WIP refactor issuance protocol text

[wip-abramson]

This is a start to address the issue I raised in #181.

Not made it all the way through the spec yet - at about Section 7. I seem to be making a fair few changes here, so comments and feedback welcome as I go.

Also, I recommend we update the following diagrams:

• AnonCreds data objects in the introduction. Suggest: remove blinded link secret from holder entirely
• 8. Issuance Flow. Suggest we add a step before 13 that is something like: Remove blinding factor

[wip-abramson]

Getting there...

I am not fully certain how the blinding factor is generated in RSA, so we should check this.

[wip-abramson]

Okay, I think the last piece is around 8.4 Receiving a Credential.

I need to look into the protocol, but I am pretty certain there should be a step here that transforms the credential signature from the issuer (A,e,v) into (A',e',v') using the blinding factor. Where the credential signature stored by the holder is (A',e',v').

[wip-abramson]

So I checked the CL signature code.

This is the line where the prover does the operation to remove the blinding from the credential signature L635. Called from L237 in process_credential_signature.

This line is

p_cred.v = v_prime.add(&p_cred.v)?;

Which mutates the V value of the credential signature. Note that v_prime, is the credential blinding factor.

[wip-abramson]

Cool! Apologies I counldn't make the call

[swcurran]

Thanks for the PR and issues, @wip-abramson. Working on the spec today — continuing the cleanup.