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Re: [TLS] TLS RSA-PSS and various versions of TLS

Dr Stephen Henson <lists@drh-consultancy.co.uk> Wed, 26 April 2017 14:16 UTC

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To: Ilari Liusvaara <ilariliusvaara@welho.com>, Martin Rex <mrex@sap.com>
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From: Dr Stephen Henson <lists@drh-consultancy.co.uk>
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Date: Wed, 26 Apr 2017 15:10:27 +0100
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Subject: Re: [TLS] TLS RSA-PSS and various versions of TLS
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On 26/04/2017 14:41, Ilari Liusvaara wrote:
> On Wed, Apr 26, 2017 at 03:23:57PM +0200, Martin Rex wrote:
>>
>> The issue with RSA-PSS digital signatures is that they were defined
>> with additional (unnecessary) parameters that are encoded (=hidden) in the
>> ASN.1 AlgorithmIdentifier, and that are therefore unspecified when
>> RSA-PSS is requested as (rsa-pss,sha-256) rather than with an ASN.1
>> AlgorithmIdentifier.
> 
> TLS 1.3 specifies what values those parameters have for various
> SignatureSchemes.
>  
>> The additional, unnecessary parameters are "saltLen" and
>> "MaskGenerationFunction" (MGF), and the commonly-used MaskGenerationFunction
>> (mgf1) adds yet another additional, unnecessary parameter (MGF-internal hash).
> 
> Also specified.
> 

For TLS message signatures yes.

For signatures on certificates I think it is far less clear. For salt lengths
the spec says:

"When used in signed TLS handshake messages, the length of the salt MUST be
 equal to the length of the digest output."

It says nothing about salt length restrictions (if any) on certificates.

Steve.
-- 
Dr Stephen N. Henson.
Core developer of the   OpenSSL project: http://www.openssl.org/
Freelance consultant see: http://www.drh-consultancy.co.uk/
Email: shenson@drh-consultancy.co.uk, PGP key: via homepage.

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