How to create IEEE P1363 signature

A SecKey instance that represents a key that is stored in a keychain can be safely cast to a SecKeychainItem for manipulation as a keychain item. On the other hand, if the key is not stored in a keychain, casting the object to a SecKeychainItem and passing it to Keychain Services functions returns errors.

Code Block 
 let tag = "com.example.keys.mykey".data(using: .utf8)!
    let getquery: [String: Any] = [kSecClass as String: kSecClassKey,
                    kSecAttrApplicationTag as String: tag,
                    kSecAttrKeyType as String: kSecAttrKeyTypeEC,
                    kSecReturnRef as String: true]
    var item: CFTypeRef?
    let status = SecItemCopyMatching(getquery as CFDictionary, &item)
    guard status == errSecSuccess else { return nil }
    if item == nil {
      return nil
    }
    let key = item as! SecKey 

Signature must be generated in IEEE P1363 format. If you generate
signatures in another format, such as the ASN.1 format, you must convert your signatures before sending

Code Block  var error1: Unmanaged<CFError>?
        guard let verified : Bool? = SecKeyVerifySignature(publicKey,
                      SecKeyAlgorithm.ecdsaSignatureMessageX962SHA256,
                      signatureString.data(using: .utf8)! as CFData,
                      signedData as CFData,
                        &error1) else {
          let e = error1!.takeRetainedValue() as Error
          print("Verify Error \(e.localizedDescription)")
          return signedString
        }