Independent Submission A. Keromytis Request for Comments: 5708 Columbia University Category: Informational January 2010 ISSN: 2070-1721
X.509 Key and Signature Encoding for the KeyNote Trust Management System
This memo describes X.509 key identifiers and signature encoding for version 2 of the KeyNote trust-management system (RFC 2704). X.509 certificates (RFC 5280) can be directly used in the Authorizer or Licensees field (or in both fields) in a KeyNote assertion, allowing for easy integration with protocols that already use X.509 certificates for authentication.
In addition, the document defines additional signature types that use other hash functions (beyond the MD5 and SHA1 hash functions that are defined in RFC 2792).
Status of This Memo
This document is not an Internet Standards Track specification; it is published for informational purposes.
This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
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KeyNote is a simple and flexible trust-management system designed to work well for a variety of large- and small-scale, Internet-based applications. It provides a single, unified language for both local policies and credentials. KeyNote policies and credentials, called 'assertions', contain predicates that describe the trusted actions permitted by the holders of specific public keys. KeyNote assertions are essentially small, highly structured programs. A signed assertion, which can be sent over an untrusted network, is also called a 'credential assertion'. Credential assertions, which also serve the role of certificates, have the same syntax as policy assertions but are also signed by the principal delegating the trust. Note that only one principal may sign a credential assertion, but trust may be delegated to multiple principals. The credential assertion may delegate trust to each of these principals separately or to groups of principals required to act together. For more details on KeyNote, see [KEYNOTE]. This document assumes reader familiarity with the KeyNote system.
Cryptographic keys may be used in KeyNote to identify principals. To facilitate interoperation between different implementations and to allow for maximal flexibility, keys must be converted to a normalized canonical form (dependent on the public key algorithm used) for the purposes of any internal comparisons between keys. For example, an RSA key may be encoded in base64 [RFC4648] ASCII in one credential and in hexadecimal ASCII in another. A KeyNote implementation must internally convert the two encodings to a normalized form that allows for comparison between them. Furthermore, the internal structure of an encoded key must be known for an implementation to correctly decode it. [RFC2792] describes the RSA and DSA (Digital Signature Algorithm) key identifier and signature encodings for use in KeyNote assertions. This document specifies a new key identifier, allowing X.509 certificates [RFC5280] to be used as a key substitute wherever an RSA or DSA key may be used in KeyNote. Specifically, KeyNote will use the key associated with the subject of an X.509 certificate. In addition, this document defines a corresponding signature encoding, to be used in conjunction with X.509 key identifiers. Finally, this document defines new signature encodings that use new hash functions beyond the MD5 and SHA1 functions defined in RFC 2792, and which in recent years have been found to be vulnerable to attack.
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].
X.509 key identifiers in KeyNote are encoded as an ASN1 Distinguished Encoding Rules (DER) encoding of the whole X.509 certificate, as defined in Section 4 of [RFC5280].
For use in KeyNote credentials, the ASN1 DER-encoded object is then ASCII-encoded (e.g., as a string of hex digits or base64 characters).
X.509 keys encoded in this way in KeyNote must be identified by the "x509-XXX:" algorithm name, where XXX is an ASCII encoding ("hex" or "base64"). Other ASCII encoding schemes may be defined in the future.
For comparison purposes, the Subject public key in X.509 certificates is used in the normalized form described in Section 2 of [RFC2792]. The resulting RSA or DSA key is then used for comparing, per [RFC2792]. All X.509 key comparisons in KeyNote occur between normalized forms. Note that this allows for comparison between a directly encoded RSA or DSA key (as specified in RFC 2792) and the same key when contained in an X.509 certificate.
X.509 key identifier signatures are defined for historical reasons. Implementers are encouraged to use the RSA- or DSA-based signature encodings instead.
X.509 key identifier signatures in KeyNote are identical to RSA- or DSA-based signatures [RFC2792]. The only difference is that the public key corresponding to the private key that generated the signatures is encoded in an X.509 certificate in the Authorizer field of the signed credential assertion. However, an RSA- or DSA-based signature encoding (depending on the Subject key contained in the X.509 certificate itself) may be used instead.
X.509 key identifier signatures in KeyNote are computed over the assertion body (starting from the beginning of the first keyword, up to and including the newline character immediately before the "Signature:" keyword) and the signature algorithm name (including the trailing colon character, e.g., "sig-x509-sha512-base64:")
X.509 key identifier signatures are encoded as an ASN1 OCTET STRING object, containing the signature value.
Keromytis Informational [Page 3]
RFC 5708 X.509 Encoding for KeyNote January 2010
For use in KeyNote credentials, the ASN1 OCTET STRING is then ASCII- encoded (as a string of hex digits or base64 characters).
X.509 key identifier signatures encoded in this way in KeyNote must be identified by the "sig-x509-XXX-YYY:" algorithm name, where XXX is a hash function name (see Section 5 and Section 7 of this document) and YYY is an ASCII encoding ("hex" or "base64").
5. Hash Functions For RSA, DSA, and X.509 Key Identifier Signatures
For historical reasons (backward compatibility), X.509 key identifier signatures SHOULD support SHA1 as the hash function, using the "sha1" keyword. In addition, SHA256, SHA512, and RIPEMD160 ([SHA256+], [SHA2-2], [RIPEMD-160]) signatures MUST be supported for use with X.509 key identifier signatures, by using the "sha256", "sha512", and "ripemd160" keywords, respectively (see Section 7).
In addition, SHA256, SHA512, and RIPEMD160 signature identifiers are defined for RSA signatures, using the "sha256", "sha512", and "ripemd160" keywords, respectively (see Section 7).
This document discusses the format of X.509 keys and signatures as used in KeyNote. The security of KeyNote credentials utilizing such keys and credentials is directly dependent on the strength of the related public key algorithms. On the security of KeyNote itself, see [KEYNOTE]. Furthermore, it is the responsibility of the application developer to ensure that X.509 certificates are valid (signed by a trusted authority, not expired, and not revoked).
The use of SHA1 as part of signatures and key identifiers is discouraged, because of the various weaknesses in the algorithm that have been identified in recent years.
[SHA256+] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.