Subject: Suggested changes for DSA2, take 3

From: David Shaw <[email-masked]>
Date: Mon, 27 Mar 2006 10:31:17 -0500

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