WebAuthn: Is it possible to get public key from credentials.get

Question

When we send request for credential.create we get back attestationObject which contains PublicKey within CBOR format, but when we call credential.get we have only authenticatorData without PublicKey data. So is it possible to retrieve PublicKey from Passkey, after credential.create call?

The issue is when we created a Passkey we can have connectivity problem so the PublicKey store request to backend will fail, but passkey itself is already created and we can't check the signature without PublicKey next time user want to SignIn!

credential.create request

{
  "challenge": "nhkQXfE59Jb97VyyNJkvDiXucMEvltduvcrDmGrODHY",
  "rp": {
    "name": "CredMan App Test",
    "id": "credential-manager-app-test.glitch.me"
  },
  "user": {
    "id": "2HzoHm_hY0CjuEESY9tY6-3SdjmNHOoNqaPDcZGzsr0",
    "name": "[email protected]",
    "displayName": "[email protected]"
  },
  "pubKeyCredParams": [
    {
      "type": "public-key",
      "alg": -7
    },
    {
      "type": "public-key",
      "alg": -257
    }
  ],
  "timeout": 1800000,
  "attestation": "none",
  "excludeCredentials": [],
  "authenticatorSelection": {
    "authenticatorAttachment": "platform",
    "requireResidentKey": true,
    "residentKey": "required",
    "userVerification": "required"
  }
}

credential.create response

{
  "id": "KEDetxZcUfinhVi6Za5nZQ",
  "type": "public-key",
  "rawId": "KEDetxZcUfinhVi6Za5nZQ",
  "response": {
    "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uY3JlYXRlIiwiY2hhbGxlbmdlIjoibmhrUVhmRTU5SmI5N1Z5eU5Ka3ZEaVh1Y01Fdmx0ZHV2Y3JEbUdyT0RIWSIsIm9yaWdpbiI6ImFuZHJvaWQ6YXBrLWtleS1oYXNoOk1MTHpEdll4UTRFS1R3QzZVNlpWVnJGUXRIOEdjVi0xZDQ0NEZLOUh2YUkiLCJhbmRyb2lkUGFja2FnZU5hbWUiOiJjb20uZ29vZ2xlLmNyZWRlbnRpYWxtYW5hZ2VyLnNhbXBsZSJ9",
    "attestationObject": "o2NmbXRkbm9uZWdhdHRTdG10oGhhdXRoRGF0YViUj5r_fLFhV-qdmGEwiukwD5E_5ama9g0hzXgN8thcFGRdAAAAAAAAAAAAAAAAAAAAAAAAAAAAEChA3rcWXFH4p4VYumWuZ2WlAQIDJiABIVgg4RqZaJyaC24Pf4tT-8ONIZ5_Elddf3dNotGOx81jj3siWCAWXS6Lz70hvC2g8hwoLllOwlsbYatNkO2uYFO-eJID6A"
  }
}

credential.get request

{
  "challenge": "T1xCsnxM2DNL2KdK5CLa6fMhD7OBqho6syzInk_n-Uo",
  "allowCredentials": [],
  "timeout": 1800000,
  "userVerification": "required",
  "rpId": "credential-manager-app-test.glitch.me"
}

credential.get response

{ 
  "id": "KEDetxZcUfinhVi6Za5nZQ",
  "type": "public-key",
  "rawId": "KEDetxZcUfinhVi6Za5nZQ",
  "response": {
    "clientDataJSON": "eyJ0eXBlIjoid2ViYXV0aG4uZ2V0IiwiY2hhbGxlbmdlIjoiVDF4Q3NueE0yRE5MMktkSzVDTGE2Zk1oRDdPQnFobzZzeXpJbmtfbi1VbyIsIm9yaWdpbiI6ImFuZHJvaWQ6YXBrLWtleS1oYXNoOk1MTHpEdll4UTRFS1R3QzZVNlpWVnJGUXRIOEdjVi0xZDQ0NEZLOUh2YUkiLCJhbmRyb2lkUGFja2FnZU5hbWUiOiJjb20uZ29vZ2xlLmNyZWRlbnRpYWxtYW5hZ2VyLnNhbXBsZSJ9",
    "authenticatorData": "j5r_fLFhV-qdmGEwiukwD5E_5ama9g0hzXgN8thcFGQdAAAAAA",
    "signature": "MEUCIQCO1Cm4SA2xiG5FdKDHCJorueiS04wCsqHhiRDbbgITYAIgMKMFirgC2SSFmxrh7z9PzUqr0bK1HZ6Zn8vZVhETnyQ",
    "userHandle": "2HzoHm_hY0CjuEESY9tY6-3SdjmNHOoNqaPDcZGzsr0"
  }
}

Can't see anything in Webauthn doc according to that problem

Answer 1

Fundamentally, the other answers are correct: if you missed the registration, you can't trust a random public key that is presented at assertion time!

Narrowly, however, if you're using ECDSA (which is extremely likely) then, given a signed message and a signature (both of which you have at assertion time), you can solve for the public key modulo the sign of the curve point. With two signatures from the same key, you can resolve the sign bit too.

Answer 2

If the server is unable to save the public key for whatever reason during the registration, the server should fail the registration ceremony with an appropriate error message. Sounds like your implementation silently fails instead.

Answer 3

No. It is only needed on a create call and is not provided on get.

If the public key and credential ID are not tied to the account during a trusted create call (e.g. bound session), the credential should not be used.