Internet Engineering Task Force (IETF) T. Heer
Request for Comments: 8002 Albstadt-Sigmaringen University
Obsoletes: 6253 S. Varjonen
Updates: 7401 University of Helsinki
Category: Standards Track October 2016
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
certificate parameter and the error signaling in case of a failed
verification. Additionally, this document specifies the
representations of Host Identity Tags (HITs) in X.509 version 3 (v3).
The concrete use cases of certificates, including how certificates
are obtained and requested and which actions are taken upon
successful or failed verification, are 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 7401 and obsoletes RFC 6253.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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) [RFC7401] 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 [RFC7401] and thus updates [RFC7401].
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. 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, each carrying one certificate. 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). Ungrouped certificates exhibit a
unique CERT group field and set the CERT count to 1. CERT parameters
with the same group number in the CERT 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 (variable length) |
Length Length in octets, excluding Type, Length, and
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. Any added padding bytes MUST be zeroed
by the sender, and their values SHOULD NOT be checked
by the receiver.
The certificates MUST use the algorithms defined in [RFC7401] as the
signature and hash algorithms.
The following certificate types are defined:
| CERT format | Type number |
| Reserved | 0 |
| X.509 v3 | 1 |
| Obsoleted | 2 |
| Hash and URL of X.509 v3 | 3 |
| Obsoleted | 4 |
| LDAP URL of X.509 v3 | 5 |
| Obsoleted | 6 |
| Distinguished Name of X.509 v3 | 7 |
| Obsoleted | 8 |
The next sections outline the use of HITs in X.509 v3. 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].
Hash and Uniform Resource Locator (URL) encoding (3) is used as
defined in Section 3.6 of [RFC7296]. Using hash and URL encodings
result 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 encoding (5) is 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) encoding (7) is 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 [RFC7343].
When the Host Identifier (HI) is used to sign the certificate, the
respective HIT SHOULD 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 an HI, the respective HIT SHOULD 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 the
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 A shows a full example 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 SHOULD 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. Revocation of Certificates
Revocation of X.509 v3 certificates is handled as defined in
Section 5 of [RFC5280] with two exceptions. First, any HIP
certificate serial number that appears on the Certificate Revocation
List (CRL) is treated as invalid regardless of the reason code.
Second, the certificateHold is not supported.
5. Error Signaling
If the Initiator does not send all the certificates 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 a CERT group and CERT ID octet
(in this order) of the CERT parameter that caused the
6. IANA Considerations
This document defines the CERT parameter for HIP [RFC7401]. The CERT
parameter type number (768) is defined in [RFC7401].
The CERT parameter has an 8-bit unsigned integer field for different
certificate types, for which IANA has created and maintains a
subregistry entitled "HIP Certificate Types" under "Host Identity
Protocol (HIP) Parameters". Values for the "HIP Certificate Types"
registry are given in Section 2. New values for the Certificate
types from the unassigned space are assigned through IETF Review.
In Section 5, this document defines two types for the "NOTIFY Message
Types" subregistry under "Host Identity Protocol (HIP) Parameters".
As this document obsoletes [RFC6253], references to [RFC6253] in IANA
registries have been replaced by references to this document. This
document changes the "HIP Certificate Types" registry in Section 2.
The following updates to the "HIP Certificate Types" registry have
The references have been updated from [RFC6253] to this document.
This document obsoleted the type numbers "2", "4", "6", and "8"
for the Simple Public Key Infrastructure (SPKI) certificates.
7. 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
Although the CERT parameter is allowed in the I1 packet, it is NOT
RECOMMENDED because it can increase the processing times of I1s,
which can be problematic when processing storms of I1s. Furthermore,
the Initiator has to take into consideration that the Responder can
drop the CERT parameter in I1 without processing the parameter.
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 X.509 v3 are discussed in [RFC5280].
8. Differences from RFC 6253
This section summarizes the technical changes made from [RFC6253].
This section is informational and is intended to help implementors of
the previous protocol version. If any text in this section
contradicts text in other portions of this specification, the text
found outside of this section should be considered normative.
The following change has been made.
o Support for SPKI certificates has been removed.
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
University of Helsinki
Gustaf Haellstroemin katu 2b