Here is round three with suggested changes from Hal and Ben.
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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.
Changes from both Hal and Ben.
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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 an even multiple of 64 bits long. 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 reminder why we really, really want people to use one of the
four DSS p/q pairs, plus a change with regards to referring people to
FIPS186 for the DSS spec. DSS has other requirements besides the four
p/q sizes and list of SHA hashes, and the original language implied
that using the right size and right hash was enough for DSS
compliance.
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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.
Change from Hal.
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David