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Re: [openpgp] "SHA-1 is a Shambles" and forging PGP WoT signatures

Marcus Brinkmann <marcus.brinkmann@rub.de> Sat, 25 January 2020 01:15 UTC

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To: Michael Richardson <mcr@sandelman.ca>, Damien Goutte-Gattat <dgouttegattat@incenp.org>
Cc: Kai Engert <kaie@kuix.de>, openpgp@ietf.org
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From: Marcus Brinkmann <marcus.brinkmann@rub.de>
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Date: Sat, 25 Jan 2020 02:15:51 +0100
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Subject: Re: [openpgp] "SHA-1 is a Shambles" and forging PGP WoT signatures
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Hi,

On 1/24/20 5:57 PM, Michael Richardson wrote:
> 
> Damien Goutte-Gattat <dgouttegattat@incenp.org> wrote:
>     > What Marcus says the author *could* have done is to generate the two keys A
>     > and B in such a way that they also have the same fingerprint. They have
>     > not
> 
> I'm not convinced that there are enough under-determined bytes that can be
> mutated in the content that goes into making the fingerprint.
> AFAIK, it's just the key.
> 
> I guess, maybe if the key is big enough (rsa 8K, bigger), that the bytes
> could be in the prime itself.  Are you saying that?  If so, I wonder what the
> smallest key for which this is true is.

The authors demonstrate a collision block within a RSA 6144 bit public
key.  The collision happens at the first 6056 bits of the modulus, and
the remaining 88 bits are used to make a valid modulus for the public
exponent, which is fixed at 2^16+1.

Thanks,
Marcus




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Dipl.-Math. Marcus Brinkmann

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