Network Working Group Matt Blaze
Internet Draft John Ioannidis
Expires in six months AT&T Labs - Research
Angelos D. Keromytis
U. of Pennsylvania
January 2000
DSA and RSA Key and Signature Encoding for the
KeyNote Trust Management System
<draft-angelos-keynote-dsa-rsa-encoding-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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Abstract
This memo describes RSA and DSA key and signature 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 [BFIK]. This document assumes reader familiarity with the
KeyNote system.
Cryptographic keys in KeyNote are used 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.
This document specifies RSA and DSA [DSA94] key and signature
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
- the p parameter
- the q parameter
- the g parameter
For an explanation of the various parameters, see [AC2]. 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.
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.
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 begining 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 [AC2].
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] 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 begining 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 [MD5] and SHA1 [SHA1]
hash algorithms respectively, may be used with RSA) and YYY is an
ASCII encoding ("hex" or "base64").
5. Security Considerations
This document discusses the format of RSA and DSA 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 [BFIK].
6. IANA Considerations
Per [BFIK], IANA should provide a registry of reserved algorithm
identifiers. The following identifiers are reserved by this
document as public key encodings:
- "rsa-hex"
- "rsa-base64"
- "dsa-hex"
- "dsa-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"
References
[AC2] Bruce Schneier, Applied Cryptography 2nd Edition, John Wiley
& Sons, New York, NY, 1996.
[BFIK] M. Blaze, J. Feigenbaum, J. Ioannidis, A D. Keromytis, "The
KeyNote Trust-Management System Version 2", RFC 2704,
September 1999.
[DSA94] NIST, FIPS PUB 186, "Digital Signature Standard", May 1994.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
MIT and RSA Data Security, Inc., 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] 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 John Ioannidis
mab@research.att.com ji@research.att.com
AT&T Labs - Research
180 Park Avenue
Florham Park, New Jersey 07932-0000
Angelos D. Keromytis
angelos@dsl.cis.upenn.edu
Distributed Systems Lab
CIS Department, University of Pennsylvania
200 S. 33rd Street
Philadelphia, Pennsylvania 19104-6389
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