1 files changed, 65 insertions, 1 deletions
diff --git a/python/helpers.py b/python/helpers.py
index 34949a4..3d741b0 100644
--- a/python/helpers.py
+++ b/python/helpers.py
@@ -1,4 +1,5 @@
-from constants import BLOCK_BYTES
+from constants import BLOCK_BITS, BLOCK_BYTES
+from lilliput_tbc import LilliputTBCEnc
 
 
 def ArrayToBlockbytesMatrix(array) :
@@ -23,3 +24,66 @@ def ArrayToBlockbytesMatrix(array) :
 def BlockbytesMatrixToBytes(matrix):
     return bytes(byte for block in matrix for byte in block)
 
+
+def XorState(state1, state2):
+    return list(
+        s1^s2 for (s1, s2) in zip(state1, state2)
+    )
+
+
+def Padding10LSB(array, number_bits):
+    shifted = 0
+    for byte in range(0, len(array)):
+        shifted |= (array[byte] << (8 * byte))
+    shifted = (shifted << (BLOCK_BITS - number_bits)) & 0xffffffffffffffffffffffffffffffff
+
+    padded = shifted | (0x1 << (BLOCK_BITS - number_bits - 1))
+
+    array_padded = [0 for byte in range(0, BLOCK_BYTES)]
+    for byte in range(0, BLOCK_BYTES):
+        array_padded[byte] = (padded & (0xff << (8 * byte))) >> (8 * byte)
+
+    return array_padded
+
+
+def _tweakAssociatedData(t, i, padded):
+    t_bytes = t//8
+    tweak = [0]*(t_bytes)
+
+    mask = 0xff
+    for byte in range(t_bytes-1):
+        tweak[byte] = (i & mask) >> (byte * 8)
+        mask = mask << 8
+
+    mask = (0xf << (8 * t_bytes-1))
+    tweak[-1] = (i & mask) >> ((t_bytes-1)*8)
+    if not padded:
+        tweak[-1] |= 0x20
+    else:
+        tweak[-1] |= 0x60
+
+    return tweak
+
+
+def BuildAuth(t, A, key):
+    Auth = [0 for byte in range(0, BLOCK_BYTES)]
+    l_a = len(A)//BLOCK_BYTES
+
+    padding_bytes = len(A)%BLOCK_BYTES
+
+    A = ArrayToBlockbytesMatrix(A)
+
+    for i in range(0, l_a):
+        tweak = _tweakAssociatedData(t, i, padded=False)
+        enc = LilliputTBCEnc(tweak, key, A[i])
+        Auth = XorState(Auth, enc)
+
+    if padding_bytes == 0:
+        return Auth
+
+    tweak = _tweakAssociatedData(t, l_a, padded=True)
+    ad_padded = Padding10LSB(A[l_a], padding_bytes*8)
+    enc = LilliputTBCEnc(tweak, key, ad_padded)
+    Auth = XorState(Auth, enc)
+
+    return Auth