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Re: [webauthn] Device-bound key extension (#1658)

From: =JeffH via GitHub <sysbot+gh@w3.org>
Date: Fri, 19 Nov 2021 19:15:18 +0000
To: public-webauthn@w3.org
Message-ID: <issue_comment.created-974338933-1637349316-sysbot+gh@w3.org>
> Can the device private key sign over just clientDataHash instead of (clientDataHash || userCredentialId)? I understand the need to "bind" the device private key to a user credential, but I was thinking the RP can just remember which user credential the device private key is associated with the first time it sees it. 

Signing over userCredentialId prudently yields a cryptographic binding to the user credential, strongly demonstrating the device private key had access to the userCredentialId.

When verified by a RP, it will help prevent the RP being possibly snookered by, say, some form of malware-induced substitution attack (although I do not have such a worked-out attack scenario at this time).

Do you anticipate or find it an implementation hardship for the device private key to sign over both clientDataHash and userCredentialId ?

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