Add signature validation method

Signed-off-by: Leonid Lygin <ionagamed@gmail.com>

iroha/iroha.py

@@ -102,6 +102,23 @@ def sign_query(query, private_key):
         query.signature.CopyFrom(signature)
         return query
 
+    @staticmethod
+    def is_signature_valid(message, signature):
+        """
+        Verify signature validity.
+        :param signature: the signature to be checked
+        :param message: message to check the signature against
+        :return: bool, whether the signature is valid for the message
+        """
+        message_hash = IrohaCrypto.hash(message)
+        try:
+            signature_bytes = binascii.unhexlify(signature.signature)
+            public_key = binascii.unhexlify(signature.public_key)
+            ed25519.checkvalid(signature_bytes, message_hash, public_key)
+            return True
+        except (ed25519.SignatureMismatch, ValueError):
+            return False
+
     @staticmethod
     def reduced_hash(transaction):
         """