94] US Congress, Office of Technology Assessment, "Information Security & Privacy in Network Environments", OTA-TCT-606, Government Printing Office, Washington, DC, September 1994. [Sch96] Jeff Schiller, "Security AD Statement on IPSEC Key Management", Email to IPsec mailing list, September 19, 1996. [Sch94] Bruce Schneier, Applied Cryptography, Section 8.6, John Wiley & Sons, New York, NY, 1994. [SDNS] SDNS Secure Data Network System, Security Protocol 3, SP3, Document SDN.301, Revision 1.5, 15 May 1989, published in NIST Publication NIST-IR-90-4250, February 1990. [STD-1] J. Postel, "Internet Official Protocol Standards", STD-1, March 1996. [VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-level Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983. [ZDESZ93] Zhang, L., Deering, S., Estrin, D., Shenker, S., and Zappala, D., "RSVP: A New Resource ReSerVation Protocol", IEEE Network magazine, September 1993. DISCLAIMER The views expressed in this note are those of the editor and are not necessarily those of his employer. The editor and his employer specifically disclaim responsibility for any problems arising from correct or incorrect implementation or use of this design. EDITOR INFORMATION: Randall Atkinson <rja@cisco.com> cisco Systems 170 West Tasman Drive San Jose, CA, 95134-1706 USA Telephone: +1 (408) 526-4000 Atkinson [Page 24]