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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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119 [RFC2119].
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
NOT 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.
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 |
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].
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:
X509v3 Subject Alternative Name:
Example X509v3 extensions:
X509v3 Issuer Alternative Name:
X509v3 Subject Alternative Name:
Appendix B shows a full example of an X.509 v3 certificate with HIP
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].
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.
NOTIFICATION PARAMETER - ERROR TYPES Value
The Responder is unwilling to set up an association,
as the Initiator did not send the needed credentials.
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)
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].
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
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,
[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.
Appendix A. SPKI Certificate Example
This section shows an SPKI certificate with encoded HITs. The
example has been indented for readability.
(hash hit 2001:15:2453:698a:9aa:253a:dcb5:981e)
(hash hit 2001:12:ccd6:4715:72a3:2ab1:77e4:4acc)
(hash sha1 |h5fC8HUMATTtK0cjYqIgeN3HCIMA|)