On 2022-03-04 8:08, Carsten Bormann wrote:
On 2022-03-04, at 07:54, Anders Rundgren <[email protected]> wrote:
- Collect key and algorithm data from the authorization signature object.
- Save and Remove FIDO "authenticatorData" and FIDO "signature" from the CBOR
container.
This is what we called the “transform” in the beloved XMLDSig.
The complexities of this step can be the basis of interesting vulnerabilities
(or interoperability failures).
Since I had not worked with low-level encoders and decoders, I spent a couple
of days in the lab (kitchen actually).
To not be dependent on my own stuff (which of course works flawlessly since it
was from the beginning designed with FIDO in mind), I applied the more
universal CSF (CBOR Signature Format) to Laurence's excellent QCBOR library.
This is what I came up with:
https://github.com/cyberphone/D-CBOR/blob/main/verify-demo/csf-verifier.c
The actual transform part is performed by FOUR LINES of C. This was a surprise
even to me.
Carsten, you should be proud; CBOR is the by far best data interchange format
for blending with cool cryptographic constructs!
Could wrapping your precious data in bstr just in order to sign it, be headed
for obsolescence? :)
Regarding the other "hot" subject, deterministic serialization, the demo does
not depend on that since it operates directly on the CBOR data. However, high-level CBOR
tools that convert streaming CBOR to internal self-serializing objects, do:
https://github.com/cyberphone/openkeystore/blob/9470afdcd4d820489b3351c21b256d994930e422/library/src/org/webpki/cbor/CBORMap.java#L328
Cheers,
Anders
- Set "authorizationData" = re-serialized CBOR container.
(Using deterministic serialization.
Fortunately, that works much better in CBOR than in other serialization
formats.)
- Verify signature using ("authenticatorData" || sha256(authorizationData) as
signed data.
Now you have verified a signature as applied to the transformed data.
COSE differs in design by signing the actual data as interchanged.
(This is not as expensive to do in CBOR as in serialization formats that cannot
efficiently encode byte strings, i.e., binary data.)
Grüße, Carsten
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