Network Working Group                                           M. Blaze
Request for Comments: 2792                                  J. Ioannidis
Category: Informational                             AT&T Labs - Research
                                                            A. Keromytis
                                                      U. of Pennsylvania
                                                              March 2000


             DSA and RSA Key and Signature Encoding for the
                    KeyNote Trust Management System

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This memo describes RSA and DSA key and signature encoding, and
   binary key encoding for version 2 of the KeyNote trust-management
   system.

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.
   For more details on KeyNote, see [BFIK99].  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 (depended on the public key algorithm used) for the
   purposes of any internal comparisons between keys.  For example, an

   RSA [RSA78] key may be encoded in base64 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.

   In some applications, other types of values, such as a passphrase or
   a random nonce, may be used as principal identifiers.  When these
   identifiers contain characters that may not appear in a string (as
   defined in [BFIK99]), a simple ASCII encoding is necessary to allow
   their use inside KeyNote assertions.  Note that if the identifier
   only contains characters that can appear in a string, it may be used
   as-is.  Naturally, such identifiers may not be used to sign an
   assertion, and thus no related signature encoding is defined.

   This document specifies RSA and DSA [DSA94] key and signature
   encodings, and binary key encodings for use in KeyNote.

2.  Key Normalized Forms

2.1  DSA Key Normalized Form

   DSA keys in KeyNote are identified by four values:

   - the public value, y
   - the p parameter
   - the q parameter
   - the g parameter

   Where the y, p, q, and g are the DSA parameters corresponding to the
   notation of [Sch96]. These four values together make up the DSA key
   normalized form used in KeyNote.  All DSA key comparisons in KeyNote
   occur between normalized forms.

2.2  RSA Key Normalized Form

   RSA keys in KeyNote are identified by two values:

   - the public exponent
   - the modulus

   These two values together make up the RSA key normalized form used in
   KeyNote.  All RSA key comparisons in KeyNote occur between normalized
   forms.

2.3  Binary Identifier Normalized Form

   The normalized form of a Binary Identifier is the binary identifier's
   data.  Thus, Binary Identifier comparisons are essentially binary-
   string comparisons of the Identifier values.

3.  Key Encoding

3.1  DSA Key Encoding

   DSA keys in KeyNote are encoded as an ASN1 SEQUENCE of four ASN1
   INTEGER objects.  The four INTEGER objects are the public value and
   the p, q, and g parameters of the DSA key, in that order.

   For use in KeyNote credentials, the ASN1 SEQUENCE is then ASCII-
   encoded (e.g., as a string of hex digits or base64 characters).

   DSA keys encoded in this way in KeyNote must be identified by the
   "dsa-XXX:" algorithm name, where XXX is an ASCII encoding ("hex" or
   "base64").  Other ASCII encoding schemes may be defined in the
   future.

3.2  RSA Key Encoding

   RSA keys in KeyNote are encoded as an ASN1 SEQUENCE of two ASN1
   INTEGER objects.  The two INTEGER objects are the public exponent and
   the modulus of the DSA key, in that order.

   For use in KeyNote credentials, the ASN1 SEQUENCE is then ASCII-
   encoded (e.g., as a string of hex digits or base64 characters).

   RSA keys encoded in this way in KeyNote must be identified by the
   "rsa-XXX:" algorithm name, where XXX is an ASCII encoding ("hex" or
   "base64").  Other ASCII encoding schemes may be defined in the
   future.

3.3  Binary Identifier Encoding

   Binary Identifiers in KeyNote are assumed to have no internal
   encoding, and are treated as a sequence of binary digits.  The Binary
   Identifiers are ASCII-encoded, similarly to RSA or DSA keys.

   Binary Identifiers encoded in this way in KeyNote must be identified
   by the "binary-XXX:" algorithm name, where XXX is an ASCII encoding
   ("hex" or "base64").  Other ASCII encoding schemes may be defined in
   the future.

4.  Signature Computation and Encoding

4.1  DSA Signature Computation and Encoding

   DSA 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-dsa-sha1-base64:")

   DSA signatures are then encoded as an ASN1 SEQUENCE of two ASN1
   INTEGER objects.  The two INTEGER objects are the r and s values of a
   DSA signature [Sch96], in that order.

   For use in KeyNote credentials, the ASN1 SEQUENCE is then ASCII-
   encoded (as a string of hex digits or base64 characters).

   DSA signatures encoded in this way in KeyNote must be identified by
   the "sig-dsa-XXX-YYY:" algorithm name, where XXX is a hash function
   name ("sha1", for the SHA1 [SHA1-95] hash function is currently the
   only hash function that may be used with DSA) and YYY is an ASCII
   encoding ("hex" or "base64").

4.2  RSA Signature Computation and Encoding

   RSA 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-rsa-sha1-base64:")

   RSA signatures are then encoded as an ASN1 OCTET STRING object,
   containing the signature value.

   For use in KeyNote credentials, the ASN1 OCTET STRING is then ASCII-
   encoded (as a string of hex digits or base64 characters).

   RSA signatures encoded in this way in KeyNote must be identified by
   the "sig-rsa-XXX-YYY:" algorithm name, where XXX is a hash function
   name ("md5" or "sha1", for the MD5 [Riv92] and SHA1 [SHA1-95] hash
   algorithms respectively, may be used with RSA) and YYY is an ASCII
   encoding ("hex" or "base64").

4.3  Binary Signature Computation and Encoding

   Binary Identifiers are unstructured sequences of binary digits, and
   are not associated with any cryptographic algorithm.  Thus, they may
   not be used to validate an assertion.

5.  Security Considerations

   This document discusses the format of RSA and DSA keys and signatures
   and of Binary principal identifiers 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 [BFIK99].

6.  IANA Considerations

   Per [BFIK99], IANA should provide a registry of reserved algorithm
   identifiers.  The following identifiers are reserved by this document
   as public key and binary identifier encodings:

   - "rsa-hex"
   - "rsa-base64"
   - "dsa-hex"
   - "dsa-base64"
   - "binary-hex"
   - "binary-base64"

   The following identifiers are reserved by this document as signature
   encodings:

   - "sig-rsa-md5-hex"
   - "sig-rsa-md5-base64"
   - "sig-rsa-sha1-hex"
   - "sig-rsa-sha1-base64"
   - "sig-dsa-sha1-hex"
   - "sig-dsa-sha1-base64"

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

7.  Acknowledgments

   This work was sponsored by the DARPA Information Assurance and
   Survivability (IA&S) program, under BAA 98-34.

References

   [Sch96]   Bruce Schneier, Applied Cryptography 2nd Edition, John
             Wiley & Sons, New York, NY, 1996.

   [BFIK99]  Blaze, M., Feigenbaum, J., Ioannidis, J. and A. Keromytis,
             "The KeyNote Trust-Management System Version 2", RFC 2704,
             September 1999.

   [DSA94]   NIST, FIPS PUB 186, "Digital Signature Standard", May 1994.

   [Riv92]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
             April 1992.

   [RSA78]   R. L. Rivest, A. Shamir, L. M. Adleman, "A Method for
             Obtaining Digital Signatures and Public-Key Cryptosystems",
             Communications of the ACM, v21n2. pp 120-126, February
             1978.

   [SHA1-95] NIST, FIPS PUB 180-1, "Secure Hash Standard", April 1995.
             http://csrc.nist.gov/fips/fip180-1.txt (ascii)
             http://csrc.nist.gov/fips/fip180-1.ps  (postscript)

Contacts

   Comments about this document should be discussed on the
   keynote-users@nsa.research.att.com mailing list.

   Questions about this document can also be directed to the authors as
   a group at the keynote@research.att.com alias, or to the individual
   authors at:

   Matt Blaze
   AT&T Labs - Research
   180 Park Avenue
   Florham Park, New Jersey 07932-0000

   EMail: mab@research.att.com


   John Ioannidis
   AT&T Labs - Research
   180 Park Avenue
   Florham Park, New Jersey 07932-0000

   EMail: ji@research.att.com


   Angelos D. Keromytis
   Distributed Systems Lab
   CIS Department, University of Pennsylvania
   200 S. 33rd Street
   Philadelphia, Pennsylvania  19104-6389

   EMail: angelos@cis.upenn.edu

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