Updating CryptographyAESKey::encrypt to generate 96 bit IVs for GCM…

… block cipher mode to adhere to the RFC for JWA in `jose/backends/cryptography_backend.py`

See https://www.rfc-editor.org/rfc/rfc7518.html#section-5.3 for the official RFC requirements for JWA

See panva/jose#678 for related discussion on this issue

jose/backends/cryptography_backend.py

@@ -439,6 +439,11 @@ class CryptographyAESKey(Key):
         ALGORITHMS.A256KW: None,
     }
 
+    IV_BYTE_LENGTH_MODE_MAP = {
+        'CBC': algorithms.AES.block_size // 8,
+        'GCM': 96 // 8
+    }
+
     def __init__(self, key, algorithm):
         if algorithm not in ALGORITHMS.AES:
             raise JWKError("%s is not a valid AES algorithm" % algorithm)
@@ -468,7 +473,8 @@ def to_dict(self):
     def encrypt(self, plain_text, aad=None):
         plain_text = ensure_binary(plain_text)
         try:
-            iv = get_random_bytes(algorithms.AES.block_size // 8)
+            iv_byte_length = self.IV_BYTE_LENGTH_MODE_MAP[self._mode.name] if self._mode.name in self.IV_BYTE_LENGTH_MODE_MAP else algorithms.AES.block_size
+            iv = get_random_bytes(iv_byte_length)
             mode = self._mode(iv)
             if mode.name == "GCM":
                 cipher = aead.AESGCM(self._key)