After an issuer has completed the "Save Schema and Cred Def" and "Issue Credential" how-tos, you have all the context for a credential holder and a relying party (verifier) to generate a zero-knowledge proof based on the credential.
In case of troubles running the how-to, please read the trouble shooting section.
Setup your workstation with an indy development virtual machine (VM). See prerequisites.
In your normal workstation operating system (not the VM), open a text editor of your
choice and paste the template code of one of the available in the list bellow into
a new file and saved it as negotiate_proof.EXT, replacing EXT with the proper file
extension (e.g for python: negotiate_proof.py, for nodejs: negotiate_proof.js, and so on).
We will be modifying this code in later steps.
[ Python template | Java template | Node.js template | Rust template]
This is a very simple app framework into which you'll plug the code you'll be writing.
This how-to builds on the work in "Issue Credential". Rather than duplicate our explanation of those steps here, we will simply copy that code as our starting point.
Copy the contents of the correspondent step2 file below into your negotiate_proof file
instead of the Step 2 code goes here placeholder comment, and save it.
[ Python step2 | Node.js step2 | Rust step2]
Proof negotiation typically begins when a verifier (also called a relying party) requests proof. (As with credential issuance, the process has three logical phases, but it is rare to begin with a proof offer. However, if an initial proof request is met with a counter-offer, the offering phase of the sequence becomes relevant.)
A proof request is a JSON file that describes what sort of proof would satisfy the relying party.
Once the proof request is received, a holder of credentials must scan their identity wallet to find out which credentials could be used to satisfy the request. (Wallet scanning is inefficient, but this does not cause problems for dozens or hundreds of credentials. At higher scale, a new mechanism is needed. Work is underway to add index-driven search to indy wallets. Visit #indy-sdk on Rocket.Chat to learn more.)
Copy the contents of the correspondent step3 file below into your negotiate_proof
file instead of the Step 3 code goes here placeholder comment.
[ Python step3 | Node.js step3 | Rust step3]
At this point, the holder becomes a prover by generating and presenting
a proof. This is done by building some JSON that selects the credentials
(out of those identified as valid candidates in the previous step),
that the prover wishes to use to satisfy the request. The prover calls
prover_create_proof function with appropriate parameters, and the
proof is created.
Copy the contents of step4 file below into your negotiate_proof file instead of
the Step 4 code goes here placeholder comment and save it.
[ Python step4 | Node.js step4 | Rust step4]
Finally, the verifier needs to check to be sure the proof that's presented
satisfies their criteria. This is easy; just call verifier_verify_proof function.
Copy the contents of step5 file below into negotiate_proof file instead of
the Step 5 code goes here placeholder comment and save it.
[ Python step5 | Node.js step5 | Rust step5]
Run the completed demo and observe the whole sequence.
[ Python complete | Node.js complete | Rust complete]
You might try the "Send a Secure Message" how-to.