How to generate a proof of posession

[jfromaniello]

I am new to jsonld signatures and as I am learning about this, I need to be able to generate a proof of possession including an audience and a challenge.

Specifically I'm trying to implement this part of this spec:

https://openid.net/specs/openid-connect-4-verifiable-credential-issuance-1_0.html#section-6.6.1

The proof element MUST incoporate a fresh nonce value generate by the credential issuer and the credential issuer's identifier (audience) in order to allow the credential issuer to detect replay. The way those data is incorporated depends on the proof type. In a LD proof, for example, the nonce is included as challenge element in the proof object and the issuer (the intended audience) is included as domain element.

For which then is given this example:

{
  "type": "RsaSignature2018",
  "created": "2018-09-14T21:19:10Z",
  "proofPurpose": "authentication",
  "verificationMethod": "did:example:ebfeb1f712ebc6f1c276e12ec21/keys/1",
  "challenge": "2H4dB9xl-FZQL-pixV-WJk0eOt4CXQ-1NXKW",
  "domain": "https://issuer.example.com",
  "jws": "eyJhbGciOiJFZERTQSIsImI2NCI6ZmFsc2UsImNyaXQiOlsiYjY0Il19..l9d0YHjcFAH2H4dB9xlWFZQLUpixVCWJk0eOt4CXQe1NXKWZwmhmn9OQp6YxX0a2LffegtYESTCJEoGVXLqWAA",
}

Thank you very much!