Adds blob encryption on both sides

pisa-btc/pisa/encrypted_blob.py

@@ -0,0 +1,23 @@
+import hashlib
+from binascii import unhexlify
+from cryptography.hazmat.primitives.ciphers.aead import AESGCM
+
+
+class EncryptedBlob:
+    def __init__(self, data):
+        self.data = data
+
+    def decrypt(self, key):
+        # Extend the key using SHA256 as a KDF
+        extended_key = hashlib.sha256(key).digest()
+
+        # The 16 MSB of the extended key will serve as the AES GCM 128 secret key. The 16 LSB will serve as the IV.
+        sk = extended_key[:16]
+        nonce = extended_key[16:]
+
+        # Decrypt
+        aesgcm = AESGCM(sk)
+        data = unhexlify(self.data.encode)
+        raw_tx = aesgcm.decrypt(nonce=nonce, data=data, associated_data=None)
+
+        return raw_tx