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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

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 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.

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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.

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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.

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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.

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 [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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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

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