Network Working Group D. Piper Request for Comments: 2407 Network Alchemy Category: Standards Track November 1998 The Internet IP Security Domain of Interpretation for ISAKMP Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. IESG Note Section 4.4.4.2 states, "All implememtations within the IPSEC DOI MUST support ESP_DES...". Recent work in the area of cryptanalysis suggests that DES may not be sufficiently strong for many applications. Therefore, it is very likely that the IETF will deprecate the use of ESP_DES as a mandatory cipher suite in the near future. It will remain as an optional use protocol. Although the IPsec working group and the IETF in general have not settled on an alternative algorithm (taking into account concerns of security and performance), implementers may want to heed the recommendations of section 4.4.4.3 on the use of ESP_3DES. 1. Abstract The Internet Security Association and Key Management Protocol (ISAKMP) defines a framework for security association management and cryptographic key establishment for the Internet. This framework consists of defined exchanges, payloads, and processing guidelines that occur within a given Domain of Interpretation (DOI). This document defines the Internet IP Security DOI (IPSEC DOI), which instantiates ISAKMP for use with IP when IP uses ISAKMP to negotiate security associations. For a list of changes since the previous version of the IPSEC DOI, please see Section 7. Piper Standards Track [Page 1] RFC 2407 IP Security Domain of Interpretation November 1998 2. Introduction Within ISAKMP, a Domain of Interpretation is used to group related protocols using ISAKMP to negotiate security associations. Security protocols sharing a DOI choose security protocol and cryptographic transforms from a common namespace and share key exchange protocol identifiers. They also share a common interpretation of DOI-specific payload data content, including the Security Association and Identification payloads. Overall, ISAKMP places the following requirements on a DOI definition: o define the naming scheme for DOI-specific protocol identifiers o define the interpretation for the Situation field o define the set of applicable security policies o define the syntax for DOI-specific SA Attributes (Phase II) o define the syntax for DOI-specific payload contents o define additional Key Exchange types, if needed o define additional Notification Message types, if needed The remainder of this document details the instantiation of these requirements for using the IP Security (IPSEC) protocols to provide authentication, integrity, and/or confidentiality for IP packets sent between cooperating host systems and/or firewalls. For a description of the overall IPSEC architecture, see [ARCH], [AH], and [ESP]. 3. Terms and Definitions The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC 2119]. 4.1 IPSEC Naming Scheme Within ISAKMP, all DOI’s must be registered with the IANA in the "Assigned Numbers" RFC [STD-2]. The IANA Assigned Number for the Internet IP Security DOI (IPSEC DOI) is one (1). Within the IPSEC DOI, all well-known identifiers MUST be registered with the IANA under the IPSEC DOI. Unless otherwise noted, all tables within this document refer to IANA Assigned Numbers for the IPSEC DOI. See Section 6 for further information relating to the IANA registry for the IPSEC DOI. All multi-octet binary values are stored in network byte order. Piper Standards Track [Page 2] RFC 2407 IP Security Domain of Interpretation November 1998 4.2 IPSEC Situation Definition Within ISAKMP, the Situation provides information that can be used by the responder to make a policy determination about how to process the incoming Security Association request. For the IPSEC DOI, the Situation field is a four (4) octet bitmask with the following values. Situation Value --------- ----- SIT_IDENTITY_ONLY 0x01 SIT_SECRECY 0x02 SIT_INTEGRITY 0x04 4.2.1 SIT_IDENTITY_ONLY The SIT_IDENTITY_ONLY type specifies that the security association will be identified by source identity information present in an associated Identification Payload. See Section 4.6.2 for a complete description of the various Identification types. All IPSEC DOI implementations MUST support SIT_IDENTITY_ONLY by including an Identification Payload in at least one of the Phase I Oakley exchanges ([IKE], Section 5) and MUST abort any association setup that does not include an Identification Payload. If an initiator supports neither SIT_SECRECY nor SIT_INTEGRITY, the situation consists only of the 4 octet situation bitmap and does not include the Labeled Domain Identifier field (Figure 1, Section 4.6.1) or any subsequent label information. Conversely, if the initiator supports either SIT_SECRECY or SIT_INTEGRITY, the Labeled Domain Identifier MUST be included in the situation payload. 4.2.2 SIT_SECRECY The SIT_SECRECY type specifies that the security association is being negotiated in an environment that requires labeled secrecy. If SIT_SECRECY is present in the Situation bitmap, the Situation field will be followed by variable-length data that includes a sensitivity level and compartment bitmask. See Section 4.6.1 for a complete description of the Security Association Payload format. If an initiator does not support SIT_SECRECY, SIT_SECRECY MUST NOT be set in the Situation bitmap and no secrecy level or category bitmaps shall be included. If a responder does not support SIT_SECRECY, a SITUATION-NOT- SUPPORTED Notification Payload SHOULD be returned and the security association setup MUST be aborted. Piper Standards Track [Page 3] RFC 2407 IP Security Domain of Interpretation November 1998 4.2.3 SIT_INTEGRITY The SIT_INTEGRITY type specifies that the security association is being negotiated in an environment that requires labeled integrity. If SIT_INTEGRITY is present in the Situation bitmap, the Situation field will be followed by variable-length data that includes an integrity level and compartment bitmask. If SIT_SECRECY is also in use for the association, the integrity information immediately follows the variable-length secrecy level and categories. See section 4.6.1 for a complete description of the Security Association Payload format. If an initiator does not support SIT_INTEGRITY, SIT_INTEGRITY MUST NOT be set in the Situation bitmap and no integrity level or category bitmaps shall be included. If a responder does not support SIT_INTEGRITY, a SITUATION-NOT- SUPPORTED Notification Payload SHOULD be returned and the security association setup MUST be aborted. 4.3 IPSEC Security Policy Requirements The IPSEC DOI does not impose specific security policy requirements on any implementation. Host system policy issues are outside of the scope of this document. However, the following sections touch on some of the issues that must be considered when designing an IPSEC DOI host implementation. This section should be considered only informational in nature. 4.3.1 Key Management Issues It is expected that many systems choosing to implement ISAKMP will strive to provide a protected domain of execution for a combined IKE key management daemon. On protected-mode multiuser operating systems, this key management daemon will likely exist as a separate privileged process. In such an environment, a formalized API to introduce keying material into the TCP/IP kernel may be desirable. The IP Security architecture does not place any requirements for structure or flow between a host TCP/IP kernel and its key management provider. Piper Standards Track [Page 4] RFC 2407 IP Security Domain of Interpretation November 1998 4.3.2 Static Keying Issues Host systems that implement static keys, either for use directly by IPSEC, or for authentication purposes (see [IKE] Section 5.4), should take steps to protect the static keying material when it is not residing in a protected memory domain or actively in use by the TCP/IP kernel. For example, on a laptop, one might choose to store the static keys in a configuration store that is, itself, encrypted under a private password. Depending on the operating system and utility software installed, it may not be possible to protect the static keys once they’ve been loaded into the TCP/IP kernel, however they should not be trivially recoverable on initial system startup without having to satisfy some additional form of authentication. 4.3.3 Host Policy Issues It is not realistic to assume that the transition to IPSEC will occur overnight. Host systems must be prepared to implement flexible policy lists that describe which systems they desire to speak securely with and which systems they require speak securely to them. Some notion of proxy firewall addresses may also be required. A minimal approach is probably a static list of IP addresses, network masks, and a security required flag or flags. A more flexible implementation might consist of a list of wildcard DNS names (e.g. ’*.foo.bar’), an in/out bitmask, and an optional firewall address. The wildcard DNS name would be used to match incoming or outgoing IP addresses, the in/out bitmask would be used to determine whether or not security was to be applied and in which direction, and the optional firewall address would be used to indicate whether or not tunnel mode would be needed to talk to the target system though an intermediate firewall. 4.3.4 Certificate Management Host systems implementing a certificate-based authentication scheme will need a mechanism for obtaining and managing a database of certificates. Secure DNS is to be one certificate distribution mechanism, however the pervasive availability of secure DNS zones, in the short term, is doubtful for many reasons. What’s far more likely is that hosts will Piper Standards Track [Page 5] RFC 2407 IP Security Domain of Interpretation November 1998 need an ability to import certificates that they acquire through secure, out-of-band mechanisms, as well as an ability to export their own certificates for use by other systems. However, manual certificate management should not be done so as to preclude the ability to introduce dynamic certificate discovery mechanisms and/or protocols as they become available. 4.4 IPSEC Assigned Numbers The following sections list the Assigned Numbers for the IPSEC DOI: Situation Identifiers, Protocol Identifiers, Transform Identifiers, AH, ESP, and IPCOMP Transform Identifiers, Security Association Attribute Type Values, Labeled Domain Identifiers, ID Payload Type Values, and Notify Message Type Values. 4.4.1 IPSEC Security Protocol Identifier The ISAKMP proposal syntax was specifically designed to allow for the simultaneous negotiation of multiple Phase II security protocol suites within a single negotiation. As a result, the protocol suites listed below form the set of protocols that can be negotiated at the same time. It is a host policy decision as to what protocol suites might be negotiated together. The following table lists the values for the Security Protocol Identifiers referenced in an ISAKMP Proposal Payload for the IPSEC DOI. Protocol ID Value ----------- ----- RESERVED 0 PROTO_ISAKMP 1 PROTO_IPSEC_AH 2 PROTO_IPSEC_ESP 3 PROTO_IPCOMP 4 4.4.1.1 PROTO_ISAKMP The PROTO_ISAKMP type specifies message protection required during Phase I of the ISAKMP protocol. The specific protection mechanism used for the IPSEC DOI is described in [IKE]. All implementations within the IPSEC DOI MUST support PROTO_ISAKMP. NB: ISAKMP reserves the value one (1) across all DOI definitions. Piper Standards Track [Page 6] RFC 2407 IP Security Domain of Interpretation November 1998 4.4.1.2 PROTO_IPSEC_AH The PROTO_IPSEC_AH type specifies IP packet authentication. The default AH transform provides data origin authentication, integrity protection, and replay detection. For export control considerations, confidentiality MUST NOT be provided by any PROTO_IPSEC_AH transform. 4.4.1.3 PROTO_IPSEC_ESP The PROTO_IPSEC_ESP type specifies IP packet confidentiality. Authentication, if required, must be provided as part of the ESP transform. The default ESP transform includes data origin authentication, integrity protection, replay detection, and confidentiality. 4.4.1.4 PROTO_IPCOMP The PROTO_IPCOMP type specifies IP payload compression as defined in [IPCOMP]. 4.4.2 IPSEC ISAKMP Transform Identifiers As part of an ISAKMP Phase I negotiation, the initiator’s choice of Key Exchange offerings is made using some host system policy description. The actual selection of Key Exchange mechanism is made using the standard ISAKMP Proposal Payload. The following table lists the defined ISAKMP Phase I Transform Identifiers for the Proposal Payload for the IPSEC DOI. Transform Value --------- ----- RESERVED 0 KEY_IKE 1 Within the ISAKMP and IPSEC DOI framework it is possible to define key establishment protocols other than IKE (Oakley). Previous versions of this document defined types both for manual keying and for schemes based on use of a generic Key Distribution Center (KDC). These identifiers have been removed from the current document. The IPSEC DOI can still be extended later to include values for additional non-Oakley key establishment protocols for ISAKMP and IPSEC, such as Kerberos [RFC-1510] or the Group Key Management Protocol (GKMP) [RFC-2093]. Piper Standards Track [Page 7] RFC 2407 IP Security Domain of Interpretation November 1998 4.4.2.1 KEY_IKE The KEY_IKE type specifies the hybrid ISAKMP/Oakley Diffie-Hellman key exchange (IKE) as defined in the [IKE] document. All implementations within the IPSEC DOI MUST support KEY_IKE. 4.4.3 IPSEC AH Transform Identifiers The Authentication Header Protocol (AH) defines one mandatory and several optional transforms used to provide authentication, integrity, and replay detection. The following table lists the defined AH Transform Identifiers for the ISAKMP Proposal Payload for the IPSEC DOI. Note: the Authentication Algorithm attribute MUST be specified to identify the appropriate AH protection suite. For example, AH_MD5 can best be thought of as a generic AH transform using MD5. To request the HMAC construction with AH, one specifies the AH_MD5 transform ID along with the Authentication Algorithm attribute set to HMAC-MD5. This is shown using the "Auth(HMAC-MD5)" notation in the following sections. Transform ID Value ------------ ----- RESERVED 0-1 AH_MD5 2 AH_SHA 3 AH_DES 4 Note: all mandatory-to-implement algorithms are listed as "MUST" implement (e.g. AH_MD5) in the following sections. All other algorithms are optional and MAY be implemented in any particular implementation. 4.4.3.1 AH_MD5 The AH_MD5 type specifies a generic AH transform using MD5. The actual protection suite is determined in concert with an associated SA attribute list. A generic MD5 transform is currently undefined. All implementations within the IPSEC DOI MUST support AH_MD5 along with the Auth(HMAC-MD5) attribute. This suite is defined as the HMAC-MD5-96 transform described in [HMACMD5]. The AH_MD5 type along with the Auth(KPDK) attribute specifies the AH transform (Key/Pad/Data/Key) described in RFC-1826. Piper Standards Track [Page 8] RFC 2407 IP Security Domain of Interpretation November 1998 Use of AH_MD5 with any other Authentication Algorithm attribute value is currently undefined. 4.4.3.2 AH_SHA The AH_SHA type specifies a generic AH transform using SHA-1. The actual protection suite is determined in concert with an associated SA attribute list. A generic SHA transform is currently undefined. All implementations within the IPSEC DOI MUST support AH_SHA along with the Auth(HMAC-SHA) attribute. This suite is defined as the HMAC-SHA-1-96 transform described in [HMACSHA]. Use of AH_SHA with any other Authentication Algorithm attribute value is currently undefined. 4.4.3.3 AH_DES The AH_DES type specifies a generic AH transform using DES. The actual protection suite is determined in concert with an associated SA attribute list. A generic DES transform is currently undefined. The IPSEC DOI defines AH_DES along with the Auth(DES-MAC) attribute to be a DES-MAC transform. Implementations are not required to support this mode. Use of AH_DES with any other Authentication Algorithm attribute value is currently undefined. 4.4.4 IPSEC ESP Transform Identifiers The Encapsulating Security Payload (ESP) defines one mandatory and many optional transforms used to provide data confidentiality. The following table lists the defined ESP Transform Identifiers for the ISAKMP Proposal Payload for the IPSEC DOI. Note: when authentication, integrity protection, and replay detection are required, the Authentication Algorithm attribute MUST be specified to identify the appropriate ESP protection suite. For example, to request HMAC-MD5 authentication with 3DES, one specifies the ESP_3DES transform ID with the Authentication Algorithm attribute set to HMAC-MD5. For additional processing requirements, see Section 4.5 (Authentication Algorithm). Piper Standards Track [Page 9] RFC 2407 IP Security Domain of Interpretation November 1998 Transform ID Value ------------ ----- RESERVED 0 ESP_DES_IV64 1 ESP_DES 2 ESP_3DES 3 ESP_RC5 4 ESP_IDEA 5 ESP_CAST 6 ESP_BLOWFISH 7 ESP_3IDEA 8 ESP_DES_IV32 9 ESP_RC4 10 ESP_NULL 11 Note: all mandatory-to-implement algorithms are listed as "MUST" implement (e.g. ESP_DES) in the following sections. All other algorithms are optional and MAY be implemented in any particular implementation. 4.4.4.1 ESP_DES_IV64 The ESP_DES_IV64 type specifies the DES-CBC transform defined in RFC-1827 and RFC-1829 using a 64-bit IV. 4.4.4.2 ESP_DES The ESP_DES type specifies a generic DES transform using DES-CBC. The actual protection suite is determined in concert with an associated SA attribute list. A generic transform is currently undefined. All implementations within the IPSEC DOI MUST support ESP_DES along with the Auth(HMAC-MD5) attribute. This suite is defined as the [DES] transform, with authentication and integrity provided by HMAC MD5 [HMACMD5]. 4.4.4.3 ESP_3DES The ESP_3DES type specifies a generic triple-DES transform. The actual protection suite is determined in concert with an associated SA attribute list. The generic transform is currently undefined. All implementations within the IPSEC DOI are strongly encouraged to support ESP_3DES along with the Auth(HMAC-MD5) attribute. This suite is defined as the [ESPCBC] transform, with authentication and integrity provided by HMAC MD5 [HMACMD5]. Piper Standards Track [Page 10]