RFC 6253
This document is obsolete. Please refer to RFC 8002.

Internet Engineering Task Force (IETF)                           T. Heer
Request for Comments: 6253                COMSYS, RWTH Aachen University
Updates: 5201                                                S. Varjonen
Category: Experimental     Helsinki Institute for Information Technology
ISSN: 2070-1721                                                 May 2011

                  Host Identity Protocol Certificates


   The Certificate (CERT) parameter is a container for digital
   certificates.  It is used for carrying these certificates in Host
   Identity Protocol (HIP) control packets.  This document specifies the
   CERT parameter and the error signaling in case of a failed
   verification.  Additionally, this document specifies the
   representations of Host Identity Tags in X.509 version 3 (v3) and
   Simple Public Key Infrastructure (SPKI) certificates.

   The concrete use of certificates, including how certificates are
   obtained, requested, and which actions are taken upon successful or
   failed verification, is specific to the scenario in which the
   certificates are used.  Hence, the definition of these scenario-
   specific aspects is left to the documents that use the CERT

   This document updates RFC 5201.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

1.  Introduction

   Digital certificates bind pieces of information to a public key by
   means of a digital signature and thus enable the holder of a private
   key to generate cryptographically verifiable statements.  The Host
   Identity Protocol (HIP) [RFC5201] defines a new cryptographic
   namespace based on asymmetric cryptography.  The identity of each
   host is derived from a public key, allowing hosts to digitally sign
   data and issue certificates with their private key.  This document
   specifies the CERT parameter, which is used to transmit digital
   certificates in HIP.  It fills the placeholder specified in
   Section 5.2 of [RFC5201] and thus updates [RFC5201].

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119 [RFC2119].

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2.  CERT Parameter

   The CERT parameter is a container for certain types of digital
   certificates.  It does not specify any certificate semantics.
   However, it defines supplementary parameters that help HIP hosts to
   transmit semantically grouped CERT parameters in a more systematic
   way.  The specific use of the CERT parameter for different use cases
   is intentionally not discussed in this document, because it is
   specific to a concrete use case.  Hence, the use of the CERT
   parameter will be defined in the documents that use the CERT

   The CERT parameter is covered and protected, when present, by the HIP
   SIGNATURE field and is a non-critical parameter.

   The CERT parameter can be used in all HIP packets.  However, using it
   in the first Initiator (I1) packet is NOT RECOMMENDED, because it can
   increase the processing times of I1s, which can be problematic when
   processing storms of I1s.  Each HIP control packet MAY contain
   multiple CERT parameters.  These parameters MAY be related or
   unrelated.  Related certificates are managed in Cert groups.  A Cert
   group specifies a group of related CERT parameters that SHOULD be
   interpreted in a certain order (e.g., for expressing certificate
   chains).  For grouping CERT parameters, the Cert group and the Cert
   count field MUST be set.  Ungrouped certificates exhibit a unique
   Cert group field and set the Cert count to 1.  CERT parameters with
   the same Cert group number in the group field indicate a logical
   grouping.  The Cert count field indicates the number of CERT
   parameters in the group.

   CERT parameters that belong to the same Cert group MAY be contained
   in multiple sequential HIP control packets.  This is indicated by a
   higher Cert count than the amount of CERT parameters with matching
   Cert group fields in a HIP control packet.  The CERT parameters MUST
   be placed in ascending order, within a HIP control packet, according
   to their Cert group field.  Cert groups MAY only span multiple
   packets if the Cert group does not fit the packet.  A HIP packet MUST
contain more than one incomplete Cert group that continues in the
   next HIP control packet.

   The Cert ID acts as a sequence number to identify the certificates in
   a Cert group.  The numbers in the Cert ID field MUST start from 1 up
   to Cert count.

   The Cert group and Cert ID namespaces are managed locally by each
   host that sends CERT parameters in HIP control packets.

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |             Type              |             Length            |
     |  Cert group   |  Cert count   |    Cert ID    |   Cert type   |
     |                          Certificate                          /
     /                               |            Padding            |

    Type          768
    Length        Length in octets, excluding Type, Length, and Padding.
    Cert group    Group ID grouping multiple related CERT parameters.
    Cert count    Total count of certificates that are sent, possibly
                  in several consecutive HIP control packets.
    Cert ID       The sequence number for this certificate.
    Cert Type     Indicates the type of the certificate.
    Padding       Any Padding, if necessary, to make the TLV a multiple
                  of 8 bytes.

   The certificates MUST use the algorithms defined in [RFC5201] as the
   signature and hash algorithms.

   The following certificate types are defined:

             |           Cert format          | Type number |
             |            Reserved            |      0      |
             |            X.509 v3            |      1      |
             |              SPKI              |      2      |
             |    Hash and URL of X.509 v3    |      3      |
             |      Hash and URL of SPKI      |      4      |
             |      LDAP URL of X.509 v3      |      5      |
             |        LDAP URL of SPKI        |      6      |
             | Distinguished Name of X.509 v3 |      7      |
             |   Distinguished Name of SPKI   |      8      |

   The next sections outline the use of Host Identity Tags (HITs) in
   X.509 v3 and in Simple Public Key Infrastructure (SPKI) certificates.
   X.509 v3 certificates and the handling procedures are defined in
   [RFC5280].  The wire format for X.509 v3 is the Distinguished
   Encoding Rules format as defined in [X.690].  The SPKI, the handling
   procedures, and the formats are defined in [RFC2693].

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   Hash and Uniform Resource Locator (URL) encodings (3 and 4) are used
   as defined in Section 3.6 of [RFC5996].  Using hash and URL encodings
   results in smaller HIP control packets than by including the
   certificate(s), but requires the receiver to resolve the URL or check
   a local cache against the hash.

   Lightweight Directory Access Protocol (LDAP) URL encodings (5 and 6)
   are used as defined in [RFC4516].  Using LDAP URL encoding results in
   smaller HIP control packets but requires the receiver to retrieve the
   certificate or check a local cache against the URL.

   Distinguished Name (DN) encodings (7 and 8) are represented by the
   string representation of the certificate's subject DN as defined in
   [RFC4514].  Using the DN encoding results in smaller HIP control
   packets, but requires the receiver to retrieve the certificate or
   check a local cache against the DN.

3.  X.509 v3 Certificate Object and Host Identities

   If needed, HITs can represent an issuer, a subject, or both in
   X.509 v3.  HITs are represented as IPv6 addresses as defined in
   [RFC4843].  When the Host Identifier (HI) is used to sign the
   certificate, the respective HIT MUST be placed into the Issuer
   Alternative Name (IAN) extension using the GeneralName form iPAddress
   as defined in [RFC5280].  When the certificate is issued for a HIP
   host, identified by a HIT and HI, the respective HIT MUST be placed
   into the Subject Alternative Name (SAN) extension using the
   GeneralName form iPAddress, and the full HI is presented as the
   subject's public key info as defined in [RFC5280].

   The following examples illustrate how HITs are presented as issuer
   and subject in the X.509 v3 extension alternative names.

       Format of X509v3 extensions:
           X509v3 Issuer Alternative Name:
               IP Address:hit-of-issuer
           X509v3 Subject Alternative Name:
               IP Address:hit-of-subject

       Example X509v3 extensions:
           X509v3 Issuer Alternative Name:
               IP Address:2001:14:6cf:fae7:bb79:bf78:7d64:c056
           X509v3 Subject Alternative Name:
               IP Address:2001:1c:5a14:26de:a07c:385b:de35:60e3

   Appendix B shows a full example of an X.509 v3 certificate with HIP

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   As another example, consider a managed Public Key Infrastructure
   (PKI) environment in which the peers have certificates that are
   anchored in (potentially different) managed trust chains.  In this
   scenario, the certificates issued to HIP hosts are signed by
   intermediate Certification Authorities (CAs) up to a root CA.  In
   this example, the managed PKI environment is neither HIP aware, nor
   can it be configured to compute HITs and include them in the

   When HIP communications are established, the HIP hosts not only need
   to send their identity certificates (or pointers to their
   certificates), but also the chain of intermediate CAs (or pointers to
   the CAs) up to the root CA, or to a CA that is trusted by the remote
   peer.  This chain of certificates MUST be sent in a Cert group as
   specified in Section 2.  The HIP peers validate each other's
   certificates and compute peer HITs based on the certificate public

4.  SPKI Cert Object and Host Identities

   When using SPKI certificates to transmit information related to HIP
   hosts, HITs need to be enclosed within the certificates.  HITs can
   represent an issuer, a subject, or both.  In the following, we define
   the representation of those identifiers for SPKI given as
   S-expressions.  Note that the S-expressions are only the human-
   readable representation of SPKI certificates.  Full HIs are presented
   in the public key sequences of SPKI certificates.

   As an example, the Host Identity Tag of a host is expressed as

       Format:  (hash hit hit-of-host)
       Example: (hash hit 2001:13:724d:f3c0:6ff0:33c2:15d8:5f50)

   Appendix A shows a full example of a SPKI certificate with HIP

5.  Revocation of Certificates

   Revocation of X.509 v3 certificates is handled as defined in
   Section 5 of [RFC5280].  Revocation of SPKI certificates is handled
   as defined in Section 5 of [RFC2693].

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6.  Error Signaling

   If the Initiator does not send the certificate that the Responder
   requires, the Responder may take actions (e.g., reject the
   connection).  The Responder MAY signal this to the Initiator by
   sending a HIP NOTIFY message with NOTIFICATION parameter error type

   If the verification of a certificate fails, a verifier MAY signal
   this to the provider of the certificate by sending a HIP NOTIFY
   message with NOTIFICATION parameter error type INVALID_CERTIFICATE.

     ------------------------------------     -----

     CREDENTIALS_REQUIRED                      48

     The Responder is unwilling to set up an association,
     as the Initiator did not send the needed credentials.

     INVALID_CERTIFICATE                       50

     Sent in response to a failed verification of a certificate.
     Notification Data MAY contain n groups of 2 octets (n calculated
     from the NOTIFICATION parameter length), in order Cert group and
     Cert ID of the Certificate parameter that caused the failure.

7.  IANA Considerations

   This document defines the CERT parameter for the Host Identity
   Protocol [RFC5201].  This parameter is defined in Section 2 with type
   768.  The parameter type number is also defined in [RFC5201].

   The CERT parameter has an 8-bit unsigned integer field for different
   certificate types, for which IANA has created and now maintains a new
   sub-registry entitled "HIP Certificate Types" under the "Host
   Identity Protocol (HIP) Parameters".  Initial values for the
   Certificate type registry are given in Section 2.  New values for the
   Certificate types from the unassigned space are assigned through IETF

   In Section 6, this document defines two new types for the "NOTIFY
   Message Types" sub-registry under "Host Identity Protocol (HIP)

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8.  Security Considerations

   Certificate grouping allows the certificates to be sent in multiple
   consecutive packets.  This might allow similar attacks, as IP-layer
   fragmentation allows, for example, the sending of fragments in the
   wrong order and skipping some fragments to delay or stall packet
   processing by the victim in order to use resources (e.g., CPU or
   memory).  Hence, hosts SHOULD implement mechanisms to discard
   certificate groups with outstanding certificates if state space is

   Checking of the URL and LDAP entries might allow denial-of-service
   (DoS) attacks, where the target host may be subjected to bogus work.

   Security considerations for SPKI certificates are discussed in
   [RFC2693] and for X.509 v3 in [RFC5280].

9.  Acknowledgements

   The authors would like to thank A. Keranen, D. Mattes, M. Komu, and
   T. Henderson for the fruitful conversations on the subject.  D.
   Mattes most notably contributed the non-HIP aware use case in
   Section 3.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2693]  Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
              B., and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
              September 1999.

   [RFC4514]  Zeilenga, K., Ed., "Lightweight Directory Access Protocol
              (LDAP): String Representation of Distinguished Names",
              RFC 4514, June 2006.

   [RFC4516]  Smith, M., Ed., and T. Howes, "Lightweight Directory
              Access Protocol (LDAP): Uniform Resource Locator",
              RFC 4516, June 2006.

   [RFC4843]  Nikander, P., Laganier, J., and F. Dupont, "An IPv6 Prefix
              for Overlay Routable Cryptographic Hash Identifiers
              (ORCHID)", RFC 4843, April 2007.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., Ed., and T.
              Henderson, "Host Identity Protocol", RFC 5201, April 2008.

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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [X.690]    ITU-T, "Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", July 2002.

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Appendix A.  SPKI Certificate Example

   This section shows an SPKI certificate with encoded HITs.  The
   example has been indented for readability.

         (e #010001#)
         (n |yDwznOwX0w+zvQbpWoTnfWrUPLKW2NFrpXbsIcH/QBSLb
           (hash hit 2001:15:2453:698a:9aa:253a:dcb5:981e)
           (hash hit 2001:12:ccd6:4715:72a3:2ab1:77e4:4acc)
         (not-before "2011-01-12_13:43:09")
         (not-after "2011-01-22_13:43:09")
         (hash sha1 |h5fC8HUMATTtK0cjYqIgeN3HCIMA|)

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Appendix B.  X.509 v3 Certificate Example

   This section shows a X.509 v3 certificate with encoded HITs.

           Version: 3 (0x2)
           Serial Number: 0 (0x0)
           Signature Algorithm: sha1WithRSAEncryption
           Issuer: CN=Example issuing host, DC=example, DC=com
               Not Before: Mar 11 09:01:39 2011 GMT
               Not After : Mar 21 09:01:39 2011 GMT
           Subject: CN=Example subject host, DC=example, DC=com
           Subject Public Key Info:
               Public Key Algorithm: rsaEncryption
               RSA Public Key: (1024 bit)
                   Modulus (1024 bit):
                   Exponent: 65537 (0x10001)
           X509v3 extensions:
               X509v3 Issuer Alternative Name:
                   IP Address:2001:13:8d83:41c5:dc9f:38ed:e742:7281
               X509v3 Subject Alternative Name:
                   IP Address:2001:1c:6e02:d3e0:9b90:8417:673e:99db
       Signature Algorithm: sha1WithRSAEncryption

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Authors' Addresses

   Tobias Heer
   Chair of Communication and Distributed Systems - COMSYS
   RWTH Aachen University
   Ahornstrasse 55

   Phone: +49 241 80 20 776
   EMail: heer@cs.rwth-aachen.de
   URI:   http://www.comsys.rwth-aachen.de/team/tobias-heer/

   Samu Varjonen
   Helsinki Institute for Information Technology
   Gustaf Haellstroemin katu 2b

   EMail: samu.varjonen@hiit.fi
   URI:   http://www.hiit.fi

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