X.509 Key and Signature Encoding for the KeyNote Trust Management System
RFC 5708

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

Abstract

   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.

   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/rfc5708.

Copyright Notice

   Copyright (c) 2010 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
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1.  Introduction

   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.

1.1. Conventions

   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].

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2.  X.509 Key Identifier Encoding

   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.

3.  X.509 Key Identifier Normalized Forms

   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.

4.  X.509 Signature Computation and Encoding

   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.

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   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).

6.  Security Considerations

   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.

7.  IANA Considerations

   Per [RFC2792], IANA has provided a registry of reserved algorithm
   identifiers.  The following are reserved by this document as KeyNote
   public key format identifiers:

   - "x509-hex"
   - "x509-base64"

   The following are reserved by this document as KeyNote signature
   algorithm identifiers:

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   - "sig-x509-sha1-hex"
   - "sig-x509-sha1-base64"
   - "sig-x509-sha256-hex"
   - "sig-x509-sha256-base64"
   - "sig-x509-sha512-hex"
   - "sig-x509-sha512-base64"
   - "sig-x509-ripemd160-hex"
   - "sig-x509-ripemd160-base64"
   - "sig-rsa-sha256-hex"
   - "sig-rsa-sha256-base64"
   - "sig-rsa-sha512-hex"
   - "sig-rsa-sha512-base64"
   - "sig-rsa-ripemd160-hex"
   - "sig-rsa-ripemd160-base64"

   Note that the double quotes are not part of the algorithm
   identifiers.

8. References

8.1.  Normative References

   [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.

8.2.  Informative References

   [KEYNOTE]    Blaze, M., Feigenbaum, J., Ioannidis, J., and A.
                Keromytis, "The KeyNote Trust-Management System Version
                2", RFC 2704, September 1999.

   [RFC2792]    Blaze, M., Ioannidis, J., and A. Keromytis, "DSA and RSA
                Key and Signature Encoding for the KeyNote Trust
                Management System", RFC 2792, March 2000.

   [RFC4648]    Josefsson, S., "The Base16, Base32, and Base64 Data
                Encodings", RFC 4648, October 2006.

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   [RIPEMD-160] 3.ISO/IEC 10118-3:1998, "Information technology -
                Security techniques - Hash-functions - Part 3: Dedicated
                hash-functions," International Organization for
                Standardization, Geneva, Switzerland, 1998.

   [SHA2-2]     NIST, "Descriptions of SHA-256, SHA-384, and SHA-512",
                May 2001, <http://csrc.nist.gov/publications/fips/
                fips180-3/fips180-3_final.pdf>.

9.  Acknowledgements

   The author would like to thank Jim Schaad for his review and comments
   on earlier versions of this document.

Author's Address

   Angelos D. Keromytis
   Department of Computer Science
   Columbia University
   Mail Code 0401
   1214 Amsterdam Avenue
   New York, New York 1007
   USA

   EMail: angelos@cs.columbia.edu

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