Re: [CFRG] Threshold Sig required - Random bit flip hits Cert Transparency Log

Ben Laurie <> Thu, 08 July 2021 10:37 UTC

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From: Ben Laurie <>
Date: Thu, 08 Jul 2021 11:37:18 +0100
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To: Phillip Hallam-Baker <>
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Subject: Re: [CFRG] Threshold Sig required - Random bit flip hits Cert Transparency Log
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On Wed, 7 Jul 2021 at 17:04, Phillip Hallam-Baker <>

> So it has actually happened, a one in a billion computing error has caused
> a cert transparency log to become corrupted due to a bit flip error. There
> is a discussion of the issue here:
> Single random bit flip causes error in certificate transparency log |
> Hacker News (
> <>
> The solution they obsess over (ECC RAM) is actually irrelevant to the
> error in that case as it was an even rarer CPU error. Which means that what
> I considered to be a more or less theoretical concern about signing append
> only logs turns out to have actually occurred. Do things billions of times
> and billion to one chances will happen.
> The only robust solution to this issue is for redundant notaries to sign
> the log.

That's not true - the solution CT uses already is to have multiple
redundant logs.

> Consider the case where we have an append only log that is authenticated
> by means of a Merkle tree with the apex of the tree being signed at 10
> minute intervals. If we have a single server doing the signing, any error
> that occurs will lead to the log becoming invalid. This condition cannot be
> distinguished from a notary default.
> But consider the case where there are three notaries each signing the log,
> which private key should they use?
> All three signers use the same key means that if an error occurs, we risk
> having a correct and incorrect version of the same log being signed. That
> means there is a real risk of the incorrect log and signature leaking
> somehow.
> All three signers using different keys is also bad because now we have
> three independent notaries and the relying party has to do the job of
> deciding which one to trust. There is an even greater risk of the wrong log
> being relied on at some point.
> A threshold scheme with three shares and a quorum of 2 solves the problem
> very neatly. The possibility of an undetected error is now much smaller as
> two signers must be hit with an error having exactly the same effect at the
> same time. That is a very remote possibility unless the error is somehow
> caused by an architectural defect in the CPU. So we should probably choose
> separate chipset architectures (Intel, AMD, ARM?) if we want to get to
> maximum robustness.

This seems no more robust to me than the existing scheme - in fact it is
probably less robust because it is more complex.

Also, I'm not sure why a quorum helps - logs are verifiably correct (or
not), so what's the quorum for? That requirement also reduces robustness.

> So how is the FROST draft coming?
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