SHA1 collision found

[Leo Gaspard]

On 02/23/2017 09:00 PM, Robert J. Hansen wrote:
> [...]
>
> To which I said, "Create two keys with the same fingerprint.  Sign a contract with one, then renege on the deal.  When you get called into court, say "I never signed that, Your Honor!" and present the second key.  This collision pretty much shatters the nonrepudiability of SHA-1 signatures."
> 
> To which Peter quite reasonably answered that the other person has a copy of the public key which was used to sign the document originally.  Why should the fraudster's denial be believed?
> 
> The answer is that to enforce a contract (at least here in the United States) you must be able to prove, based on a preponderance of the evidence, that the other person entered into a contract with you.  So imagine this conversation:
> 
> PLAINTIFF: "Your Honor, the defendant reneged on a $10,000 contract.  Make him pay up."
> DEFENDANT: "I never signed anything, Your Honor."
> PLAINTIFF: "I have his key, it's right here."
> DEFENDANT: "That's not my key.  This is my key."
> PLAINTIFF: "Of course that's what he claims!  They have the same SHA-1 fingerprint!  He did that in order to deny his signature!"
> JUDGE: "So these keys are uniquely identified by the fingerprint?"
> (both parties agree)
> JUDGE: "And you have two keys that are identified by the same fingerprint?"
> (both parties agree)
> JUDGE: "And there's no way to tell which key is real?"
> (both parties agree)
> JUDGE: "Then we're stuck.  There's no reason to prefer one key over another.  Plaintiff, you have failed your burden of proof in establishing the defendant signed the contract."

I'd like to respectfully disagree on this point. SHA1 is currently
vulnerable only to collision attacks, which means that in order to have
two keys with the same fingerprint they both have to be created by the
same person (up to a random collision). Thus the defendant. And this is
enough to prove that he did sign the contract with the key he claims he
doesn't own.

Is there any flaw in this logic?

> [...]

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