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definition of a webauthn varsig type #11

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5c80c39
WIP definition of a webauthn varsig type
chunningham Aug 8, 2023
677c0f7
re-use varsig body in webauthn varsig, slim it down
chunningham Oct 4, 2023
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26 changes: 26 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -390,6 +390,28 @@ sig-bytes = 128(OCTET)
| `es512-varsig-header` | `%x1202` | `%x8224` | P-521 [multicodec] prefix |
| `es512-hash-algorithm` | `%x13` | `%x13` | SHA2-512 [multicodec] prefix |

### 5.3.4 Example: Webauthn

``` abnf
webauthn-varsig = webauthn-varsig-header client-data-json authenticator-data-json sig-bytes

webauthn-varsig-header = TODO
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@Gozala Gozala Feb 4, 2026

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Can we put some placeholder code here ?

client-data-json = JSON
authenticator-data-length = 1*unsigned-varint
authenticator-data = OCTET
sig-bytes = OCTET
```

The Webauthn varsig header notifies the consumer that the signature is generated via webauthn. Verification must therefor rely on the [`client-data-json`][Webauthn Client Data JSON] JSON object and the [`authenticator-data`][Webauthn Authenticator Data] bytestring. The `client-data-json` object is self-describing and thus does not need to have a length prefix, however it must include the required fields as specified by the Webauthn spec. The `authenticator-data` can vary in length but must have at least 37 bytes, and so requires the `authenticator-data-length` varint to specify it's length. The length of `sig-bytes` can be known via the [`attestedCredentialData`][Webauthn Attested Credential Data] portian of the [`authenticator-data`][Webauthn Authenticator Data] byte string. In order to keep the varsig verifiable and concise, the signed payload MUST be included via CID as the `challenge` field of the [`client-data-json`][Webauthn Client Data JSON].
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@bumblefudge bumblefudge Aug 8, 2023 •
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reminder: say this isn't the PRF thing that's currently an extension but will be core webauthn some day (as per brook's comment in today's meeting)


Verification of this varsig follows these steps:
1. extraction of the `client-data-json` and `authenticator-data` from the varsig.
2. extraction of a CID from `client-data-json`
3. matching of the extracted CID with the signed-over payload
4. extraction of the credential public key from the [`attestedCredentialData`][Webauthn Attested Credential Data] in `authenticator-data`.
5. `sha256` hashing of `client-data-json` and concatenation with the [`authenticator-data`][Webauthn Authenticator Data] (i.e. `concat(authenticator-data, sha256(client-data-json))`), according to the process defined as [Webauthn Signature Creation][Webauthn Signature Creation]
6. verification of the `sig-bytes` with the credential public key and the byte string resulting from step 5, according to the public key type specified by the credential public key object.

# 6 Further Reading

* [Canonicalization Attacks Against MACs and Signatures][canonicalization attacks]
Expand All @@ -416,3 +438,7 @@ sig-bytes = 128(OCTET)
[multicodec]: https://github.com/multiformats/multicodec
[raw binary multicodec]: https://github.com/multiformats/multicodec/blob/master/table.csv#L40
[unsigned varint]: https://github.com/multiformats/unsigned-varint
[Webauthn Client Data JSON]: https://w3c.github.io/webauthn/#dictionary-client-data
[Webauthn Authenticator Data]: https://w3c.github.io/webauthn/#sctn-authenticator-data
[Webauthn Signature Creation]: https://w3c.github.io/webauthn/#fig-signature
[Webauthn Attested Credential Data]: https://w3c.github.io/webauthn/#sctn-attested-credential-data