Network Working Group R. Housley Request for Comments: 1457 Xerox Special Information Systems May 1993
Security Label Framework for the Internet
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited.
Acknowledgements
The members of the Privacy and Security Research Group and the attendees of the invitational Security Labels Workshop (hosted by the National Institute of Standards and Technology) helped me organize my thoughts on this subject. The ideas of these professionals are scattered throughout the memo.
1.0 Introduction
This memo presents a security labeling framework for the Internet. The framework is intended to help protocol designers determine what, if any, security labeling should be supported by their protocols. The framework should also help network architects determine whether or not a particular collection of protocols fulfill their security labeling requirements. The Open Systems Interconnection Reference Model [1] provides the structure for the presentation, therefore OSI protocol designers may also find this memo useful.
2.0 Security Labels
Data security is the set of measures taken to protect data from accidental, unauthorized, intentional, or malicious modification, destruction, or disclosure. Data security is also the condition that results from the establishment and maintenance of protective measures [2]. Given this two-pronged definition for data security, this memo examines security labeling as one mechanism which provides data security. In general, security labeling by itself does not provide sufficient data security; it must be complemented by other security mechanisms.
In data communication protocols, security labels tell the protocol processing how to handle the data transferred between two systems. That is, the security label indicates what measures need to be taken to preserve the condition of security. Handling means the activities
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performed on data such as collecting, processing, transferring, storing, retrieving, sorting, transmitting, disseminating, and controlling [3].
The definition of data security includes protection from modification and destruction. In computer systems, this is protection from writing and deleting. These protections implement the data integrity service defined in the OSI Security Architecture [4].
Biba [5] has defined a data integrity model which includes security labels. The Biba model specifies rule-based controls for writing and deleting necessary to preserve data integrity. The model also specifies rule-based controls for reading to prevent a high integrity process from relying on data that has less integrity than the process.
The definition of data security also includes protection from disclosure. In computer systems, this is protection from reading. This protection is the data confidentiality service defined in the OSI Security Architecture [4].
Bell and LaPadula [6] defined a data confidentiality model which includes security labels. The Bell and LaPadula model specifies rule-based controls for reading necessary to preserve data confidentiality. The model also specifies rule-based controls for writing to ensure that data is not copied to a container where confidentiality can not be guaranteed.
In both the Biba model and the Bell and LaPadula model, the security label is an attribute of the data. In general, the security label associated with the data remains constant. Exceptions will be discussed later in the memo, but relabeling is always the result of some network entity handling the data. Since the security label is an attribute of data, it should be bound to the data. When data moves through the network, the integrity security service [4] is generally used to accomplish this binding. If the communications environment does not include a protocol which provides the integrity security service to bind the security label to the data, then the communications environment should include other mechanisms to preserve this binding.
2.1 Integrity Labels
Integrity labels are security labels which support data integrity models, like the Biba model. The integrity label tells the degree of confidence that may be placed in the data and also indicates which measures the data requires for protection from modification and destruction.
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As data moves through the network, the confidence that may be placed in that data may change as a result of being handled by various network components. Therefore, the integrity label is a function of the integrity of the data before being transmitted on the network and the path that the data takes through the network. The confidence that may be placed in data does not increase because it was transferred across a network, but the confidence that may be placed in data may decrease as a result of being handled by arbitrary network components. Entities are assigned integrity labels which indicate how much confidence may be placed in data that is handled by them. Thus, when data is handled by an entity with an integrity label lower than the integrity label of the data, the data is relabeled with the integrity label of the entity. Such relabeling should be avoided by limiting the possible paths that data may take through the network to those where the data will be handled only by entities with the same or a higher integrity label than the data.
When integrity labels are used, each of the systems on a network must implement the integrity model and the protocol suite must transfer the integrity label with the data, if the confidence of the data is to be maintained throughout the network. Each of the systems on a network may have its own internal representation for a integrity label, but the protocols must provide common syntax and semantics for the transfer of the integrity label, as well as the data itself. To date, no protocols have been standardized which include integrity labels in the protocol control information.
2.2 Sensitivity Labels
Sensitivity labels are security labels which support data confidentiality models, like the Bell and LaPadula model. The sensitivity label tells the amount of damage that will result from the disclosure of the data and also indicates which measures the data requires for protection from disclosure. The amount of damage that results from unauthorized disclosure depends on who obtains the data; the sensitivity label should reflect the worst case.
As data moves through the network, it is processed by various network components and may be mixed with data of differing sensitivity. If these network components are not trusted to segregate data of differing sensitivities, then all of the data processed by those components must be handled as the most sensitive data processed by those network components. For example, poor buffer management may append highly sensitive data to the end of a protocol data unit that was otherwise publicly releasable. Therefore, the sensitivity label is a function of the sensitivity of the data before being transmitted on the network and the most sensitive data handled by the network components, and the trustworthiness of those network components. The
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amount of damage that will result from the disclosure of the data does not decrease because it was transferred across a network, but the amount of damage that will result from the disclosure of the data may increase as a result of being mixed with more sensitive data by arbitrary network components. Thus, when data is handled by an untrusted entity with a sensitivity label higher than the sensitivity label of the data, the data is relabeled with the higher sensitivity label. Such relabeling should be avoided by limiting the possible paths that data may take through the network to those where the data will be handled only by entities with the same sensitivity label as the data or by using trustworthy network components. Entities with lower sensitivity labels may not handle the data because this would be disclosure.
When sensitivity labels are used, each of the systems on a network must implement the sensitivity model and the protocol suite must transfer the sensitivity label with the data, if the protection from disclosure is to be maintained throughout the network. Each of the systems on a network may have its own internal representation for a sensitivity label, but the protocols must provide common syntax and semantics for the transfer of the sensitivity label, as well as the data itself. Sensitivity labels, like the ones provided by the IP Security Option (IPSO) [7], have been used in a few networks for years.
3.0 Security Label Usage
The Internet includes two major types of systems: end systems and intermediate systems [1]. These terms should be familiar to the reader. For this discussion, the definition of intermediate system is understood to include routers, packet switches, and bridges. End systems and intermediate systems use security labels differently.
3.1 End System Security Label Usage
When two end systems communicate, common security label syntax and semantics are needed. The security label, as an attribute of the data, indicates what measures need to be taken to preserve the condition of security. The security label must communicate all of the integrity and confidentiality handling requirements. These requirements can become very complex.
Some operating systems label the data they process. These security labels are not part of the data; they are attributes of the data. Some database management systems (DBMSs) perform similar labeling. The format of these security labels is a local matter, but they are usually in a format different than the one used by the data communication protocols. Security labels must be translated by these
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operating systems and DBMSs between the local format and the format used in the data communication protocols without any loss of meaning.
Trusted operating systems that implement rule-based access control policies require security labels on the data they import [8,9]. These security labels permit the Trusted Computing Base (TCB) in the end system to perform trusted demultiplexing. That is, the traffic is relayed from the TCB to a process only if the process has sufficient authorization for the data. In most cases, the TCB must first translate the security label into the local syntax before it can make the access control decision.
3.2 Intermediate System Security Label Usage
This section discusses "user" data security labels within the intermediate system. The labeling requirements associated with intermediate system-to-end system (IS-ES) traffic, intermediate system-to-intermediate system (IS-IS) traffic, and intermediate system-to-network management (IS-NM) traffic are not included in this discussion.
Intermediate systems may make routing choices or discard traffic based on the security label. The security label used by the intermediate system should contain only enough information to make the routing/discard decision and may be a subset of the security label used by the end system. Some portions of the label may not effect routing decisions, but they may effect processing done within the end system.
In the Internet today, very few intermediate systems actually make access control decisions. For performance reasons, only those intermediate systems which do make access control decisions should be burdened with parsing the security label. That is, information hiding principles apply. Further, security labels which are to be parsed only by end systems should not be visible to physical, data link, or network layer protocols, where intermediate systems will have to examine them.
Intermediate systems do not usually translate the security labels to a local format. They use them "as is" to make their routing/discard decisions. However, when two classification authorities share a network by bilateral agreement, the intermediate systems may be required to perform security label translation. Security label translations should be avoided whenever possible by using a security label format that is supported by all systems that will process the security label. Since end systems do not generally know which intermediate systems will process their traffic, security label translation cannot always be avoided.
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Since security labels which are to be parsed only by end systems should not be carried by protocols interpreted by intermediate systems, such security labels should be carried by upper layer protocols, and end systems which use different formats for such security labels cannot rely on an intermediate systems to perform security label translation. Neither the Internet nor the OSI architecture includes such transformation functions in the transport, session, or presentation layer, which means that application layer gateways should be used to translate between different end system security label formats. Such application gateways should be avoided because they impinge on operation, especially when otherwise compatible protocols are used. This complication is another reason why the use of a security label format that is supported by all systems is desirable. A standard label syntax with registered security label semantics goes a long way toward avoiding security label translation [10].
4.0 Approaches to Labeling
There are several tradeoffs to be made when determining how a particular network will perform security labeling. Explicit or implicit labels can be used. Also, security labels can either be connectionless or connection-oriented. A combination of these alternatives may be appropriate.
4.1 Explicit Versus Implicit Security Labels
Explicit security labels are actual bits in the protocol control information (PCI). The IP Security Option (IPSO) is an example of an explicit security label [7]. Explicit security labels may be either connectionless or connection-oriented. The syntax and semantics of the explicit security label may be either tightly or loosely coupled. If the syntax and semantics are tightly coupled, then the explicit security label format supports a single security policy. If the syntax and semantics are loosely coupled, then the explicit security label format can support multiple security policies through registration. In both cases, software enforces the security policy, but the label parsing software can be written once if the syntax and semantics are loosely coupled. Fixed length explicit security label format parsers are generally faster than parsers for variable length formats. Intermediate systems suffer less performance impact when fixed length explicit security labels can be used, but end systems often need variable length explicit security labels to express data handling requirements.
Implicit security labels are not actual bits in the PCI; instead, some attribute is used to determine the security label. For example, the choice of cryptographic key in the SP4 protocol [11] can
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determine the security label. Implicit security labels may be either connectionless or connection-oriented.
4.2 Connectionless Versus Connection-oriented Security Labels
When connectionless security labels are used, the security label appears in every protocol data unit (PDU). The IP Security Option (IPSO) [7] is an example of connectionless labeling. All protocols have limits on the size of their PCI, and the explicit security label cannot exceed this size limit. It cannot use the entire PCI space either; the protocol has other fields that must be transferred as well. This size limitation may prohibit explicit connectionless security labels from meeting the requirements of end systems. However, the requirements of intermediate systems are more easily satisfied by explicit connectionless security labels.
Connection-oriented security labels are attributes of virtual circuits, connections, and associations. For simplicity, all of these are subsequently referred to as connections. Connection- oriented security labels are used when the SDNS Key Management Protocol (KMP) [12] is used to associate security labels with each of the transport connection protected by the SP4 protocol [10,11] (using SP4C). The security label is defined at connection establishment, and all data transferred over the connection inherits that security label. This approach is more compatible with end system requirements than intermediate system requirements. One noteworthy exception is X.25 packets switches; these intermediate systems could associate connection-oriented labels with each virtual circuit.
Connectionless security labels may be used in conjunction with connectionless or connection-oriented data transfer protocols. However, connection-oriented security labels may only be used in conjunction with connection-oriented data transfer protocols.
5.0 Labeling Within the OSI Reference Model
This section examines each of the seven OSI layers with respect to security labels.
5.1 Layer 1, The Physical Layer
Explicit security labels are not possible in the Physical Layer. The Physical Layer does not include any protocol control information (PCI), so there is no place to include the bits which represent the label.
Implicit security labels are possible in the Physical Layer. For example, all of the data that comes in through a particular physical
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port could inherit one security label. Most Physical Layer communication is connectionless, supporting only bit-at-a-time or byte-at-a-time operations. Thus, these implicit security labels are connectionless.
Implicit security labels in the Physical Layer may be used to meet the requirements of either end systems or intermediate systems so long as the communication is single level. That is, only one security label is associated with all of the data received or transmitted through the physical connection.
5.2 Layer 2, The Data Link Layer
Explicit security labels are possible in the Data Link Layer. In fact, the IEEE 802.2 Working Group is currently working on an optional security label standard for the Logical Link Control (LLC) protocol (IEEE 802.2) [13]. These labels will optionally appear in each LLC frame. These are connectionless security labels.
Explicit connection-oriented security labels are also possible in the Data Link Layer. One could imagine a security label standard which worked with LLC Type II.
Of course, implicit security labels are also possible in the Data Link Layer. Such labels could be either connectionless or connection-oriented. One attribute that might be used in IEEE 802.3 (CSMA/CD) [14] to determine the implicit security label is the source address of the frame.
Security labels in the Data Link Layer may be used to meet the requirements of end systems and intermediate systems (especially bridges). Explicit security labels in this layer tend to be small because the protocol headers for data link layer protocols are themselves small. Therefore, when end systems require large security labels, a higher protocol layer should used to carry them. However, when end systems do not require large security labels, the data link layer is attractive because in many cases the data link layer protocol supports several protocol suites simultaneously. Label- based routing/relay decisions made by bridges are best supported in this layer.
5.3 Layer 3, The Network Layer
Explicit security labels are possible in the Network Layer. In fact, the IP Security Option (IPSO) [7] has been used for many years. These labels optionally appear in each IP datagram. IPSO labels are obviously connectionless security labels.
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Explicit connection-oriented security labels are also possible in the Network Layer. One could easily imagine a security label standard for X.25 [15], but none exists.
Of course, implicit security labels are also possible in the Network Layer. These labels could be either connectionless or connection- oriented. One attribute that might be used to determine the implicit security label is the X.25 virtual circuit.
Security labels in the Network Layer may be used to meet the requirements of end systems and intermediate systems. Explicit security labels in this layer tend to be small because the protocol headers for network layer protocols are themselves small. Small fixed size network layer protocol headers allow efficient router implementations. Therefore, when end systems require large security labels, a higher protocol layer should used to carry them. Alternatively, the Network Layer (especially the Subnetwork Independent Convergence Protocol (SNICP) sublayer) is an excellent place to carry a security label to support trusted demultiplexing, because many implementations demultiplex from an system-wide daemon to a user process after network layer processing. The SNICP is end- to-end, yet it is low enough in the protocol stack to aid trusted demultiplexing.
Label-based routing/relay decisions made by routers and packet switches are best supported in the Network Layer. Routers can also add labels at subnetwork boundaries. However, placement of these security labels must be done carefully to ensure that their addition does not degrade overall network performance by forcing routers that do not make label-based routing decisions to parse the security label. Also, performance will suffer if the addition of security labels at subnet boundaries induces fragmentation/segmentation.
5.4 Layer 4, The Transport Layer
Explicit security labels are possible in the Transport Layer. For example, the SP4 protocol [10,11] includes them. These labels can be either connectionless (using SP4E) or connection-oriented (using SP4C). SP4 is an addendum to the TP [16] and CLTP [17] protocols.
Implicit security labels are also possible in the Transport Layer. Such labels could be either connectionless or connection-oriented. One attribute that might be used to determine the implicit label in the SP4 protocol (when explicit labels are not used as discussed above) is the choice of cryptographic key.
Security labels in the Transport Layer may be used to meet the requirements of end systems. The Transport Layer cannot be used to
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meet the requirements of intermediate systems because intermediate systems, by definition, do not process protocols above the Network Layer. Connection-oriented explicit security labels in this layer are especially good for meeting end system requirements where large labels are required. The security label is transmitted only at connection establishment, so overhead is kept to a minimum. Of course, connectionless transport protocols may not take advantage of this overhead reduction technique. Yet, in many implementations the Transport Layer is low enough in the protocol stack to aid trusted demultiplexing.
5.5 Layer 5, The Session Layer
Explicit security labels are possible in the Session Layer. Such labels could be either connectionless or connection-oriented. However, it is unlikely that a standard will ever be developed for such labels because the OSI Security Architecture [4] does not allocate any security services to the Session Layer, and the Internet protocol suite does not have a Session Layer.
Implicit security labels are also possible in the Session Layer. These implicit labels could be either connectionless or connection- oriented. Again, the OSI Security Architecture makes this layer an unlikely choice for security labeling.
Security labels in the Session Layer may be used to meet the requirements of end systems, but the Session Layer is too high in the protocol stack to support trusted demultiplexing. The Session Layer cannot be used to meet the requirements of intermediate systems because intermediate systems, by definition, do not process protocols above the Network Layer. Security labels in the Session Layer do not offer any advantages to security labels in the Transport Layer.
5.6 Layer 6, The Presentation Layer
Explicit security labels are possible in the Presentation Layer. The presentation syntax may include a security label. This approach naturally performs translation to the local label format and supports both connectionless and connection-oriented security labeling.
Implicit security labels are also possible in the Presentation Layer. Such labels could be either connectionless or connection-oriented.
Security labels in the Presentation Layer may be used to meet the requirements of end systems, but the Presentation Layer is too high in the protocol stack to support trusted demultiplexing. The Presentation Layer cannot be used to meet the requirements of intermediate systems because intermediate systems, by definition, do
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not process protocols above the Network Layer. To date, no Presentation Layer protocols have been standardized which include security labels.
5.7 Layer 7, The Application Layer
Explicit security labels are possible in the Application Layer. The CCITT X.400 message handling system includes security labels in message envelopes [18]. Other Application Layer protocols will probably include security labels in the future. These labels could be either connectionless or connection-oriented. Should security labels be incorporated into transaction processing protocols and message handling protocols, these will most likely be connectionless security labels; should security labels be incorporated into other application protocols, these will most likely be connection-oriented security labels. Application layer protocols are unique in that they can include security label information which is specific to a particular application without burdening other applications with the syntax or semantics of that security label.
Store and forward application protocols, like electronic messaging and directory protocols, deserve special attention. In terms of the OSI Reference Model, they are end system protocols, but multiple end systems cooperate to provide the communications service. End systems may use security labels to determine which end system should be next in a chain of store and forward interactions; this use of security labels is very similar to the label-based routing/relay decisions made by routers except that the security labels are carried in an Application Layer protocol. Also, Application Layer protocols must be used to carry security labels in a store and forward application when sensitivity labels must be concealed from some end systems in the chain or when some end systems in the chain are untrustworthy.
Implicit security labels are also possible in the Application Layer. These labels could be either connectionless or connection-oriented. Application title or well know port number might be used to determine the implicit label.
Security labels in the Application Layer may be used to meet the requirements of end systems, but the Application Layer is too high in the protocol stack to support trusted demultiplexing. The Application Layer cannot be used to meet the requirements of intermediate systems because intermediate systems, by definition, do not process protocols above the Network Layer.
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6.0 Summary
Very few hard rules exist for security labels. Internet architects and protocol designers face many tradeoffs when making security label placement decisions. However, a few guidelines can be derived from the preceding discussion:
First, security label-based routing decisions are best supported by explicit security labels in the Data Link Layer and the Network Layer. When bridges are making the routing decisions, the Data Link Layer should carry the explicit security label; when routers are making the routing decisions, the Network Layer should carry the explicit security label.
Second, when security labels are specific to a particular application it is wise to define them in the application protocol, so that these security labels will not burden other applications on the network.
Third, when trusted demultiplexing is a concern, the Network Layer (preferably the SNICP) or Transport Layer should be used to carry the explicit security label. The SNICP or transport protocol are especially attractive when combined with a cryptographic protocol that binds the security label to the data and protects the both against undetected modification.
Fourth, to avoid explicit security label translation, a common explicit security label format should be defined for the Internet. Registration of security label semantics should be used so that many security policies can be supported by the common explicit security label syntax.
References
[1] ISO Open Systems Interconnection - Basic Reference Model (ISO 7498). International Organization for Standardization, 1981.
[2] Dictionary of Military and Associated Terms (JCS Pub 1). Joint Chiefs of Staff. 1 April 1984.
[3] Security Requirements for Automatic Data Processing (ADP) Systems (DODD 5200.28). Department of Defense. 21 March 1988.
[4] Information Processing Systems - Open Systems Interconnection Reference Model - Security Architecture (ISO 7498-2). Organization for Standardization, 1988.
[5] Biba, K. J. "Integrity Considerations for Secure Computer Systems", MTR-3153, The Mitre Corporation, April 1977.
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[6] Bell, D. E.; LaPadula, L. J. "Secure Computer System: Unified Exposition and Multics Interpretation", MTR-2997, The MITRE Corporation, March 1976.
[7] Kent, S. "U.S. Department of Defense Security Options for the Internet Protocol", RFC 1108, BBN Communications, November 1992.
[8] Trusted Computer System Evaluation Criteria (DoD 5200.28-STD) National Computer Security Center, 26 December 1985.
[9] Trusted Network Interpretation of the Trusted Computer System Evaluation Criteria, (NCSC-TG-005, Version-1). National Computer Security Center, 31 July 1987.
[10] Nazario, Noel (Chairman). "Standard Security Label for GOSIP An Invitational Workshop", NISTIR 4614, June 1991.
[11] Dinkel, Charles (Editor). "Secure Data Network System (SDNS) Network, Transport, and Message Security Protocols", NISTIR 90- 4250, February 1990, pp 39-62.
[12] Dinkel, Charles (Editor). "Secure Data Network System (SDNS) Key Management Documents", NISTIR 90-4262, February 1990.
[13] IEEE Standards for Local Area Networks: Logical Link Control, IEEE 802.2. The Institute of Electrical and Electronics Engineers, Inc, 1984.
[14] IEEE Standards for Local Area Networks: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specification, IEEE 802.3. The Institute of Electrical and Electronics Engineers, Inc, 1985.
[15] Recommendation X.25, Interface Between Data Terminal Equipment (DTE) and Data Circuit Terminating Equipment (DCE) for Terminals Operating in the Packet Mode on Public Data Networks. Consultative Committee, International Telephone and Telegraph (CCITT), 1984.
[16] Information Processing Systems - Open Systems Interconnection - Connection oriented transport protocol specification (ISO 8073). Organization for Standardization, 1985. [Also ISO 8208]
[17] Information Processing Systems - Open Systems Interconnection - Protocol for providing the connectionless-mode transport service (ISO 8602). Organization for Standardization, 1986.
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[18] Recommendation X.411, Message Handling Systems: Message Transfer System: Abstract Service Definition and Procedures. Consultative Committee, International Telephone and Telegraph (CCITT), 1988. [Also ISO 8883-1]
Security Considerations
This entire memo is devoted to a discussion of a Framework for labeling information for security purposes in network protocols.
Author's Address
Russell Housley Xerox Special Information Systems 7900 Westpark Drive McLean, Virginia 22102
