added sign and verify func

goodchain/src/helpers/SignatureHelper.py

@@ -1,6 +1,8 @@
 from cryptography.exceptions import *
 from cryptography.hazmat.primitives.asymmetric import rsa
 from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives import hashes
+from cryptography.hazmat.primitives.asymmetric import padding
 
 def generateKeys():
     private_key = rsa.generate_private_key(public_exponent=65537,key_size=2048)
@@ -38,3 +40,30 @@ def bytesToKeys(private_ser, public_ser):
         public_ser
     )
     return private_key, public_key
+
+def sign(message, private_key):
+    message = bytes(str(message), 'utf-8')
+    signature = private_key.sign(
+        message,
+        padding.PSS(mgf=padding.MGF1(hashes.SHA256()), salt_length=padding.PSS.MAX_LENGTH),
+        hashes.SHA256()
+        )
+    return signature
+
+def verify(message, signature, pbc_ser):
+    message = bytes(str(message), 'utf-8')
+    public_key = serialization.load_pem_public_key(pbc_ser)
+    try:
+        public_key.verify(
+            signature,
+            message,
+            padding.PSS(mgf=padding.MGF1(hashes.SHA256()),
+            salt_length=padding.PSS.MAX_LENGTH),
+            hashes.SHA256()
+            )
+        return True
+    except InvalidSignature:
+         return False
+    except:
+        print("Error executing 'public_key.verify'")
+        return False