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                                                           Brian Korver
                                                        Xythos Software
                                                          Eric Rescorla
INTERNET-DRAFT                                                RTFM, Inc.
<draft-ietf-ipsec-pki-profile-01.txt>  October 2002 (Expires April 2003)

        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 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 mate-
   rial or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the

   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).


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 specifica-
   tions such as IKE, which assume the existence of public key certifi-
   cates and related keying materials, but which do not address PKI
   issues explicitly. This document addresses those issues.


Table of Contents

1           Introduction                                                    4
2           Terms and Definitions                                           5
3           Profile of ISAKMP                                               6



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3.1         Background                                                      6
3.1.1       Certificate-Related Payloads in ISAKMP                          6
3.1.1.1     Identification Payload                                          6
3.1.1.2     Certificate Payload                                             6
3.1.1.3     Certificate Request Payload                                     6
3.1.1.4     Hash Payload                                                    6
3.1.2       Endpoint Identification                                         7
3.1.2.1     Identification Payload Only                                     7
3.1.2.2     Certificate Payload Only                                        7
3.2         Identification Payload                                          7
3.2.1       ID_IPV4_ADDR and ID_IPV6_ADDR                                   7
3.2.2       ID_FQDN                                                         8
3.2.3       ID_USER_FQDN                                                    8
3.2.4       ID_IPV4_ADDR_SUBNET, ID_IPV6_ADDR_SUBNET, ID_IPV4_ADD...        8
3.2.5       ID_DER_ASN1_DN                                                  8
3.2.6       ID_DER_ASN1_GN                                                  9
3.2.7       ID_KEY_ID                                                       9
3.2.8       Using Peer Source IP Address to Bind Identity to Poli...        9
3.2.9       Securely Binding Identity to Policy                            10
3.2.9.1     Single Address Identification Data                             10
3.2.9.2     Identification Data other than a Single Address                10
3.2.10      Selecting an Identity from a Certificate                       10
3.2.11      Transitively Binding Identity to Policy                        11
3.3         Certificate Request Payload                                    11
3.3.1       Certificate Type                                               11
3.3.2       X.509 Certificate - Signature                                  12
3.3.3       X.509 Certificate - Key Exchange                               12
3.3.4       Certificate Revocation List (CRL)                              12
3.3.5       Authority Revocation List (ARL)                                12
3.3.6       PKCS #7 wrapped X.509 certificate                              12
3.3.7       Presence or Absence of Certificate Request Payloads            12
3.3.8       Certificate Requests                                           13
3.3.8.1     Specifying Certificate Authorities                             13
3.3.8.2     Empty Certificate Authority Field                              13
3.3.9       CRL Requests                                                   13
3.3.9.1     Specifying Certificate Authorities                             13
3.3.9.2     Empty Certificate Authority Field                              13
3.3.10      Robustness                                                     14
3.3.10.1    Unrecognized or Unsupported Certificate Types                  14
3.3.10.2    Undecodable Certificate Authority Fields                       14
3.3.10.3    Ordering of Certificate Request Payloads                       14
3.3.11      Optimizations                                                  14
3.3.11.1    Duplicate Certificate Request Payloads                         14
3.3.11.2    Name Lowest 'Common' Certification Authorities                 14
3.3.11.3    Example                                                        15
3.4         Certificate Payload                                            15
3.4.1       Certificate Type                                               15
3.4.2       X.509 Certificate - Signature                                  16



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3.4.3       X.509 Certificate - Key Exchange                               16
3.4.4       Certificate Revocation List (CRL)                              16
3.4.5       Authority Revocation List (ARL)                                16
3.4.6       PKCS #7 wrapped X.509 certificate                              16
3.4.7       Certificate Payloads Not Mandatory                             16
3.4.8       Response to Multiple Certificate Authority Proposals...        17
3.4.9       Using Local Keying Materials                                   17
3.4.10      Robustness                                                     17
3.4.10.1    Unrecognized or Unsupported Certificate Types                  17
3.4.10.2    Undecodable Certificate Data Fields                            17
3.4.10.3    Ordering of Certificate Payloads                               17
3.4.10.4    Duplicate Certificate Payloads                                 18
3.4.10.5    Irrelevant Certificates                                        18
3.4.11      Optimizations                                                  18
3.4.11.1    Duplicate Certificate Payloads                                 18
3.4.11.2    Send Lowest 'Common' Certificates                              18
3.4.11.3    Drop Duplicate Certificate Payloads                            18
4           Profile of PKIX                                                18
4.1         X.509 Certificates                                             18
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
4.1.2.2.1   Non-FQDN Host Names                                            19
4.1.2.2.2   FQDN Host Names                                                19
4.1.2.3     EmailAddress                                                   19
4.1.3       X.509 Certificate Extensions                                   20
4.1.3.1     AuthorityKeyIdentifier                                         20
4.1.3.2     SubjectKeyIdentifier                                           20
4.1.3.3     KeyUsage                                                       20
4.1.3.4     PrivateKeyUsagePeriod                                          20
4.1.3.5     Certificate Policies                                           21
4.1.3.6     PolicyMappings                                                 21
4.1.3.7     SubjectAltName                                                 21
4.1.3.7.1   Permitted Choices                                              21
4.1.3.7.1.1 dNSName                                                        21
4.1.3.7.1.2 iPAddress                                                      21
4.1.3.7.1.3 rfc822Name                                                     22
4.1.3.8     IssuerAltName                                                  22
4.1.3.9     SubjectDirectoryAttributes                                     22
4.1.3.10    BasicConstraints                                               22
4.1.3.11    NameConstraints                                                22
4.1.3.12    PolicyConstraints                                              22
4.1.3.13    ExtendedKeyUsage                                               22
4.1.3.14    CRLDistributionPoint                                           23
4.1.3.15    InhibitAnyPolicy                                               23
4.1.3.16    FreshestCRL                                                    23
4.1.3.17    AuthorityInfoAccess                                            23



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4.1.3.18    SubjectInfoAccess                                              23
4.2         X.509 Certificate Revocation Lists                             23
4.2.1       Certificate Revocation Requirement                             24
4.2.2       Multiple Sources of Certificate Revocation Informatio...       24
4.2.3       X.509 Certificate Revocation List Extensions                   24
4.2.3.1     AuthorityKeyIdentifier                                         24
4.2.3.2     IssuerAltName                                                  24
4.2.3.3     CRLNumber                                                      24
4.2.3.4     DeltaCRLIndicator                                              24
4.2.3.4.1   If Delta CRLs Are Unsupported                                  24
4.2.3.4.2   Delta CRL Recommendations                                      25
4.2.3.5     IssuingDistributionPoint                                       25
4.2.3.6     FreshestCRL                                                    25
5           Configuration Data Exchange Conventions                        25
5.1         Certificates                                                   25
5.2         Public Keys                                                    26
5.3         PKCS#10 Certificate Signing Requests                           26
6           IKE                                                            26
6.1         IKE Phase 1 Authenticated With Signatures                      26
6.1.1       Identification Payload                                         26
6.1.2       X.509 Certificate Extensions                                   26
6.1.2.1     KeyUsage                                                       26
6.1.3       Obtaining Peer Certificates and CRLs                           27
6.2         IKE Phase 1 Authenticated With Public Key Encryption...        27
6.2.1       Identification Payload                                         27
6.2.2       Hash Payload                                                   27
6.2.3       X.509 Certificate Extensions                                   27
6.2.3.1     KeyUsage                                                       27
6.2.4       Obtaining Peer Certificates and CRLs                           27
6.3         IKE Phase 1 Authenticated With a Revised Mode of Publ...       27
7           Security Considerations                                        28
7.1         Identity Payload                                               28
7.2         Certificate Request Payload                                    28
7.3         Certificate Payload                                            28
7.4         IKE Main Mode                                                  28
7.5         IKE Aggressive Mode                                            28
8           Intellectual Property Rights                                   29
9           IANA Considerations                                            29

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]. Unfortu-
   nately, the combination of these standards leads to an underspecified
   set of requirements for the use of certificates in the context of
   IPsec.




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   ISAKMP references PKIX but in many cases merely specifies the con-
   tents of various messages without specifying their syntax or seman-
   tics. Meanwhile, PKIX provides a large set of certificate mechanisms
   which are generally applicable for Internet protocols, but little
   specific guidance for IPsec. Given the numerous underspecified
   choices, interoperability is hampered if all implementors do not make
   similar choices, or at least fail to account for implementations
   which have choosen differently.

   This profile of the ISAKMP and PKIX frameworks is intended to provide
   an agreed upon standard for using PKI technology in the context of
   IPsec by profiling the PKIX framework for use with ISAKMP and IPsec,
   and by documenting the contents of the relevant ISAKMP payloads and
   further specifying their semantics.

   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 "01" of this document is intended as a "straw man" to encour-
   age 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 mail-
   ing 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 source 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].



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3. Profile of ISAKMP

3.1. Background

3.1.1. Certificate-Related Payloads in ISAKMP

   ISAKMP has three primary certificate-related payloads: Identifica-
   tion, Certificate, and Certificate Request. Additionally, IKE speci-
   fies 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 gener-
   ate 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 implemen-
   tation to request that a peer provide some set of certificates or
   certificate revocation lists. 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 implementa-
   tions to communicate hash values to a peer. The meaning of the con-
   tents of such payloads is left undefined by ISAKMP.







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3.1.2. Endpoint Identification

   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 require-
   ments 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

   The ID payload requirements in this document cover only 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 source address is
   permitted by the relevant policy are not addressed here as they are
   out of the scope of this document.

   The [DOI] defines the 11 types of Identification Data that can be
   used and specifies the syntax for these types. All of these are dis-
   cussed immediately below.

3.2.1. 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]. 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, the payload MUST contain exactly four octets
   [RFC791]. For the ID_IPV6_ADDR type, the payload MUST contain exactly
   sixteen octets [RFC1883]. When comparing the contents of ID with the
   iPAddress field in the subjectAltName extension for equality, binary
   comparison is performed.



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3.2.2. 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] is employed. When comparing
   the contents of ID with the dNSName field in the subjectAltName
   extension for equality, caseless string comparison is performed. Sub-
   string, wildcard, or regular expression matching MUST NOT be per-
   formed.

3.2.3. 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 ID with the rfc822Name field
   in the subjectAltName extension for equality, caseless string compar-
   ison is performed. Substring, wildcard, or regular expression match-
   ing MUST NOT be performed.

3.2.4. ID_IPV4_ADDR_SUBNET, ID_IPV6_ADDR_SUBNET, ID_IPV4_ADDR_RANGE,
       ID_IPV6_ADDR_RANGE

   As there is currently no standard method for putting address subnet
   or range identity information into certificates, the use of these ID
   types is currently undefined.

3.2.5. ID_DER_ASN1_DN

   Implementations MAY support receiving the ID_DER_ASN1_DN ID type,
   although implementations SHOULD NOT generate this type. Implementa-
   tions which generate this type SHOULD populate the contents of ID
   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 that MUST be used to populate the ID payload: implemen-
   tations MUST NOT re-encode the DN for the purposes of making it DER
   if it does not appear in the certificate as DER. Implementations MUST
   NOT populate ID with the Subject Name from the end entity certificate
   if it is empty, as described in the "Subject" section of PKIX.






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3.2.6. 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 understanding
   this type. Implementations which generate this type MUST populate the
   contents of ID with the a GeneralName from the SubjectAltName exten-
   sion 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 General-
   Name MUST be used to populate the ID payload: implementations MUST
   NOT re-encode the GeneralName for the purposes of making it DER if it
   does not appear in the certificate as DER.

3.2.7. ID_KEY_ID

   Type ID_KEY_ID type used to specify pre-shared keys and thus is not
   relevant to this document.

3.2.8. Using Peer Source IP Address to Bind Identity to Policy

   Because implementations sometimes use ID as a lookup key to determine
   which policy to use, all implementations MUST be especially careful
   to verify the truthfulness of the contents by verifying that they
   correspond to some keying material demonstrably held by the peer.
   Failure to do so may result in the use of an inappropriate or inse-
   cure policy. The following sections describe the methods for perform-
   ing this binding.

   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 the ID payload. Although packet IP
   addresses are inherently untrustworthy and must therefore be indepen-
   dently 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 speci-
   fies a particular CA that is trusted to issue certificates which con-
   tain a valid rfc822Name which can 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).





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   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 IPsec traffic.

   As a general comment, however, it may be easier to spoof the contents
   of an ID payload than it is to spoof a peer source address because
   the peer source address must exist on the route to the peer, while ID
   can contain essentially random identification information. Implemen-
   tations MUST validate the Identity Data provided by a peer, but
   implementations MAY favor unauthenticated peer source addresses over
   an unauthenticated ID for initial policy lookup.

3.2.9. Securely Binding Identity to Policy

3.2.9.1. Single Address Identification Data

   In the case where ID contains ID_IPV4_ADDR or ID_IPV6_ADDR (that is,
   is not an address range or subnet), implementations MUST verify that
   this address is the same as the peer source address. If the end
   entity certificate contains addresses identities, then the peer
   source address must match at least one of those identities. If either
   of the above do not match, this MUST be treated as an error and secu-
   rity association setup MUST be aborted. This event SHOULD be
   auditable. The definition of "match" is specific to each ID type and
   was discussed above. In addition, implementations MUST allow adminis-
   trators to configure a local policy that requires that the peer
   source address exist in the certificate. Implementations SHOULD allow
   administrators to configure a local policy that does not enforce this
   requirement.

3.2.9.2. Identification Data other than a Single Address

   In the case where ID contains an identity type other than a single
   address, implementations MUST verify that the identity contained in
   the ID payload matches identity information contained in the peer end
   entity certificate, either in the Subject Name field or subjectAlt-
   Name 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 was discussed above.

3.2.10. Selecting an Identity from a Certificate

   Implementations MUST support certificates that contain more than a
   single identity. In many cases a certificate will contain an identity
   such as an IP address in the subjectAltName extension in addition to
   a non-empty Subject Name.



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   Which identity an implementations chooses to populate ID with is a
   local matter. For compatibility with non-conformant implementations,
   implementations SHOULD populate ID with whichever identity is likely
   to be named in the peer's policy. In practice, this generally means
   IP address, FQDN, or USER-FQDN.

3.2.11. 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, implemen-
   tations SHOULD select the most appropriate identity to use from the
   identities contained in the certificate when determining the appro-
   priate policy.

   For example, imagine that a peer is configured with a certificate
   that contains both a non-empty Subject Name and an FQDN. Independent
   of which identity is used to populate ID, the host implementation
   MUST locate the proper policy. For instance, if ID contains the peer
   Subject Name, then the peer end entity certificate may be found using
   the Subject Name as a key. Once the certificate has been located and
   then validated, the FQDN in the certificate can be used to locate the
   appropriate policy.

3.3. Certificate Request Payload

3.3.1. 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 the
   following types are relevant:

   * X.509 Certificate - Signature
   * X.509 Certificate - Key Exchange
   * Certificate Revocation List (CRL)
   * Authority Revocation List (ARL)
   * 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



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   * Kerberos Tokens
   * SPKI Certificate
   * X.509 Certificate - Attribute

   are out of the scope of this document.

3.3.2. 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.

3.3.3. 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.

3.3.4. Certificate Revocation List (CRL)

   This type requests that X.509 CRLs be provided, along with any cer-
   tificates that may be needed to validate those CRLs.

3.3.5. Authority Revocation List (ARL)

   This ID type SHOULD be treated as synonymous with the CRL type.
   Implementations SHOULD NOT generate CERTREQ payloads with this type,
   but should instead generate CRL CERTREQs.

3.3.6. 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. Implementa-
   tions which receive CERTREQs which contain this ID type MAY ignore
   such payloads.

3.3.7. 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.



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3.3.8. Certificate Requests

3.3.8.1. Specifying Certificate Authorities

   Implementations MUST generate CERTREQs for every peer root that local
   policy explicitly deems trusted during a given exchange. Implementa-
   tions MUST populate the Certificate Authority field with the Subject
   Name of the trusted root, populated such that binary comparison of
   the Subject Name and the Certificate Authority will succeed.

   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. Imple-
   mentations MAY send other certificates from the chain.

3.3.8.2. Empty Certificate Authority Field

   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 from a non-conformant implementation, 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 heuris-
   tics are out of the scope of this document.

3.3.9. CRL Requests

3.3.9.1. Specifying Certificate Authorities

   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 vali-
   date those CRLs.

3.3.9.2. Empty Certificate Authority Field

   Implementations MAY generate CERTREQs where the Certificate Type is
   "Certificate Revocation List (CRL)" with an empty Certificate



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   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.

3.3.10. Robustness

3.3.10.1. Unrecognized or Unsupported Certificate Types

   Implementations MUST be able to deal with receiving CERTREQs with
   unsupported Certificate Types. Absent any recognized and supported
   CERTREQs, implementations MAY treat them as if they are of a sup-
   ported type with the Certificate Authority field left empty, depend-
   ing on local policy. ISAKMP Section 5.10 "Certificate Request Payload
   Processing" specifies additional processing.

3.3.10.2. 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.

3.3.10.3. Ordering of Certificate Request Payloads

   Implementations MUST NOT assume that CERTREQs are ordered in any way.

3.3.11. Optimizations

3.3.11.1. Duplicate Certificate Request Payloads

   Implementations SHOULD NOT send duplicate CERTREQs during an
   exchange.

3.3.11.2. Name Lowest 'Common' Certification Authorities

   When a peer's certificate keying materials have been cached, an
   implementation can send a hint to the peer to elide some of the cer-
   tificates 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 suffi-
   ciently convey all of the information required by the peer. Specifi-
   cally, either the peer may not support this optimization or there may
   be additional chains that could be used in this context but will not



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   be specified if only supplying the issuer of the end entity
   certificate.

   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 Certifi-
   cation 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 has just expired,
   which will result in a failure because the needed keying materials
   are not available to validate the new end entity certificate. Imple-
   mentations which implement this optimization MUST recognize when the
   end entity certificate has changed and respond to it by not perform-
   ing this optimization when the exchange is retried.

3.3.11.3. 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 Sub-
   ject Name of CA2, the implementation is providing a hint that only 1
   certificate needs to be sent: EE.

3.4. Certificate Payload

3.4.1. Certificate Type

   The Certificate Type field identifies to the peer the type of
   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 the follow-
   ing types are relevant:

   * X.509 Certificate - Signature
   * X.509 Certificate - Key Exchange
   * Certificate Revocation List (CRL)
   * Authority Revocation List (ARL)
   * 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



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   Revocation List (CRL)".

   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.

3.4.2. 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.

3.4.3. X.509 Certificate - Key Exchange

   This type specifies that Certificate Data contains an end entity
   certificate used for either key exchange (or key encipherment).

3.4.4. Certificate Revocation List (CRL)

   This type specifies that Certificate Data contains an X.509 CRL.

3.4.5. Authority Revocation List (ARL)

   This type specifies that Certificate Data contains an X.509 CRL that
   revokes CAs.

3.4.6. 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 as specified in ISAKMP.
   Implementations MAY accept CERTs that contain this Certificate type.

3.4.7. 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



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   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.

3.4.8. 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 a CA that matches any of the identities
   provided by the peer.

3.4.9. Using 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.

3.4.10. Robustness

3.4.10.1. Unrecognized or Unsupported Certificate Types

   Implementations MUST be able to deal with receiving CERTs with unrec-
   ognized or unsupported Certificate Types. Implementations MAY discard
   such payloads, depending on local policy. ISAKMP Section 5.10
   "Certificate Request Payload Processing" specifies additional pro-
   cessing.

3.4.10.2. Undecodable Certificate Data Fields

   Implementations MUST be able to deal with receiving CERTs with unde-
   codable Certificate Data fields. Implementations MAY discard such
   payloads, depending on local policy. ISAKMP specifies other actions
   which may be taken.

3.4.10.3. Ordering of Certificate Payloads

   Implementations MUST NOT assume that CERTs are ordered in any way.





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3.4.10.4. Duplicate Certificate Payloads

   Implementations MUST support receiving multiple identical CERTs dur-
   ing an exchange.

3.4.10.5. 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.

3.4.11. Optimizations

3.4.11.1. Duplicate Certificate Payloads

   Implementations SHOULD NOT send duplicate CERTs during an exchange.
   Such payloads should be suppressed.


3.4.11.2. Send Lowest 'Common' Certificates

   When multiple CERTREQs are received which specify certificate author-
   ities within the end entity certificate chain, implementations MAY
   send the shortest chain possible. However, implementations SHOULD
   always send the end entity certificate. See section 3.3.12.2 for more
   discussion of this optimization.

3.4.11.3. Drop Duplicate Certificate Payloads

   Implementations MAY employ local 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.

4. Profile of PKIX

4.1. X.509 Certificates







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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 certifi-
   cates. 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 Sub-
   ject Name field, as specified in PKIX. Identity information in such
   certificates will be contained entirely in the SubjectAltName exten-
   sion.

4.1.2.2. Specifying Hosts in Subject Name

4.1.2.2.1. Non-FQDN Host Names

   Implementations which desire to place host names that are not FQDNs
   (for instance "Gateway Router") in the Subject Name MUST use the com-
   monName 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 pop-
   ulating 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 some implementations will reject
   certificates that contain this attribute.

4.1.2.3. EmailAddress

   As specified in PKIX, implementations MUST NOT populate Distin-
   guishedNames with the EmailAddress attribute.




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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 certifi-
   cates MAY ignore the value of the criticality bit for extensions that
   are supported by that implementation, but MUST support the critical-
   ity 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 generate
   certificate hierarchies which overly complex to process in the
   absence of this extension, such that those that require possibly ver-
   ifying a signature against a large number of similarly named CA cer-
   tificates in order to find the CA certificate which contains the 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 generate
   certificate hierarchies which overly complex to process in the
   absence of this extension, such that those that require possibly ver-
   ifying a signature against a large number of similarly named CA cer-
   tificates 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 Pri-
   vateKeyUsageExtension is intended to be used when signatures will
   need to be verified long past the time when signatures using the pri-
   vate keypair may be generated. Since ISAKMP SAs are short-lived rela-
   tive to the intended use of this extension in addition to the fact



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   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 gen-
   erate 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

   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 function-
   ality.

4.1.3.7.1.1. dNSName

   This field MUST contain a fully qualified domain name. Implementa-
   tions 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. Implementa-
   tions 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 Con-
   straints" section of PKIX is explicitly not permitted by that speci-
   fication for conveying identity information. In other words, the CIDR



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   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 priv-
   ilege information, in a manner analogous to privileges carried in
   Attribute Certificates. Implementations MAY ignore this extension as
   PKIX mandates it be marked non-critical.

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 pre-
   sent, 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 pre-
   sent, 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 exten-
   sion is present and marked critical, use of this certificate for



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   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 CRLDis-
   tributionPoint 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's certificates.

4.1.3.15. InhibitAnyPolicy

   Many implementations do not support the InhibitAnyPolicy extension.
   Since PKIX mandates that this extension be marked critical when pre-
   sent, implementations which intend to be maximally interoperable
   SHOULD NOT generate certificates which contain this extension.

4.1.3.16. FreshestCRL

   Most implementations expect to exchange CRLs in band via the ISAKMP
   Certificate Payload. Implementations MUST NOT assume that the Fresh-
   estCRL extension will exist in peer extensions and therefore imple-
   mentations SHOULD request that peers send CRLs in the absence knowl-
   edge 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 imple-
   mentations SHOULD ignore this extension when present.

4.2. X.509 Certificate Revocation Lists

   Implementations SHOULD send CRLs, unless non-CRL certificate revoca-
   tion 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



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   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 revoca-
   tion 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.

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 generate
   certificate hierarchies which overly complex to process in the
   absence of this extension.

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 accord-
   ing 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.



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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, implementa-
   tions 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 specifica-
   tion RECOMMENDS against issuing delta CRLs at this time. On the other
   hand, failure to issue delta CRLs exposes a larger window of vulnera-
   bility. See the Security Considerations section of PKIX for addi-
   tional discussion. Implementors as well as administrators are encour-
   aged to consider these issues.

4.2.3.5. IssuingDistributionPoint

   Given the recommendations against implementations generating delta
   CRLs, this specification RECOMMENDS that implementations do not popu-
   late CRLs with the IssuingDistributionPoint extension, which must be
   marked critical if present according to PKIX but is generally only
   useful in the context of delta CRLs.

4.2.3.6. FreshestCRL

   Given the recommendations against implementations generating delta
   CRLs, this specification RECOMMENDS that implementations do not popu-
   late 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 beginning and end of any line, and MUST support the
   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-----





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   -----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
   same form as above. A raw key is only the SubjectPublicKeyInfo por-
   tion of the certificate, and MUST be Base64 encoded and appear
   between the following delimiters:

   -----BEGIN PUBLIC KEY-----

   -----END PUBLIC KEY-----

5.3. PKCS#10 Certificate Signing Requests

   A PKCS#10 Certificiate Signing Request MUST be Base64 encoded and
   appear between the following delimeters:

   -----BEGIN CERTIFICATE REQUEST-----

   -----END CERTIFICATE REQUEST-----

6. IKE

6.1. IKE Phase 1 Authenticated With Signatures

6.1.1. Identification Payload

   IKE mandates the use of the ID payload in Phase 1.

   Implementations SHOULD populate ID with identity information that is
   contained within the end entity certificate. This enables recipients
   to use ID as a lookup key to find the peer end entity certificate.
   The only case where implementations MAY populate ID with information
   that is not contained in the end entity certificate is when ID con-
   tains the peer source address (a single address, not a subnet or
   range). This means that implementations MUST be able to map a peer
   source address to a peer end entity certificate, even when the
   certificate does not contain that address. The exact method for per-
   forming this mapping is out of the scope of this document.

6.1.2. X.509 Certificate Extensions

6.1.2.1. KeyUsage

   If the KeyUsage extension is present in an end entity certificate,
   the digitalSignature bit must be asserted.



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6.1.3. Obtaining Peer Certificates and CRLs

   IKE implementations MUST assume all necessary certificates and CRLs
   will be exchanged in-band.

6.2. IKE Phase 1 Authenticated With Public Key Encryption

6.2.1. Identification Payload

   IKE mandates the use of the ID payload in Phase 1.

   If certificates are not being used, the contents of ID are out of
   scope for this document.

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 should
   use for encrypting payloads during the exchange.

   Implementations SHOULD include this payload whenever the public por-
   tion of the keypair has been placed in a certificate.

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.

6.2.4. Obtaining Peer Certificates and CRLs

   Certificates are generally not exchanged in-band, but rather are
   exchanged out-of-band, with direct trust of the peer certificate
   being most prevalent. CRLs SHOULD be obtained out-of-band from a
   directory or other repository.

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 Encryp-
   tion has the same requirements as IKE Phase 1 Authenticated With Pub-
   lic Key Encryption. See section 6.2 for these requirements.






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7. Security Considerations

7.1. Identity Payload

   Depending on the exchange type, ID may be passed in the clear. Admin-
   istrators in some environments may wish to use the empty Certifica-
   tion Authority option to prevent such information from leaking, at
   the possible cost of some performance.

7.2. Certificate Request Payload

   The Contents of CERTREQ are not encrypted in IKE. In some environ-
   ments this may leak private information. Administrators in some envi-
   ronments may wish to use the empty Certification Authority option to
   prevent such information from leaking, at the cost of performance.

7.3. Certificate Payload

   Depending on the exchange type, CERTs may be passed in the clear and
   therefore may leak identity information.

7.4. IKE Main Mode

   Implementations may not wish to respond with CERTs in the second mes-
   sage, thereby violating the identity protection feature of Main Mode
   IKE. ISAKMP allows CERTs to be included in any message, and therefore
   implementations may wish to respond with CERTs in a message that
   offers privacy protection in this case.

7.5. IKE Aggressive Mode

   The contents of ID are not encrypted in Aggressive Mode when authen-
   tication is performed with signatures. In some environments this may
   leak private information. The solutions to this problem if such a
   leak is unacceptable are:

   * Use Main Mode instead of Aggressive Mode.
   * Populate ID Data with the address of the host.

   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 solutions to this problem if
   such a leak is unacceptable are:

   * Use Main Mode instead of Aggressive Mode.






Korver, Rescorla                                                [Page 28]Internet-Draft         PKI Profile for ISAKMP/PKIX               10/2002


8. Intellectual Property Rights

   No new intellectual property rights are introduced by this document.

9. IANA Considerations

   There are no known numbers which IANA will need to manage.


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 Security Extensions",
              RFC 2535, March 1999.

   [DOI]      Piper, D., "The Internet IP Security Domain of
              Interpretation for ISAKMP", RFC 2407, November 1998.

   [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, Rescorla                                                [Page 29]Internet-Draft         PKI Profile for ISAKMP/PKIX               10/2002


Acknowledgments

   The authors would like to acknowledge the expired draft-ietf-ipsec-
   pki-req-05.txt for providing valuable materials for this document.
   The authors would like to thank Greg Carter for his valuable comments
   on an early draft of 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


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 docu-
   ment 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 develop-
   ing 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.





Korver, Rescorla                                                [Page 30]Internet-Draft         PKI Profile for ISAKMP/PKIX               10/2002


   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 MER-
   CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



Appendix A. Change History

   * October 2002, Reorganization
   * June 2002, Initial Draft


Appendix 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 imple-
   mentations 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 may fail to decode
   such CRLs due to the critical DeltaCRLIndicator extension. The algo-
   rithm that is implemented in this case is approximately:

     fetch newest CRL
     check validity of CRL signature
     if CRL signature is valid then
     if CRL does not contain unrecognized critical extensions
     and certificate is on CRL then
     set certificate status to revoked

   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.












Korver, Rescorla                                                [Page 31]


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