Create common package

Moves cryptographer to common. Also adds constants and defines the leghtn of the locator to avoid hardcoding it in almost every file

common/cryptographer.py

@@ -0,0 +1,51 @@
+from hashlib import sha256
+from binascii import unhexlify, hexlify
+from cryptography.exceptions import InvalidTag
+from cryptography.hazmat.primitives.ciphers.aead import ChaCha20Poly1305
+
+from pisa.logger import Logger
+
+logger = Logger("Cryptographer")
+
+
+# FIXME: Cryptographer is assuming AES-128-GCM and SHA256 since they are the only pair accepted by the encrypted blob
+#   and the only pair programmed so far.
+class Cryptographer:
+    @staticmethod
+    # ToDo: #20-test-tx-decrypting-edge-cases
+    def decrypt(encrypted_blob, key, rtype="hex"):
+        if rtype not in ["hex", "bytes"]:
+            raise ValueError("Wrong return type. Return type must be 'hex' or 'bytes'")
+
+        if len(encrypted_blob.data) % 2:
+            logger.info(
+                "Incorrect (Odd-length) value to be decrypted.", encrypted_blob=encrypted_blob.data, dispute_txid=key
+            )
+            return None
+
+        # sk is the H(txid) (32-byte) and nonce is set to 0 (12-byte)
+        sk = sha256(unhexlify(key)).digest()
+        nonce = bytearray(12)
+
+        logger.info(
+            "Creating new blob.",
+            sk=hexlify(sk).decode(),
+            nonce=hexlify(nonce).decode(),
+            encrypted_blob=encrypted_blob.data,
+        )
+
+        # Decrypt
+        cipher = ChaCha20Poly1305(sk)
+        data = unhexlify(encrypted_blob.data.encode())
+
+        try:
+            blob = cipher.decrypt(nonce=nonce, data=data, associated_data=None)
+
+            # Change the blob encoding to hex depending on the rtype (default)
+            if rtype == "hex":
+                blob = hexlify(blob).decode("utf8")
+
+        except InvalidTag:
+            blob = None
+
+        return blob