feat(identity): add generate commitment function (#877)

This static method is particularly useful after signature verification, as it allows retrieval of
the corresponding commitment associated with the public key.

re #873

packages/identity/src/index.ts

@@ -138,4 +138,20 @@ export class Identity {
     static verifySignature(message: BigNumberish, signature: Signature, publicKey: Point): boolean {
         return verifySignature(message, signature, publicKey)
     }
+
+    /**
+     * Generates the commitment from the given public key.
+     * This static method is particularly useful after signature verification,
+     * as it allows retrieval of the corresponding commitment associated with the public key.
+     *
+     * @example
+     * const identity = new Identity()
+     * Identity.generateCommitment(identity.publicKey)
+     *
+     * @param publicKey The public key to generate the commitment.
+     * @returns The Semaphore identity commitment.
+     */
+    static generateCommitment(publicKey: Point): bigint {
+        return poseidon2(publicKey)
+    }
 }

packages/identity/tests/index.test.ts

@@ -138,4 +138,14 @@ describe("Identity", () => {
             expect(Identity.verifySignature("message", signature, identity.publicKey)).toBeTruthy()
         })
     })
+
+    describe("# generateCommitment", () => {
+        it("Should generate the identity commitment from the public key", () => {
+            const identity = new Identity(privateKeyText)
+
+            const commitment = Identity.generateCommitment(identity.publicKey)
+
+            expect(identity.commitment).toBe(commitment)
+        })
+    })
 })