1 files changed, 15 insertions, 11 deletions
diff --git a/src/add_python/lilliput/ae_mode_1.py b/src/add_python/lilliput/ae_mode_1.py
index 23f4c7b..4a40b78 100644
--- a/src/add_python/lilliput/ae_mode_1.py
+++ b/src/add_python/lilliput/ae_mode_1.py
@@ -56,21 +56,25 @@ def _byte_from_nibbles(lower, upper):
     return upper<<4 | lower
 
 
-def _tweak_message(N, j, padding):
-    j = integer_to_byte_array(j, (TWEAK_BITS-NONCE_BITS-4)//8+1)
-
-    middle_byte = _byte_from_nibbles(
-        _lower_nibble(j[-1]), _lower_nibble(N[0])
-    )
-
-    shifted_N = [
+def _tweak_message(N, j, prefix):
+    # j is encoded on 68 bits; get 72 and clear the upper 4.
+    j_len = (TWEAK_BITS-NONCE_BITS-4)//8 + 1
+    tweak = integer_to_byte_array(j, j_len)
+    tweak[-1] &= 0b00001111
+
+    # Add nonce.
+    tweak[-1] |= _lower_nibble(N[0]) << 4
+    tweak.extend(
         _byte_from_nibbles(_upper_nibble(N[i-1]), _lower_nibble(N[i]))
         for i in range(1, NONCE_BITS//8)
-    ]
+    )
 
-    last_byte = _byte_from_nibbles(_upper_nibble(N[-1]), padding.value)
+    # Add last nibble from nonce and prefix.
+    tweak.append(
+        _byte_from_nibbles(_upper_nibble(N[-1]), prefix.value)
+    )
 
-    return j[:-1] + [middle_byte] + shifted_N + [last_byte]
+    return tweak
 
 
 def _treat_message_enc(M, N, key):