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

Marcus Brinkmann <marcus.brinkmann@rub.de> Sat, 25 January 2020 01:21 UTC

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To: Michael Richardson <mcr@sandelman.ca>
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From: Marcus Brinkmann <marcus.brinkmann@rub.de>
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Date: Sat, 25 Jan 2020 02:21:14 +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 6:00 PM, Michael Richardson wrote:
> 
> Marcus Brinkmann <marcus.brinkmann=40rub.de@dmarc.ietf.org> wrote:
>     >> Does this mean, comparing a 20 bytes (40 hex digits) fingerprint, as
>     >> printed by e.g. GnuPG 2.2.x, is no longer a reliable way to verify you
>     >> have obtained the correct key?
> 
>     > The answer to this would formally be "yes", because after creating two
>     > such keys, the attacker could first show you one key, and, later on show
>     > you the other key and if the only thing you remember about the first key
>     > is the fingerprint, you have no way to notice the swap.
> 
> Would the attacker have to control the private keys of both generated keys to
> accomplish this?  I don't entirely see why.

As the collision I am thinking of happens in the modulus MPI, the
attacker would control the modulus and thus the private exponent (public
exponent fixed at 2^16+1).

> Clearly the signatures generated by the two keys (with identical
> fingerprints) would also be different (assume that the signatures were
> calculated on a SHA256 hash, to remove an attack from that side).

Yes. Any signatures made by these keys would be different.

Thanks,
Marcus

-- 
Dipl.-Math. Marcus Brinkmann

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