In the FCL_Webauthn.sol contract, the WebAuthn_format function currently encodes the provided clientChallenge using Base64. However, according to the updated WebAuthn specification, the challenge within the clientData should be base64url encoded. This discrepancy can lead to a mismatch when comparing the hash of the two, especially if the challenge contains URL-safe characters.
"challenge, of type DOMString
This member contains the base64url encoding of the challenge provided by the Relying Party. See the §13.1 Cryptographic Challenges security consideration."
"The term Base64url Encoding refers to the base64 encoding using the URL- and filename-safe character set defined in Section 5 of [RFC4648], with all trailing '=' characters omitted (as permitted by Section 3.2) and without the inclusion of any line breaks, whitespace, or other additional characters."
Potential Solution:
The encoding method used for clientChallenge in the WebAuthn_format function should be updated to use base64url without trailing = characters instead of plain base64. This would ensure that the resulting hashes are equivalent given the clientChallenge contains URL-safe characters and follows the specification precisely.
rdubois-crypto
commented
11 months ago
Description:
Issue:
In the
FCL_Webauthn.solcontract, theWebAuthn_formatfunction currently encodes the providedclientChallengeusing Base64. However, according to the updated WebAuthn specification, the challenge within theclientDatashould be base64url encoded. This discrepancy can lead to a mismatch when comparing the hash of the two, especially if the challenge contains URL-safe characters.Reference:
In the WebAuthn Level 1 specification:
Under section 5.10: Supporting Data Structures, specifically 5.10.1 "Client Data Used in WebAuthn Signatures", the challenge is defined as:
Additionally, in Section 3: Dependencies, Base64url encoding is specified as:
Potential Solution:
The encoding method used for
clientChallengein theWebAuthn_formatfunction should be updated to use base64url without trailing=characters instead of plain base64. This would ensure that the resulting hashes are equivalent given theclientChallengecontains URL-safe characters and follows the specification precisely.