Internet Engineering Task Force (IETF) T. Heer

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

Abstract

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
parameter.

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
evaluation.

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
http://www.rfc-editor.org/info/rfc6253.

Heer & Varjonen Experimental [Page 1]

RFC 6253 HIP CERT May 2011

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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119 [RFC2119].

Heer & Varjonen Experimental [Page 2]

RFC 6253 HIP CERT May 2011

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
parameter.

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.

Heer & Varjonen Experimental [Page 3]

RFC 6253 HIP CERT May 2011

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].

Heer & Varjonen Experimental [Page 4]

RFC 6253 HIP CERT May 2011

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
content.

Heer & Varjonen Experimental [Page 5]

RFC 6253 HIP CERT May 2011

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
certificates.

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
keys.

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
follows:

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
content.

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].

Heer & Varjonen Experimental [Page 6]

RFC 6253 HIP CERT May 2011

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
CREDENTIALS_REQUIRED.

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

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
Review.

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

Heer & Varjonen Experimental [Page 7]

RFC 6253 HIP CERT May 2011

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
scarce.

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.

Heer & Varjonen Experimental [Page 8]

RFC 6253 HIP CERT May 2011

[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.

Heer & Varjonen Experimental [Page 9]

RFC 6253 HIP CERT May 2011

Appendix A. SPKI Certificate Example

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

(sequence
(public_key
(rsa-pkcs1-sha1
(e #010001#)
(n |yDwznOwX0w+zvQbpWoTnfWrUPLKW2NFrpXbsIcH/QBSLb
k1RKTZhLasFwvtSHAjqh220W8gRiQAGIqKplyrDEqSrJp
OdIsHIQ8BQhJAyILWA1Sa6f5wAnWozDfgdXoKLNdT8ZNB
mzluPiw4ozc78p6MHElH75Hm3yHaWxT+s83M=|
)
)
)
(cert
(issuer
(hash hit 2001:15:2453:698a:9aa:253a:dcb5:981e)
)
(subject
(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")
)
(signature
(hash sha1 |h5fC8HUMATTtK0cjYqIgeN3HCIMA|)
|u8NTRutINI/AeeZgN6bngjvjYPtVahvY7MhGfenTpT7MCgBy
NoZglqH5Cy2vH6LrQFYWx0MjWoYwHKimEuBKCNd4TK6hrCyAI
CIDJAZ70TyKXgONwDNWPOmcc3lFmsih8ezkoBseFWHqRGISIm
MLdeaMciP4lVfxPY2AQKdMrBc=|
)
)

Heer & Varjonen Experimental [Page 10]

RFC 6253 HIP CERT May 2011

Appendix B. X.509 v3 Certificate Example

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

Certificate:
Data:
Version: 3 (0x2)
Serial Number: 0 (0x0)
Signature Algorithm: sha1WithRSAEncryption
Issuer: CN=Example issuing host, DC=example, DC=com
Validity
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):
00:c0:db:38:50:8e:63:ed:96:ea:c6:c4:ec:a3:36:
62:e2:28:e9:74:9c:f5:2f:cb:58:0e:52:54:60:b5:
fa:98:87:0d:22:ab:d8:6a:61:74:a9:ee:0b:ae:cd:
18:6f:05:ab:69:66:42:46:00:a2:c0:0c:3a:28:67:
09:cc:52:27:da:79:3e:67:d7:d8:d0:7c:f1:a1:26:
fa:38:8f:73:f5:b0:20:c6:f2:0b:7d:77:43:aa:c7:
98:91:7e:1e:04:31:0d:ca:94:55:20:c4:4f:ba:b1:
df:d4:61:9d:dd:b9:b5:47:94:6c:06:91:69:30:42:
9c:0a:8b:e3:00:ce:49:ab:e3
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
83:68:b4:38:63:a6:ae:57:68:e2:4d:73:5d:8f:11:e4:ba:30:
a0:19:ca:86:22:e9:6b:e9:36:96:af:95:bd:e8:02:b9:72:2f:
30:a2:62:ac:b2:fa:3d:25:c5:24:fd:8d:32:aa:01:4f:a5:8a:
f5:06:52:56:0a:86:55:39:2b:ee:7a:7b:46:14:d7:5d:15:82:
4d:74:06:ca:b7:8c:54:c1:6b:33:7f:77:82:d8:95:e1:05:ca:
e2:0d:22:1d:86:fc:1c:c4:a4:cf:c6:bc:ab:ec:b8:2a:1e:4b:
04:7e:49:9c:8f:9d:98:58:9c:63:c5:97:b5:41:94:f7:ef:93:
57:29

Heer & Varjonen Experimental [Page 11]

RFC 6253 HIP CERT May 2011

Authors' Addresses

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

Phone: +49 241 80 20 776
EMail: [email protected]
URI: http://www.comsys.rwth-aachen.de/team/tobias-heer/

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

EMail: [email protected]
URI: http://www.hiit.fi

Heer & Varjonen Experimental [Page 12]