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Re: [Cfrg] Hash substitution in PSS

"Manger, James H" <James.H.Manger@team.telstra.com> Wed, 01 May 2013 00:44 UTC

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From: "Manger, James H" <James.H.Manger@team.telstra.com>
To: Russ Housley <housley@vigilsec.com>, IRTF CFRG <cfrg@irtf.org>
Date: Wed, 01 May 2013 10:44:31 +1000
Thread-Topic: [Cfrg] Hash substitution in PSS
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Subject: Re: [Cfrg] Hash substitution in PSS
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> I did not intend to pick the scab off the absent vs NULL discussion.
> My point was that a shorthand for four default values would have been
> desirable, but it is way too late.

Surely a shorthand for RSASSA-PSS-params that use the 4 default values is precisely what the use of DEFAULT in its ASN.1 definition provides.

The DER-encoding of RSSASA-PSS-params that use SHA-1, MGF1 with SHA-1, a 20-byte salt, and the 0xBC trailer byte is:

  30 00

Those 2 bytes are just as short as the DER-encoding of NULL (05 00).

--
James Manger


>> RFC 4055 says "RSASSA-PSS-params  ::=  SEQUENCE  {
>>        hashAlgorithm      [0] HashAlgorithm DEFAULT
>>                                  sha1Identifier,
>>        maskGenAlgorithm   [1] MaskGenAlgorithm DEFAULT
>>                                  mgf1SHA1Identifier,
>>        saltLength         [2] INTEGER DEFAULT 20,
>>        trailerField       [3] INTEGER DEFAULT 1  }"

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