Signature from Challenge

[richard redpath]

I got  the registration of the Publickey working fine with the Authenticator hardware device and Now the Authentication is next.
The only issue I have here is what is exactly signed?
The challenge is received from the FIDO server.
I fix the challenge to "*richardredpath*" so tracing is easy. Its always the same.
I take this challenge  "*richardredpath*"and 256 hash and sign the hash, thats my signature.
Its not really  clear anywhere what the signature is on other than I assume the challenge is signed via 256hash it and then sign.

  "response": {
                   "authenticatorData": bufferToBase64url(authData),
                   "clientDataJSON":    base64URLencode(JSON.stringify( sampleClientDataJSON ) ),
                   "signature":         bufferToBase64url(hexToBytes( signature).buffer),
                 },

So this should be easy to reply.

[My1]

Hi,

According to the webAuthn Specs the sig is formed over the Authenticator Data and the Client Data Hash

https://www.w3.org/TR/webauthn-2/#fig-signature

also please dont forget to verify the signature when registering too.

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[richard redpath]

Yes I did see that URL which talks about the concatenation

https://www.w3.org/TR/webauthn-2/#fig-signature

so if my AuthenticatorData is this
"49960de5880e8c687434170f6476605b8fe4aeb9a28632c7995cf3ba831d97631d00000000"

which is really this in its parts
  var RPIDhash = "49960de5880e8c687434170f6476605b8fe4aeb9a28632c7995cf3ba831d9763";  //256 hash of "localhost"  (32 bytes)
  var flags        = "1d";   //1 byte
  var counter    = "00000000"; //do not need one 4 bytes


and my clientData is this data string.

{"type":"webauthn.get","challenge":"KnJpY2hhcmRyZWRwYXRoKg","origin":"http://localhost:5000","crossOrigin":false,"other_keys_can_be_added_here":"do not compare clientDataJSON against a template. See https://goo.gl/yabPex"}

which is hashed to this
78bb32a61d3e7e48c1b2e161ac086a9b3b1d626c24e9ce115b533782a1aa833a

The concatenation is this for the message.
49960de5880e8c687434170f6476605b8fe4aeb9a28632c7995cf3ba831d97631d0000000078bb32a61d3e7e48c1b2e161ac086a9b3b1d626c24e9ce115b533782a1aa833a

which is what I see as a dump

message
b'49960de5880e8c687434170f6476605b8fe4aeb9a28632c7995cf3ba831d97631d0000000078bb32a61d3e7e48c1b2e161ac086a9b3b1d626c24e9ce115b533782a1aa833a'

Now Do I take this message (binary data) and sign it for the Signature?

 I would take this binary data 

b'49960de5880e8c687434170f6476605b8fe4aeb9a28632c7995cf3ba831d97631d0000000078bb32a61d3e7e48c1b2e161ac086a9b3b1d626c24e9ce115b533782a1aa833a'

and hash256 it to this
cf6a03cee7ee0340986f9cd02a50ef3a442a809b737ba6aa74d23711102a2165

and then sign the Hash256 for the signature.
I assume the FIDO server takes the message and makes a hash256 to verify it with the signature.

Thanks in advance. 

[richard redpath]

And that all worked nicely

Thanks for the help. Register and Authenticate sandbox works fine for the roaming Authenticator

[My1]

nice to hear that all worked out

[Muhammad Bilal]

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