python3: struct-packed C strings are Python bytes (#31171)

src/authentic2/crypto.py

@@ -87,7 +87,7 @@ def aes_base64_decrypt(key, payload, raise_on_error=True):
 def add_padding(msg, block_size):
     '''Pad message with zero bytes to match block_size'''
     pad_length = block_size - (len(msg) + 2) % block_size
-    padded = struct.pack('<h%ds%ds' % (len(msg), pad_length), len(msg), msg, '\0' * pad_length)
+    padded = struct.pack('<h%ds%ds' % (len(msg), pad_length), len(msg), msg, b'\0' * pad_length)
     assert len(padded) % block_size == 0
     return padded
 
@@ -139,7 +139,7 @@ def aes_base64url_deterministic_encrypt(key, data, salt, hash_name='sha256', cou
 
     hmac = prf(key, crypted)[:hmac_size]
 
-    raw = struct.pack('<2sBH', 'a2', mode, count) + crypted + hmac
+    raw = struct.pack('<2sBH', b'a2', mode, count) + crypted + hmac
     return base64url_encode(raw)