Start adding derive() API.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # @digitalbazaar/vc ChangeLog
 
+## 6.1.0 - 2023-11-dd
+
+### Added
+- Add `derive()` API for deriving new verifiable credentials from
+  existing ones, for the purpose of selective disclosure or
+  unlinkable presentation.
+
 ## 6.0.2 - 2023-08-04
 
 ### Fixed

lib/index.js

@@ -42,7 +42,7 @@ export const defaultDocumentLoader =
   jsigs.extendContextLoader(_documentLoader);
 import * as credentialsContext from 'credentials-context';
 
-const {AuthenticationProofPurpose} = jsigs.purposes;
+const {AssertionProofPurpose, AuthenticationProofPurpose} = jsigs.purposes;
 const {constants: {CREDENTIALS_CONTEXT_V1_URL}} = credentialsContext;
 
 export {CredentialIssuancePurpose};
@@ -99,7 +99,7 @@ export const dateRegex = new RegExp('^(\\d{4})-(0[1-9]|1[0-2])-' +
  *
  * @param {object} options.credential - Base credential document.
  * @param {LinkedDataSignature} options.suite - Signature suite (with private
- *   key material), passed in to sign().
+ *   key material or an API to use it), passed in to sign().
  *
  * @param {ProofPurpose} [options.purpose] - A ProofPurpose. If not specified,
  *   a default purpose will be created.
@@ -144,6 +144,45 @@ export async function issue({
   return jsigs.sign(credential, {purpose, documentLoader, suite});
 }
 
+/**
+ * Derives a proof from the given verifiable credential, resulting in a new
+ * verifiable credential. This method is usually used to generate selective
+ * disclosure and / or unlinkable proofs.
+ *
+ * @param {object} [options={}] - The options to use.
+ *
+ * @param {object} options.verifiableCredential - The verifiable credential
+ *   containing a base proof to derive another proof from.
+ * @param {LinkedDataSignature} options.suite - Derived proof signature suite.
+ *
+ * Other optional params passed to `derive()`:
+ * @param {object} [options.documentLoader] - A document loader.
+ *
+ * @throws {Error} If missing required properties.
+ *
+ * @returns {Promise<VerifiableCredential>} Resolves on completion.
+ */
+export async function derive({
+  verifiableCredential, suite,
+  documentLoader = defaultDocumentLoader
+} = {}) {
+  if(!verifiableCredential) {
+    throw new TypeError('"credential" parameter is required for deriving.');
+  }
+  if(!suite) {
+    throw new TypeError('"suite" parameter is required for deriving.');
+  }
+
+  // run common credential checks
+  _checkCredential({credential: verifiableCredential, mode: 'issue'});
+
+  return jsigs.derive(verifiableCredential, {
+    purpose: AssertionProofPurpose,
+    documentLoader,
+    suite
+  });
+}
+
 /**
  * Verifies a verifiable presentation:
  *   - Checks that the presentation is well-formed