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Re: [webauthn] FIDO U2F Attestation Statement Format doesn't say what to do with Counter

From: Adam Langley via GitHub <sysbot+gh@w3.org>
Date: Wed, 16 Aug 2017 17:58:02 +0000
To: public-webauthn@w3.org
Message-ID: <issue_comment.created-322850700-1502906277-sysbot+gh@w3.org>
I believe that the signature counter was a mistake because of its limited utility and privacy risks.

The FIDO spec hints that the signature counter may be used to identify duplicated keys, but provides no guidance about how they see that happening. If a token fails to write to flash and thus repeats a counter, should sites reject that token and require a reenrollment? How is the latter to be authenticated? It seems to me that the base-rate of false positives would completely swamp the _extremely_ rare true-positives and thus nobody would bother.

On the other hand, in order to only need a constant amount of storage, U2F tokens generally maintain a single, token-wide counter. Thus the counter is a privacy leak between sites and adds a requirement that tokens maintain state.

Current U2F devices emit this counter and that has to be taken into account, but in #453 I suggest repurposing it as a randomisation field. This gives tokens (that want to use it as such) a defense against side-channel attacks and would allow future tokens to fix this privacy leak.

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Received on Wednesday, 16 August 2017 17:58:07 UTC

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