fix(identity): convert hex private key before signing messages

The ZK-Kit EdDSA Poseidon package only supports the following private key types: text, buffer. The
Semaphore identity supports text and hexadecimal strings. If the identity private key is an
hexadicimal string it needs to be converted before being passed to any ZK-Kit function.

re #733

packages/identity/src/index.ts

@@ -112,7 +112,11 @@ export class Identity {
      * @returns A {@link https://zkkit.pse.dev/types/_zk_kit_eddsa_poseidon.Signature.html | Signature} object containing the signature components.
      */
     public signMessage(message: BigNumberish): Signature<bigint> {
-        return signMessage(this.privateKey, message)
+        const privateKey = isHexadecimal(this.privateKey, false)
+            ? hexadecimalToBuffer(this.privateKey)
+            : this.privateKey
+
+        return signMessage(privateKey, message)
     }
 
     /**

packages/identity/tests/index.test.ts

@@ -54,12 +54,20 @@ describe("Identity", () => {
     })
 
     describe("# verifySignature", () => {
-        it("Should verify a signature", () => {
+        it("Should verify a signature with a text private key", () => {
             const identity = new Identity(privateKeyText)
 
             const signature = identity.signMessage("message")
 
             expect(Identity.verifySignature("message", signature, identity.publicKey)).toBeTruthy()
         })
+
+        it("Should verify a signature hexadecimal private key", () => {
+            const identity = new Identity(privateKeyHexadecimal)
+
+            const signature = identity.signMessage("message")
+
+            expect(Identity.verifySignature("message", signature, identity.publicKey)).toBeTruthy()
+        })
     })
 })