On 29 Mar 2006, at 8:37 AM, David Shaw wrote:

Here is round four. Only little fiddle changes at this point.
==================================
Section 5.2.2 (Version 3 Signature Packet Format) says:

` DSA signatures MUST use hashes with a size of 160 bits, to
``match q,
` the size of the group generated by the DSA key's generator value.
The hash function result is treated as a 160 bit number and used
directly in the DSA signature algorithm.
change to:
DSA signatures MUST use hashes that are equal in size to the
number of bits of q, the group generated by the DSA key's
generator value. If the output size of the chosen hash is larger
than the number of bits of q, the hash result is truncated to fit
by taking the number of leftmost bits equal to the number of bits
of q. This (possibly truncated) hash function result is treated
as a number and used directly in the DSA signature algorithm.

Done.

No change.
==================================
Section 12.5. (DSA) says:
An implementation SHOULD NOT implement DSA keys of size less than

` 1024 bits. Note that present DSA is limited to a maximum of
``1024 bit
` keys, which are recommended for long-term use. Also, DSA keys MUST
be an even multiple of 64 bits long.
change to:
An implementation SHOULD NOT implement DSA keys of size less than
1024 bits or with a q size of less than 160 bits. DSA keys MUST
also be a multiple of 64 bits, and the q size MUST be a multiple
of 8 bits. The Digital Signature Standard (DSS) [FIPS186]
specifies that DSA be used in one of the following ways:

` * 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384 or
``SHA-512 hash
`` * 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384 or SHA-512
``hash
` * 2048-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
* 3072-bit key, 256-bit q, SHA-256, SHA-384 or SHA-512 hash
The above key and q size pairs were chosen to best balance
the strength of the key with the strength of the hash.
Implementations SHOULD use one of the above key and q size pairs
when generating DSA keys. If DSS compliance is desired, one
of the specified SHA hashes must be used as well. [FIPS186]
is the ultimate authority on DSS, and should be consulted for all
questions of DSS compliance.
Note that earlier versions of this standard only allowed a
160-bit q with no truncation allowed, so earlier implementations
may not be able to handle signatures with a different q size or a
truncated hash.
Added a MUST that the q size is a multiple of 8. I don't think any of
us want to deal with hashes that don't end on a byte boundary.

Done, but I said that you MUST not use a q less than 160 bits.

==================================
Section 13. (Security Considerations) says:
* The DSA algorithm will work with any 160-bit hash, but it is
sensitive to the quality of the hash algorithm, if the hash
algorithm is broken, it can leak the secret key. The Digital
Signature Standard (DSS) specifies that DSA be used with SHA-1.

` RIPEMD-160 is considered by many cryptographers to be as
``strong.
` An implementation should take care which hash algorithms are
used with DSA, as a weak hash can not only allow a signature to
be forged, but could leak the secret key.
change to:
* The DSA algorithm will work with any hash, but is sensitive to
the quality of the hash algorithm. Verifiers should be aware
that even if the signer used a strong hash, an attacker could
have modified the signature to use a weak one. Only signatures
using acceptably strong hash algorithms should be accepted as
valid.
Also add:
* As OpenPGP combines many different asymmetric, symmetric, and
hash algorithms, each with different measures of strength, care
should be taken that the weakest element of an OpenPGP message
is still sufficiently strong for the purpose at hand. While
consensus about the the strength of a given algorithm may
evolve, at publication time, NIST Special Publication 800-57
[SP800-57] recommended the following list of equivalent
strengths:
Asymmetric | Hash | Symmetric
key size | size | key size
------------+--------+-----------
1024 160 80
2048 224 112
3072 256 128
7680 384 192
15360 512 256
Added the key size reminder.

`Done with various small edits. I had to fight with the formatting
``program. Here's what I did:
`
* As OpenPGP combines many different asymmetric, symmetric, and
hash algorithms, each with different measures of strength, care
should be taken that the weakest element of an OpenPGP message
is still sufficiently strong for the purpose at hand. While
consensus about the the strength of a given algorithm may
evolve, NIST Special Publication 800-57 [SP800-57] recommends
the following list of equivalent strengths:
Asymmetric | Hash | Symmetric
key size | size | key size
------------+--------+-----------
1024 160 80
2048 224 112
3072 256 128
7680 384 192
15360 512 256

==================================
David

Added in reference to SP800-57.
Jon