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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RFC 2792 Key and Signature Encoding for KeyNote March 2000
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
Blaze, et al. Informational [Page 6]RFC 2792 Key and Signature Encoding for KeyNote March 2000
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