Network Working Group Internet Draft B. Korver Document: draft-ietf-ipsec-pki-profile-00.txt Xythos, Inc. Expires: October 2002 E. Rescorla RTFM, Inc. June 2002 The Internet IP Security PKI Profile of ISAKMP and PKIX Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC-2026 [RFC2026]. 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 six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract ISAKMP and PKIX both provide frameworks that must be profiled for use in a given application. This document provides a profile of ISAKMP and PKIX that defines the requirements for using PKI technology in the context of IPsec. The document compliments protocol specifications such as IKE, which assume the existence of public key certificates and related keying materials, but which do not address PKI issues explicitly. This document addresses these issues. Korver Expires - December 2002 [Page 1] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Table of Contents 1. Introduction ..................................................3 2. Terms and Definitions .........................................4 3. Profile of ISAKMP .............................................5 3.1 Background .................................................5 3.1.1 Certificate-Related Payloads in ISAKMP ...............5 3.1.1.1 Identification Payload ...........................5 3.1.1.2 Certificate Payload ..............................5 3.1.1.3 Certificate Request Payload ......................5 3.1.1.4 Hash Payload .....................................5 3.1.2 Endpoint Identification ..............................5 3.1.2.1 Identification Payload Only ......................6 3.1.2.2 Certificate Payload Only .........................6 3.2 Identification Payload .....................................6 3.2.1 Securely Binding Identity to Policy ..................6 3.2.1.1 Using Peer IP Address to Bind Identity to Policy .7 3.2.2 Using a PKI ..........................................7 3.2.2.1 Securely Binding Identity to Policy ..............8 3.2.2.2 ID Type ..........................................8 3.2.2.3 Local Policy Regarding ID Types .................10 3.3 Certificate Request Payload ...............................11 3.3.1 Using a PKI .........................................11 3.3.1.1 Certificate Type ................................11 3.3.1.2 Certificate Request Payload Semantics ...........12 3.4 Certificate Payload .......................................15 3.4.1 Using a PKI .........................................16 3.4.1.1 Certificate Payloads Not Mandatory ..............16 3.4.1.2 Response to Multiple Certificate Authority Proposals ................................................16 3.4.1.3 Local Keying Materials ..........................16 3.4.1.4 Certificate Type ................................16 3.4.1.5 Optimizations ...................................18 3.4.1.6 Robustness ......................................18 3.4.2 Leaking Identity Information ........................19 4. Profile of PKIX ..............................................19 4.1 X.509 Certificates ........................................19 4.1.1 Versions ............................................19 4.1.2 Subject Name ........................................19 4.1.2.1 Empty Subject Name ..............................19 4.1.2.2 Specifying Hosts in Subject Name ................19 4.1.2.3 EmailAddress ....................................20 4.1.3 X.509 Certificate Extensions ........................20 4.2 X.509 Certificate Revocation Lists ........................24 4.2.1 Certificate Revocation Requirement ..................24 4.2.2 Multiple Sources of Certificate Revocation Information24 4.2.3 X.509 Certificate Revocation List Extensions ........25 5. Configuration Data Exchange Conventions ......................26 Korver Expires - December 2002 [Page 2] The IP Security PKI Profile of ISAKMP and PKIX June 2002 5.1 Certificates ..............................................26 5.2 Public Keys ...............................................26 6. IKE ..........................................................26 6.1 IKE Phase 1 Authenticated With Signatures .................26 6.1.1 Identification Payload ..............................27 6.1.1.1 When the Identification Payload is Mandatory ....27 6.1.1.2 Leaking Identity Information ....................27 6.1.2 Certificate Request Payload .........................27 6.1.3 Certificate Payload .................................27 6.1.3.1 Leaking Identity Information ....................27 6.1.4 X.509 Certificate Extensions ........................28 6.1.4.1 KeyUsage ........................................28 6.2 IKE Phase 1 Authenticated With Public Key Encryption ......28 6.2.1 Identification Payload ..............................28 6.2.1.1 Without Certificates ............................28 6.2.1.2 With Certificates ...............................28 6.2.1.3 When the Identification Payload is Mandatory ....28 6.2.2 Hash Payload ........................................28 6.2.3 X.509 Certificate Extensions ........................29 6.2.3.1 KeyUsage ........................................29 6.3 IKE Phase 1 Authenticated With a Revised Mode of Public Key Encryption ...............................................29 Intellectual Property Rights ....................................29 Security Considerations .........................................29 References ......................................................29 Acknowledgments .................................................31 Author's Addresses ..............................................31 Full Copyright Statement ........................................32 A. Change History ..............................................32 B. The Possible Dangers of Delta CRLs ..........................32 C. Mapping A Peer Address to a Peer Certificate ................33 1. Introduction IKE [IKE] and ISAKMP [ISAKMP] provide a secure key exchange mechanism for use with IPsec [IPSEC]. In many cases the peers authenticate using digital certificates as specified in PKIX [PKIX]. Unfortunately, the combination of these standards leads to an underspecified set of requirements for the use of certificates in the context of IPsec. PKIX provides a large set of certificate mechanisms which are generally applicable for Internet protocols, but no specific guidance for IPsec. This document profiles the PKIX framework for use with ISAKMP and IPsec. ISAKMP references PKIX but in many cases merely specifies the contents of various messages without specifying their semantics. In other cases, contents may be specified no more precisely than to require some form of X.509 certificate. This document profiles the Korver Expires - December 2002 [Page 3] The IP Security PKI Profile of ISAKMP and PKIX June 2002 contents of the relevant ISAKMP payloads and further specifies their semantics. Since the ISAKMP and PKIX frameworks present the implementor with numerous underspecified choices, interoperability is hampered if all implementors do not make similar choices, or at least fail to account for implementations which choose different options. This profile of ISAKMP and PKIX is intended to provide an agreed upon standard for using PKI technology in the context of IPsec. In addition to providing a profile of ISAKMP and PKIX, this document attempts to incorporate lessons learned from recent experience with both implementation and deployment, as well as the current state of related protocols and technologies. Material from ISAKMP and PKIX is not repeated here, and readers of this document are assumed to have read and understood both documents. The requirements and security aspects of those documents are fully relevant to this document as well. Version "00" of this document is intended as a "straw man" to encourage comments from implementors of IPsec and to encourage discussion of the issues which the authors hope to address this document. This document is being discussed on the ipsec@lists.tislabs.com mailing list, which is the mailing list for the IPsec Working Group. 2. Terms and Definitions Except for those terms which are defined immediately below, all PKI terms used in this document are defined in either the PKIX, ISAKMP, or DOI [DOI] documents. . Peer Address: The source address in packets from a peer. This address may be different from any addresses asserted as the "identity" of the peer. . FQDN: Fully qualified domain name. . Root CA: A CA that is directly trusted by an end entity. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119 [RFC2119]. Korver Expires - December 2002 [Page 4] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3. Profile of ISAKMP 3.1 Background 3.1.1 Certificate-Related Payloads in ISAKMP ISAKMP has three primary certificate-related payloads: Identification, Certificate, and Certificate Request. Additionally, IKE specifies the optional use of the Hash Payload to carry a pointer to a certificate in either of the Phase 1 public key encryption modes. In this section we provide a short introduction to these payload types. 3.1.1.1 Identification Payload The Identification (ID) Payload is used to indicate the identity that the agent claims to be speaking for. The receiving agent can then use the ID as a lookup key for policy and whatever certificate store or directory that it has available. Our primary concern in this document is to profile the ID payload so that it can be safely used to generate or lookup policy. 3.1.1.2 Certificate Payload The Certificate (CERT) Payload allows the peer to transmit a single certificate or CRL. Multiple certificates are transmitted in multiple payloads. However, not all certificate forms that are legal in PKIX make sense in the context of ISAKMP or IPsec. The issue of how to represent ISAKMP-meaningful name-forms in a certificate is especially problematic. This memo provides a profile for a subset of PKIX that makes sense for ISAKMP. 3.1.1.3 Certificate Request Payload The Certificate Request (CERTREQ) Payload allows an ISAKMP implementation to request that a peer provide some set of certificates. It is not clear from ISAKMP exactly how that set should be specified or how the peer should respond. We describe the semantics on both sides. 3.1.1.4 Hash Payload The Hash (HASH) Payload is a generic mechanism for ISAKMP implementations to communicate hash values to a peer. The meaning of the contents of such payloads is left undefined by ISAKMP. 3.1.2 Endpoint Identification Korver Expires - December 2002 [Page 5] The IP Security PKI Profile of ISAKMP and PKIX June 2002 ISAKMP contains two different payloads that allow the specification of endpoint identity, the ID payload and the CERT payload. According to ISAKMP, these payloads can be used separately or together, although specific profiles of ISAKMP may place additional requirements on implementations. 3.1.2.1 Identification Payload Only If one peer presents only the ID payload, it is expected that the peer will be able to recover whatever keying material is required to verify the peer's identity. How to do so is out of the scope of this document but might include a local cache, an LDAP directory, or DNS. 3.1.2.2 Certificate Payload Only If a peer presents only a CERT payload, this creates an ambiguity, since ISAKMP does not specify which of potentially many certificates corresponds to the end-entity and which are chaining certificates. Implementations SHOULD compare whatever local hints they have about peer identity to each certificate until they find one that appears acceptable. 3.2 Identification Payload According to ISAKMP: "The Identification Payload contains DOI-specific data used to exchange identification information. This information is used for determining the identities of communicating peers and may be used for determining authenticity of information." Note, the Identity Payload requirements in this document only cover the portion of the explicit policy checks that deal with the Identity Payload specifically. For instance, in the case where ID does not contain an IP address, checks such as verifying that the peer address is permitted by the relevant policy are not addressed here as they are out of the scope of this document. 3.2.1 Securely Binding Identity to Policy Because implementations sometimes use Identification Data as a lookup key to determine which policy to use, all implementations MUST be especially careful to verify the truthfulness of the Identification Data by verifying that the Identification Data corresponds to some keying material demonstrably held by the peer. Failure to do so may result in the use of an inappropriate or insecure policy. Korver Expires - December 2002 [Page 6] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3.2.1.1 Using Peer IP Address to Bind Identity to Policy Implementations MAY use the IP address found in the header of packets received from the peer to lookup the policy, but such implementations MUST still perform verification of Identification Data. 3.2.1.1.1 Peer IP Address Considerations Although packet IP addresses are inherently untrustworthy and must therefore be independently verified, it is often useful to use the apparent IP address of the peer to locate a general class of policies that will be used until the mandatory identity-based policy lookup can be performed. For instance, if the IP address of the peer is unrecognized, a VPN gateway device might load a general "road warrior" policy that specifies a specific CA that is trusted to issue certificates which contain a valid rfc822Name which will be used by that implementation to perform authorization based on access control lists (ACLs). The rfc822Name can then be used to determine the policy that provides specific authorization to access resources (such as IP addresses, ports, and so forth) that the peer has been granted access to. As another example, if the IP address of the peer is recognized to be a known peer VPN endpoint, the general policy and the identity- based policy may be identical, but until the identity is validated, the policy MUST not be used to authorize any traffic. Implementations may want to consider their deployment environments when considering local Identity Payload policy, although such considerations are environment-specific and thus out of scope of this document. In general, however, it is easier to spoof the contents of an ID payload than it is to spoof a peer address. Implementations MUST validate the Identity Data provided by a peer, but implementations SHOULD favor unauthenticated peer addresses over unauthenticated Identity Data for policy lookup, unless an implementation has been configured to do otherwise. 3.2.2 Using a PKI For the purposes of this section, it is assumed that there exists a peer certificate chain that has been validated and which is trusted in the context of the policy being discussed. Korver Expires - December 2002 [Page 7] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3.2.2.1 Securely Binding Identity to Policy 3.2.2.1.1 Single Address Identification Data In the case where Identification Data contains an address (not an address range or subnet), implementations MUST verify that this address is the same as the peer address. If the end entity certificate contains addresses identity, then the peer address must match one of those identities. If either of the above do not match, this MUST be treated as an error and security association setup MUST be aborted. This event SHOULD be auditable. The definition of "match" is specific to each ID type and is discussed below. In addition, implementations MUST allow administrators to configure a local policy that requires that the peer address exist in the certificate. Implementations SHOULD allow administrators to configure a local policy that not enforce this requirement. 3.2.2.1.2 Identification Data Not Containing a Single Address In the case where Identification Data does not contain a single address, Implementations MUST verify that the Identification Data contained in the Identification Payload matches identity information contained in the peer end entity certificate, either in the Subject Name field or subjectAltName extension. If there is not a match, this MUST be treated as an error and security association setup MUST be aborted. This event SHOULD be auditable. The definition of "match" is specific to each ID type and is discussed below. 3.2.2.2 ID Type The DOI defines the 11 types of Identification Data that can be used and specifies the syntax for these types. All of these except for the ID_KEY_ID type, which is not relevant to this document, are discussed below. 3.2.2.2.1 ID_IPV4_ADDR and ID_IPV6_ADDR Implementations MUST support either the ID_IPV4_ADDR or ID_IPV6_ADDR ID type. These addresses MUST be stored in "network byte order," as specified in RFC-791 [RFC791]. The least significant bit (LSB) of each octet is the LSB of the corresponding byte in the network address. For the ID_IPV4_ADDR type, Identity Data MUST contain exactly four octets. For the ID_IPV6_ADDR type, Identity Data MUST contain exactly sixteen octets [RFC1883]. When comparing the Korver Expires - December 2002 [Page 8] The IP Security PKI Profile of ISAKMP and PKIX June 2002 contents of Identification Data with the iPAddress field in the subjectAltName extension for equality, binary comparison is performed. 3.2.2.2.2 ID_FQDN Implementations MAY support the ID_FQDN ID type, generally to support host-based access control lists for hosts without fixed IP addresses. However, implementations SHOULD NOT use the DNS to map the FQDN to IP addresses for input into any policy decisions, unless that mapping is known to be secure, such as when DNSSEC [DNSSEC] is employed. When comparing the contents of Identification Data with the dNSName field in the subjectAltName extension for equality, caseless string comparison is performed. Substring, wildcard, or regular expression matching MUST NOT be performed. 3.2.2.2.3 ID_USER_FQDN Implementations MAY support the ID_USER_FQDN ID type, generally to support user-based access control lists for users without fixed IP addresses. However, implementations SHOULD NOT use the DNS to map the FQDN portion to IP addresses for input into any policy decisions, unless that mapping is known to be secure, such as when DNSSEC is employed. When comparing the contents of Identification Data with the rfc822Name field in the subjectAltName extension for equality, caseless string comparison is performed. Substring, wildcard, or regular expression matching MUST NOT be performed. 3.2.2.2.4 ID_IPV4_ADDR_SUBNET, ID_IPV6_ADDR_SUBNET, ID_IPV4_ADDR_RANGE, ID_IPV6_ADDR_RANGE Implementations MUST NOT use these ID types unless they appear identically in the end entity certificate. As there is currently no standard method for putting address subnet or range identity information into certificates, implementations SHOULD NOT use these ID types. Use of these ID types is currently undefined. 3.2.2.2.5 ID_DER_ASN1_DN Implementations MAY support the ID_DER_ASN1_DN ID type, although implementations SHOULD NOT generate this type. Implementations which generate this ID type SHOULD populate the contents of Identity Data with the Subject Name from the end entity certificate, and MUST do so such that a binary comparison of the two will succeed. For instance, if the certificate was erroneously created such that the encoding of the Subject Name DN varies from the constraints set by DER, that non-conformant DN is the one that MUST be used to populate ID Data. In other words, implementations MUST NOT re-encode the DN Korver Expires - December 2002 [Page 9] The IP Security PKI Profile of ISAKMP and PKIX June 2002 for the purposes of making it DER if it does not appear in the certificate as DER. Implementations MUST NOT populate Identity Data with the Subject Name from the end entity certificate if it is empty, as described in the "Subject" section of PKIX. 3.2.2.2.6 ID_DER_ASN1_GN Implementations MAY support the ID_DER_ASN1_GN ID type, although implementations SHOULD NOT generate this type unless it is known through out-of-band means that the peer is capable of making use of this ID type. Implementations which generate this ID type MUST populate the contents of Identity Data with the a GeneralName from the SubjectAltName extension in the end entity certificate, and MUST do so such that a binary comparison of the two will succeed. For instance, if the certificate was erroneously created such that the encoding of the GeneralName varies from the constraints set by DER, that non-conformant GeneralName is the one that MUST be used to populate ID Data. In other words, implementations MUST NOT re- encode the GeneralName for the purposes of making it DER if it does not appear in the certificate as DER. 3.2.2.2.7 ID_KEY_ID Type ID_KEY_ID type is not relevant to this document. 3.2.2.3 Local Policy Regarding ID Types 3.2.2.3.1 Transitively Binding Identity to Policy In the presence of certificates that contain multiple identities, implementations SHOULD NOT assume that a peer will choose the most appropriate identity with which to populate ID. Therefore, implementations SHOULD iterate over the identities contained in the certificate to locate the appropriate policy. For example, imagine that a host is configured with a certificate that contains both an IP address (for when the host is on the corporate LAN) and a "username" (either ID_FQDN or ID_USER_FQDN) (for when the host is connecting via dialup). Assume also that the peer will enforce different policies depending on whether the host is local or remote. Independent of which identity is used to populate ID, the peer implementation MUST locate the proper policy. For instance, if the lookup key for the policy in the remote case is an FQDN but ID contained the IP address identity in the certificate, the peer will likely use the IP address in ID to find the certificate, and once the certificate has been validated the FQDN in the certificate will be used to locate the appropriate policy. Korver Expires - December 2002 [Page 10] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Determining which policy is most appropriate in a given context is a local policy matter and is therefore out of the scope of this document. 3.2.2.3.2 Opportunistic IPsec TODO 3.3 Certificate Request Payload According to ISAKMP: "The Certificate Request (CERTREQ) Payload provides a means to request certificates via ISAKMP and can appear in any message. Certificate Request payloads SHOULD be included in an exchange whenever an appropriate directory service (e.g. Secure DNS [DNSSEC]) is not available to distribute certificates." 3.3.1 Using a PKI For the purposes of this section, it is assumed that there exists a peer certificate chain that has been validated and which is trusted in the context of the policy being discussed. 3.3.1.1 Certificate Type The Certificate Type field identifies to the peer the type of certificate keying materials that are desired. ISAKMP defines 10 types of Certificate Data that can be requested and specifies the syntax for these types. For the purposes of this document, only 4 types are relevant: . X.509 Certificate - Signature . X.509 Certificate - Key Exchange . Certificate Revocation List (CRL) . PKCS #7 wrapped X.509 certificate For example, if CRLs are desired, an implementation will populate the Certificate Type field with the value associated with "Certificate Revocation List (CRL)". The use of the other types: . PGP Certificate . DNS Signed Key . Kerberos Tokens . Authority Revocation List (ARL) . SPKI Certificate Korver Expires - December 2002 [Page 11] The IP Security PKI Profile of ISAKMP and PKIX June 2002 . X.509 Certificate - Attribute are out of the scope of this document. 3.3.1.1.1 X.509 Certificate - Signature This type requests that the end entity certificate be a signing certificate. Implementations that receive CERTREQs which contain this ID type in a context in which end entity signature certificates are not used SHOULD ignore such CERTREQs. 3.3.1.1.2 X.509 Certificate - Key Exchange This type requests that the end entity certificate be a key exchange certificate. Implementations that receive CERTREQs which contain this ID type in a context in which end entity key exchange certificates are not used SHOULD ignore such CERTREQs. 3.3.1.1.3 Certificate Revocation List (CRL) This ID type requests that X.509 CRLs be provided, along with any certificates that may be needed to validate those CRLs. 3.3.1.1.4 PKCS #7 wrapped X.509 certificate This ID type defines a particular encoding (not a particular certificate or CRL type), some current implementations may ignore CERTREQs they receive which contain this ID type, and the authors are unaware of any implementations that generate such CERTREQ messages. Therefore, the use of this type is deprecated. Implementations SHOULD NOT generate CERTREQs that contain this Certificate Type. Implementations which receive CERTREQs which contain this ID type MAY ignore such payloads. 3.3.1.2 Certificate Request Payload Semantics 3.3.1.2.1 Presence or Absence of Certificate Request Payloads When in-band exchange of certificate keying materials is desired, implementations MUST inform the peer of this by sending at least one CERTREQ. An implementation which does not send any CERTREQs during an exchange SHOULD NOT expect to receive any CERT payloads. 3.3.1.2.2 Empty Certificate Authority Field If no certificate authority is specified in the Certificate Korver Expires - December 2002 [Page 12] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Authority field, ISAKMP states that "no specific certificate authority [is] requested". The following two sections describe how to behave when the Certificate Authority field is empty. 3.3.1.2.2.1 Certificate Requests Implementations MUST NOT generate CERTREQs where the Certificate Type is either "X.509 Certificate - Signature" or "X.509 Certificate - Key Exchange" with an empty Certificate Authority field. Upon receipt of such a CERTREQ, implementations SHOULD send just the certificate chain associated with the end entity certificate, not including any CRLs or the certificates that would be needed to validate those CRLs. Note, in the case where multiple end entity certificates may be available, implementations SHOULD resort to local heuristics to determine which end entity is most appropriate to use. Such heuristics are out of the scope of this document. 3.3.1.2.2.2 CRL Requests Implementations MAY generate CERTREQs where the Certificate Type is "Certificate Revocation List (CRL)" with an empty Certificate Authority field to signify that the peer should send all CRLs that are possessed by that peer, whether relevant to the current exchange or not. Upon receipt of such a CERTREQ, implementations SHOULD send all CRLs that are possessed but MUST send all CRLs that are relevant to the current exchange, including the certificates that are needed to validate those CRLs. 3.3.1.2.3 Specifying Certificate Authorities Implementations MUST generate CERTREQs for every peer root that local policy explicitly deems trusted during a given exchange. Implementations MUST populate the Certificate Authority field with the Subject Name of the trusted root. 3.3.1.2.3.1 Certificate Requests Upon receipt of a CERTREQ where the Certificate Type is either "X.509 Certificate - Signature" or "X.509 Certificate - Key Exchange", implementations MUST respond by sending each certificate in the chain from the end entity certificate to the certificate whose Issuer Name matches the name specified in the Certificate Authority field. Implementations MAY send other certificates from the chain. Korver Expires - December 2002 [Page 13] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3.3.1.2.3.2 CRL Requests Upon receipt of a CERTREQ where the Certificate Type is "Certificate Revocation List (CRL)", implementations MUST respond by sending the CRL issued by the issuer of each certificate in the chain between the end entity certificate and the certificate whose Issuer Name matches the name specified in the Certificate Authority field. In additional, implementations MUST send any certificates that are needed to validate those CRLs, 3.3.1.2.4 Optimizations 3.3.1.2.4.1 Duplicate Certificate Request Payloads Implementations SHOULD NOT send duplicate CERTREQs during an exchange in order to reduce bandwidth usage. 3.3.1.2.4.2 Name Lowest "Common" Certificate Authorities When a peer's certificate keying materials have been cached, an implementation can send a hint to the peer to elide some (or all) of the certificates and CRLs the peer would normally respond with. In addition to the normal set of CERTREQs that are sent, an implementation MAY send CERTREQs containing the Issuer Name of the relevant cached end entity certificates. When sending these hints, it is still necessary to send the normal set of CERTREQs because the hints do not sufficiently convey all of the information required by the peer. Specifically, an implementation may not know about all of the peer chains and therefore will fail to send CERTREQ for all possible peer chains. No special processing is required on the part of the recipient of such a CERTREQ, and the end entity certificates will still be sent. On the other hand, the recipient MAY elect to elide certificates based on receipt of such hints. ISAKMP mandates that CERTREQs contain the Subject Name of a Certification Authority, which results in the peer always sending at least the end entity certificate. This mechanism allows implementations to determine unambiguously when a new certificate is being used by the peer, perhaps because the previous certificate was about to expire, which will result in a failure because the needed keying materials are not available to validate the new end entity certificate. Implementations which implement this optimization MUST recognize when the end entity certificate has changed and respond to it by not performing this optimization when the exchange is retried. Korver Expires - December 2002 [Page 14] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3.3.1.2.4.2.1 Example Imagine that an implementation has previously received and cached the peer certificate chain R->CA1->CA2->EE. If during a subsequent exchange this implementation sends a CERTREQ containing the Subject Name in certificate R, this implementation is requesting that the peer send at least 3 certificates: CA1, CA2, and EE. On the other hand, if this implementation also sends a CERTREQ containing the Subject Name of CA2, the implementation is providing a hint that only 1 certificate need be sent: EE. 3.3.1.2.5 Robustness 3.3.1.2.5.1 Unrecognized or Unsupported Certificate Types Implementations MUST be able to deal with receiving CERTREQs with unrecognized or unsupported Certificate Types. Absent any recognized and supported CERTREQs, implementations MAY treat them as if they are of a supported type with the Certificate Authority field left empty, depending on local policy. ISAKMP section 5.10 Certificate Request Payload Processing" specifies additional processing. 3.3.1.2.5.2 Undecodable Certificate Authority Fields Implementations MUST be able to deal with receiving CERTREQs with undecodable Certificate Authority fields. Implementations MAY treat such fields as if there were empty, depending on local policy. ISAKMP specifies other actions which may be taken. 3.3.1.2.5.3 Ordering of Certificate Request Payloads Implementations MUST NOT assume that CERTREQs are ordered in any way. 3.3.1.2.6 Leaking Identity Information Depending on the exchange type, CERTREQs may be passed in the clear. Administrators in some environments may wish to use the empty Certification Authority option to prevent such information from leaking (at the cost of performance). 3.4 Certificate Payload According to ISAKMP: "The Certificate (CERT) Payload provides a means to transport certificates or other certificate-related information via ISAKMP and can appear in any ISAKMP message. Certificate payloads SHOULD be Korver Expires - December 2002 [Page 15] The IP Security PKI Profile of ISAKMP and PKIX June 2002 included in an exchange whenever an appropriate directory service (e.g. Secure DNS [DNSSEC]) is not available to distribute certificates. The Certificate payload MUST be accepted at any point during an exchange." 3.4.1 Using a PKI For the purposes of this section, it is assumed that there exists a peer certificate chain that has been validated and which is trusted in the context of the policy being discussed. 3.4.1.1 Certificate Payloads Not Mandatory An implementation which does not receive any CERTREQs during an exchange SHOULD NOT send any CERT payloads, except when explicitly configured to proactively send CERT payloads in order to interoperate with non-compliant implementations. In this case, an implementation MUST send the all certificate chains and CRLs associated with the end entity certificate. This MUST NOT be the default behavior of implementations. Implementations which are configured to expect that a peer must receive certificates through out-of-band means SHOULD ignore any CERTREQ messages that are received. Implementations that receive CERTREQs from a peer which contain only unrecognized Certification Authorities SHOULD NOT continue the exchange, in order to avoid unnecessary and potentially expensive cryptographic processing. 3.4.1.2 Response to Multiple Certificate Authority Proposals In response to multiple CERTREQs which contain different Certificate Authority identities, implementations MAY respond using an end entity certificate which chains to any of the identities provided by the peer. 3.4.1.3 Local Keying Materials Implementations MAY elect not to use keying materials contained in a given set of CERTs if preferable keying materials are available. For instance, the contents of a CERT may be available from a previous exchange, or a newer CRL may be available through some out- of-band means. 3.4.1.4 Certificate Type The Certificate Type field identifies to the peer the type of Korver Expires - December 2002 [Page 16] The IP Security PKI Profile of ISAKMP and PKIX June 2002 certificate keying materials that are included. ISAKMP defines 10 types of Certificate Data that can be sent and specifies the syntax for these types. For the purposes of this document, only 5 types are relevant: . X.509 Certificate - Signature . X.509 Certificate - Key Exchange . Certificate Revocation List (CRL) . PKCS #7 wrapped X.509 certificate . Authority Revocation List (ARL) [TBD] For example, if CRLs are desired, an implementation will populate the Certificate Type field with the value associated with "Certificate Revocation List (CRL)", or possibly ARL [TBD]. The use of the other types: . PGP Certificate . DNS Signed Key . Kerberos Tokens . SPKI Certificate . X.509 Certificate - Attribute are out of the scope of this document. 3.4.1.4.1 X.509 Certificate - Signature This type specifies that Certificate Data contains a certificate used for signing, whether an end entity signature certificate or a CA certificate signing certificate. 3.4.1.4.2 X.509 Certificate - Key Exchange This type specifies that Certificate Data contains an end entity certificate used for either key exchange (or key encipherment). 3.4.1.4.3 Certificate Revocation List (CRL) This type specifies that Certificate Data contains an X.509 CRL. 3.4.1.4.4 PKCS #7 wrapped X.509 certificate This type defines a particular encoding, not a particular certificate or CRL type. Implementations MUST NOT generate CERTs that contain this Certificate Type. Implementations which violate this requirement SHOULD note that this is a single certificate ISAKMP. Implementations MAY accept CERTs that contain this Certificate type. Korver Expires - December 2002 [Page 17] The IP Security PKI Profile of ISAKMP and PKIX June 2002 3.4.1.5 Optimizations 3.4.1.5.1 Duplicate Certificate Payloads Implementations SHOULD NOT send duplicate CERTs during an exchange in order to reduce bandwidth usage. Such payloads should be suppressed. 3.4.1.5.2 Send Lowest "Common" Certificates When multiple CERTREQs are received which specify certificate authorities within the end entity certificate chain, implementations MAY send the shortest chain possible. However, implementations SHOULD always send the end entity certificate. See section 3.3.1.2.4.2 for more discussion of this optimization. 3.4.1.5.3 Drop Duplicate Certificate Payloads Implementations MAY employ means to recognize CERTs that have been received in the past, whether part of the current exchange or not, for which keying material is available and may discard these duplicate CERTs. 3.4.1.6 Robustness 3.4.1.6.1 Unrecognized or Unsupported Certificate Types Implementations MUST be able to deal with receiving CERTs with unrecognized or unsupported Certificate Types. Implementations MAY discard such payloads, depending on local policy. ISAKMP section 5.10 "Certificate Request Payload Processing" specifies additional processing. 3.4.1.6.2 Undecodable Certificate Data Fields Implementations MUST be able to deal with receiving CERTs with undecodable Certificate Data fields. Implementations MAY discard such payloads, depending on local policy. ISAKMP specifies other actions which may be taken. 3.4.1.6.3 Ordering of Certificate Payloads Implementations MUST NOT assume that CERTs are ordered in any way. 3.4.1.6.4 Duplicate Certificate Payloads Korver Expires - December 2002 [Page 18] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Implementations MUST support receiving multiple identical CERTs during an exchange. 3.4.1.6.5 Irrelevant Certificates Implementations MUST be prepared to receive certificates and CRLs which are not relevant to the current exchange. Implementations MAY discard such keying materials. Implementations MAY include certificates which are irrelevant to an exchange. One reason for including certificates which are irrelevant to an exchange is to minimize the threat of leaking identifying information in exchanges where CERT is not encrypted. It should be noted, however, that this probably provides rather poor protection against leaking the identity. 3.4.2 Leaking Identity Information Depending on the exchange type, CERTs may be passed in the clear and therefore may leak identity information. 4. Profile of PKIX 4.1 X.509 Certificates 4.1.1 Versions Although PKIX states that "implementations SHOULD be prepared to accept any version certificate", in practice this profile requires certain extensions that necessitate the use of Version 3 certificates. Implementations that conform to this document MAY therefore reject Version 1 and Version 2 certificates. 4.1.2 Subject Name 4.1.2.1 Empty Subject Name Implementations MUST accept certificates which contain an empty Subject Name field, as specified in PKIX. Identity information in such certificates will be contained entirely in the SubjectAltName extension. 4.1.2.2 Specifying Hosts in Subject Name 4.1.2.2.1 Non-FQDN Host Names Korver Expires - December 2002 [Page 19] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Implementations which desire to place host names that are not FQDNs (for instance "Gateway Router") in the Subject Name MUST use the commonName attribute. 4.1.2.2.2 FQDN Host Names Implementations which desire to place host names that are FQDNs (for instance "gateway.xythos.com") in the Subject Name field SHOULD use the commonName attribute type. Implementations SHOULD NOT populate the Subject Name in place of populating the dNSName field of the SubjectAltName extension. Host names that appear in the Subject Name cannot be unambiguously determined to be a host name. Note, PKIX defines a domainComponent attribute for representing FQDNs in DistinguishedNames such as Subject Name. As an alternative to using commonName, implementations MAY use the domainComponent attribute type. However, note that support for the domainComponent attribute is far from universal and many implementations will reject certificates that contain this attribute. 4.1.2.3 EmailAddress As specified in PKIX, implementations MUST NOT populate DistinguishedNames with the EmailAddress attribute. 4.1.3 X.509 Certificate Extensions Conforming applications MUST recognize extensions which must or may be marked critical according to this specification. These extensions are: KeyUsage, SubjectAltName, and BasicConstraints. Implementations SHOULD generate certificates such that the extension criticality bits are set in accordance with PKIX and this document. With respect to PKIX compliance, implementations processing certificates MAY ignore the criticality bit for extensions that are supported by that implementation, but MUST support the criticality bit for extensions that are not supported by that implementation. 4.1.3.1 AuthorityKeyIdentifier Implementations SHOULD NOT assume that other implementations support the AuthorityKeyIdentifier extension, and thus should not generate certificate hierarchies which are overly complex to process in the absence of this extension (such as those that require possibly verifying a signature against a large number of similarly named CA certificates in order to find the CA certificate which contains the Korver Expires - December 2002 [Page 20] The IP Security PKI Profile of ISAKMP and PKIX June 2002 key that was used to generate the signature). 4.1.3.2 SubjectKeyIdentifier Implementations SHOULD NOT assume that other implementations support the SubjectKeyIdentifier extension, and thus should not generate certificate hierarchies which are overly complex to process in the absence of this extension (such as those that require possibly verifying a signature against a large number of similarly named CA certificates in order to find the CA certificate which contains the key that was used to generate the signature). 4.1.3.3 KeyUsage The meaning of the nonRepudiation bit is undefined in the context of IPsec. Implementations SHOULD ignore this bit if present. See PKIX for general guidance on which of the other KeyUsage bits should be set in any given certificate. 4.1.3.4 PrivateKeyUsagePeriod PKIX recommends against the use of this extension. The PrivateKeyUsageExtension is intended to be used when signatures will need to be verified long past the time when signatures using the private keypair may be generated. Since ISAKMP SAs are short-lived relative to the intended use of this extension in addition to the fact that each signature is validated only a single time, the meaning of this extension in the context of ISAKMP is unclear. Therefore, the PrivateKeyUsagePeriod is inappropriate in the context of ISAKMP and therefore implementations MUST NOT generate certificates that contain the PrivateKeyUsagePeriod extension. 4.1.3.5 Certificate Policies Many IPsec implementations do not currently provide support for the Certificate Policies extension. Therefore, implementations that generate certificates which contain this extension SHOULD mark the extension as non-critical. 4.1.3.6 PolicyMappings Many implementations do not support the PolicyMappings extension. 4.1.3.7 SubjectAltName 4.1.3.7.1 Permitted Choices Korver Expires - December 2002 [Page 21] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Implementations SHOULD generate only the following GeneralName choices in the subjectAltName extension, as these choices map to legal ISAKMP Identity Payload types ISAKMP: rfc822Name, dNSName, or iPAddress. Although it is possible to specify any GeneralName choice in the ISAKMP Identity Payload by using the ID_DER_ASN1_GN ID type, implementations SHOULD NOT assume that a peer supports such functionality. 4.1.3.7.1.1 dNSName This field MUST contain a fully qualified domain name. Implementations MUST NOT generate names that contain wildcards. Implementations MAY treat certificates that contain wildcards in this field as syntactically invalid. Although this field is in the form of an FQDN, implementations SHOULD NOT assume that the this field contains an FQDN that will resolve via the DNS, unless this is known by way of some out-of-band mechanism. Such a mechanism is out of the scope of this document. Implementations SHOULD NOT treat the failure to resolve as an error. 4.1.3.7.1.2 iPAddress Note that the CIDR [CIDR] notation permitted in the "Name Constraints" section of PKIX is explicitly not permitted by that specification for conveying identity information. In other words, the CIDR notation MUST NOT be used in the subjectAltName extension. 4.1.3.7.1.3 rfc822Name Although this field is in the form of an Internet mail address, implementations SHOULD NOT assume that the this field contains a valid email address, unless this is known by way of some out-of-band mechanism. Such a mechanism is out of the scope of this document. 4.1.3.8 IssuerAltName Implementations SHOULD NOT assume that other implementations support the IssuerAltName extension, and especially should not assume that information contained in this extension will be displayed to end users. 4.1.3.9 SubjectDirectoryAttributes The SubjectDirectoryAttributes extension is intended to contain privilege information, in a manner analogous to privileges carried in Attribute Certificates. Implementations MAY ignore this extension as PKIX mandates it be marked non-critical. Korver Expires - December 2002 [Page 22] The IP Security PKI Profile of ISAKMP and PKIX June 2002 4.1.3.10 BasicConstraints PKIX mandates that CA certificates contain this extension and that it be marked critical. For backwards compatibility, implementations SHOULD accept CA certificates that do not contain this extension or that contain this extension marked non-critical. 4.1.3.11 NameConstraints Many implementations do not support the NameConstraints extension. Since PKIX mandates that this extension be marked critical when present, implementations which intend to be maximally interoperable SHOULD NOT generate certificates which contain this extension. 4.1.3.12 PolicyConstraints Many implementations do not support the PolicyConstraints extension. Since PKIX mandates that this extension be marked critical when present, implementations which intend to be maximally interoperable SHOULD NOT generate certificates which contain this extension. 4.1.3.13 ExtendedKeyUsage No ExtendedKeyUsage usages are defined for IPsec, so if this extension is present and marked critical, use of this certificate for IPsec MUST be treated as an error. Implementations MUST NOT generate this extension in certificates which are being used for IPsec. 4.1.3.14 CRLDistributionPoint Most implementations expect to exchange CRLs in band via the ISAKMP Certificate Payload. Implementations MUST NOT assume that the CRLDistributionPoint extension will exist in peer extensions and therefore implementations SHOULD request that peers send CRLs in the absence of knowledge that this extension exists in the peer certificates. 4.1.3.15 InhibitAnyPolicy Many implementations do not support the InhibitAnyPolicy extension. Since PKIX mandates that this extension be marked critical when present, implementations which intend to be maximally interoperable SHOULD NOT generate certificates which contain this extension. 4.1.3.16 FreshestCRL Korver Expires - December 2002 [Page 23] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Most implementations expect to exchange CRLs in band via the ISAKMP Certificate Payload. Implementations MUST NOT assume that the FreshestCRL extension will exist in peer extensions and therefore implementations SHOULD request that peers send CRLs in the absence knowledge that this extension exists in the peer certificates. 4.1.3.17 AuthorityInfoAccess PKIX defines the AuthorityInfoAccess extension, which is used to "how to access CA information and services for the issuer of the certificate in which the extension appears." This extension has no known use in the context of IPsec. Conformant implementations SHOULD ignore this extension when present. 4.1.3.18 SubjectInfoAccess PKIX defines the SubjectInfoAccess private certificate extension, which is used to indicate "how to access information and services for the subject of the certificate in which the extension appears." This extension has no known use in the context of IPsec. Conformant implementations SHOULD ignore this extension when present. 4.2 X.509 Certificate Revocation Lists Implementations SHOULD send CRLs, unless non-CRL certificate revocation information is known to be preferred by all interested parties in the application environment that the implementation is used. Implementations MUST send CRLs if non-CRL certificate revocation information may not be available to all interested parties. 4.2.1 Certificate Revocation Requirement Implementations which validate certificates MUST make use of certificate revocation information, and SHOULD support such revocation information in the form of CRLs, unless non-CRL revocation information is known to be the only method for transmitting this information. 4.2.2 Multiple Sources of Certificate Revocation Information Implementations which support multiple sources of obtaining certificate revocation information MUST act conservatively when the information provided by these sources is inconsistent: when a certificate is reported as revoked by one source, the certificate MUST be considered revoked. Korver Expires - December 2002 [Page 24] The IP Security PKI Profile of ISAKMP and PKIX June 2002 4.2.3 X.509 Certificate Revocation List Extensions 4.2.3.1 AuthorityKeyIdentifier Implementations SHOULD NOT assume that other implementations support the AuthorityKeyIdentifier extension, and thus should not generate certificate hierarchies which are overly complex to process in the absence of this extension (such as those that require possibly verifying a signature against a large number of similarly named CA certificates in order to find the CA certificate which contains the key that was used to generate the signature). 4.2.3.2 IssuerAltName Implementations SHOULD NOT assume that other implementations support the IssuerAltName extension, and especially should not assume that information contained in this extension will be displayed to end users. 4.2.3.3 CRLNumber As stated in PKIX, all issuers conforming to PKIX MUST include this extension in all CRLs. 4.2.3.4 DeltaCRLIndicator 4.2.3.4.1 If Delta CRLs Are Unsupported Implementations that do not support delta CRLs MUST reject CRLs which contain the DeltaCRLIndicator (which MUST be marked critical according to PKIX) and MUST make use of a base CRL if it is available. Such implementations MUST ensure that a delta CRL does not "overwrite" a base CRL, for instance in the keying material database. 4.2.3.4.2 Delta CRL Recommendations Since some implementations that do not support delta CRLs may behave incorrectly or insecurely when presented with delta CRLs, implementations SHOULD consider whether issuing delta CRLs increases security before issuing such CRLs. The authors are aware of several implementations which behave in an incorrect or insecure manner when presented with delta CRLs. See Appendix B for a description of the issue. Therefore, this specification RECOMMENDS against issuing delta CRLs at this time. On the other hand, failure to issue delta CRLs exposes a larger window of vulnerability. See the Security Considerations section of PKIX for additional discussion. Implementors as well as administrators are encouraged to consider these issues. Korver Expires - December 2002 [Page 25] The IP Security PKI Profile of ISAKMP and PKIX June 2002 4.2.3.5 IssuingDistributionPoint TODO 4.2.3.6 FreshestCRL Given the recommendations against implementations generating delta CRLs, this specification RECOMMENDS that implementations do not populate CRLs with the FreshestCRL extension, which is used to obtain delta CRLs. 5. Configuration Data Exchange Conventions Below we present a common format for exchanging configuration data. Implementations MUST support these formats, MUST support arbitrary whitespace at the start and end of any line, and MUST support all three line-termination disciplines: LF (US-ASCII 10), CR (US-ASCII 13), and CRLF. 5.1 Certificates Certificates MUST be Base64 encoded and appear between the following delimiters: -----BEGIN CERTIFICATE----- -----END CERTIFICATE----- 5.2 Public Keys Implementations MUST support two forms of public keys: certificates and so-called "raw" keys. Certificates should be transferred in the exact same form as above. A raw key is only the SubjectPublicKeyInfo portion of the certificate, and should be Base64 encoded and appear between the following delimiters: -----BEGIN PUBLIC KEY----- -----END PUBLIC KEY----- 6. IKE 6.1 IKE Phase 1 Authenticated With Signatures 6.1.1 Identification Payload Implementations SHOULD populate Identification Data with identity Korver Expires - December 2002 [Page 26] The IP Security PKI Profile of ISAKMP and PKIX June 2002 information that is contained within the end entity certificate. This enables recipients to use the Identification Data as a lookup key to find the peer end entity certificate. The only case where implementations MAY populate the Identification Data with information that is not contained in the end entity certificate is when the Identification Data contains the peer address (a single address, not a subnet or range). This means that implementations MUST be able to map a peer address to a peer end entity certificate. The exact method for performing this mapping is out of the scope of this document. See Appendix C for an example of how this mapping might be implemented. 6.1.1.1 When the Identification Payload is Mandatory IKE mandates the use of the ID payload in Phase 1. 6.1.1.2 Leaking Identity Information 6.1.1.2.1 IKE Aggressive Mode The contents of ID are not encrypted in Aggressive Mode when authentication is performed with signatures. In some environments this may leak private information. The solutions to this problem if such a leak is unacceptable are: (1) Use Main Mode instead of Aggressive Mode. (2) Populate ID Data with the address of the host. 6.1.2 Certificate Request Payload The Contents of CERTREQ are not encrypted in IKE. In some environments this may leak private information. Administrators in some environments may wish to use the empty Certification Authority option to prevent such information from leaking, at the cost of performance. 6.1.3 Certificate Payload 6.1.3.1 Leaking Identity Information 6.1.3.1.1 IKE Main Mode Implementations may not wish to respond with CERTs in the second message, thereby violating the identity protection feature of Main Mode IKE. ISAKMP allows CERTs to be included in any message, and Korver Expires - December 2002 [Page 27] The IP Security PKI Profile of ISAKMP and PKIX June 2002 therefore implementations may wish to respond with CERTs in a message that offers privacy protection in this case. 6.1.3.1.2 IKE Aggressive Mode The contents of CERT are not encrypted in Aggressive Mode when authentication is performed with signatures. In some environments this may leak private information. The solution to this problem if such a leak is unacceptable is to use Main Mode instead of Aggressive Mode. 6.1.4 X.509 Certificate Extensions 6.1.4.1 KeyUsage If the KeyUsage extension is present in an end entity certificate, the digitalSignature bit must be asserted. 6.2 IKE Phase 1 Authenticated With Public Key Encryption 6.2.1 Identification Payload 6.2.1.1 Without Certificates If certificates are not being used, the contents of Identification Data in this case are out of scope for this document. 6.2.1.2 With Certificates No additional requirements exist. 6.2.1.3 When the Identification Payload is Mandatory IKE mandates the use of the ID payload in Phase 1. 6.2.2 Hash Payload IKE specifies the optional use of the Hash Payload to carry a pointer to a certificate in either of the Phase 1 public key encryption modes. This pointer is used by an implementation to locate the end entity certificate that contains the public key that a peer will use for encrypting payloads during the exchange. Implementations SHOULD include this payload whenever the public portion of the keypair has been placed in a certificate. Korver Expires - December 2002 [Page 28] The IP Security PKI Profile of ISAKMP and PKIX June 2002 6.2.3 X.509 Certificate Extensions 6.2.3.1 KeyUsage If the KeyUsage extension is present in an end entity certificate, the keyEncipherment bit must be asserted. 6.3 IKE Phase 1 Authenticated With a Revised Mode of Public Key Encryption IKE Phase 1 Authenticated With a Revised Mode of Public Key Encryption has the same requirements as IKE Phase 1 Authenticated With Public Key Encryption. See section 6.2 for these requirements. Intellectual Property Rights No new intellectual property rights are introduced by this document. Security Considerations Using Peer IP Address to Bind Identity to Policy In some environments, the peer address is more resistant to spoofing than will be the identity contents of certificates, and thus may be more trustworthy than the contents of certificates. In other environments, the opposite may be true. Implementations may want to consider their deployment environments when considering local Identity Payload policy, although such considerations are environment-specific and thus out of scope of this document. See section 3.2.1.1.1 for more background. Cleartext ID, CERT, or CERTREQ Payloads Depending on the exchange, the contents of ID, CERT, or CERTREQ may be passed in the clear. This document provides suggestions for avoiding such leaks. References [CIDR] Fuller, V., et al., "Classless Inter-Domain Routing (CIDR): An Address Assignment and Aggregation Strategy", RFC 1519, September 1993. [DNSSEC] Eastlake, D., "Domain Name System Protocol Security Extensions", RFC 2535, March 1999. [DOI] Piper, D., "The Internet IP Security Domain of Interpretation for ISAKMP", RFC 2407, November 1998. Korver Expires - December 2002 [Page 29] The IP Security PKI Profile of ISAKMP and PKIX June 2002 [IKE] Harkins, D. and Carrel, D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [IPSEC] Kent, S. and Atkinson, R., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [ISAKMP] Maughan, D., et. al., "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [PKIX] Housley, R., et al., "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1883] Deering, S. and Hinden, R. "Internet Protocol, Version 6 (IPv6) Specification", RFC 1883, December 1995. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [ROADMAP] Arsenault, A., and Turner, S., "PKIX Roadmap", draft-ietf-pkix-roadmap-08.txt Korver Expires - December 2002 [Page 30] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Acknowledgments The authors would like to acknowledge the expired draft-ietf-ipsec- pki-req-05.txt for providing valuable materials for this document. Author's Addresses Brian Korver Xythos Software, Inc. 25 Maiden Lane, 6th Floor San Francisco, CA 94108 USA Phone: +1 415 248-3800 EMail: briank@xythos.com Eric Rescorla RTFM, Inc. 2064 Edgewood Drive Palo Alto, CA 94303 USA Phone: +1 650 320-8549 EMail: ekr@rtfm.com Korver Expires - December 2002 [Page 31] The IP Security PKI Profile of ISAKMP and PKIX June 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. A. Change History June 2002, Initial Draft B. The Possible Dangers of Delta CRLs The problem is that the CRL processing algorithm is often written with the assumption that all CRLs are base CRLs and it is assumed that CRLs will pass content validity tests. Specifically, such implementations fail to check the certificate against all possible CRLs: if the first CRL that is obtained from the keying material database fails to decode, no further revocation checks are performed for the relevant certificate. This problem is compounded by the fact that implementations which do not understand delta CRLs will fail to decode such CRLs due to the critical DeltaCRLIndicator extension. The insecure algorithm that is implemented in this case is approximately: Korver Expires - December 2002 [Page 32] The IP Security PKI Profile of ISAKMP and PKIX June 2002 fetch newest CRL check validity of CRL signature if CRL signature is invalid then if CRL is contains unrecognized critical extensions then if certificate is on CRL then set certificate status to revoked when the algorithm should be approximately: fetch all up-to-date CRLs set CRLchecked status to false for each CRL check validity of CRL if CRL is valid then set CRLchecked status to true if certificate is on CRL then set certificate status to revoked goto done done: if CRLchecked status is false not valid CRL failure The authors note that a number of PKI toolkits do not even provide a method for obtaining anything but the newest CRL, which in the presence of delta CRLs may in fact be a delta CRL, not a base CRL. C. Mapping A Peer Address to a Peer Certificate TODO Korver Expires - December 2002 [Page 33]