1 files changed, 92 insertions, 0 deletions
diff --git a/python/lilliput/helpers.py b/python/lilliput/helpers.py
new file mode 100644
index 0000000..8677f06
--- /dev/null
+++ b/python/lilliput/helpers.py
@@ -0,0 +1,92 @@
+from .constants import BLOCK_BITS, BLOCK_BYTES
+from . import tbc
+
+
+def ArrayToBlockbytesMatrix(array):
+    vector = list(array)
+
+    blocks_nb = len(vector)//BLOCK_BYTES
+
+    block_starts = (
+        i*BLOCK_BYTES for i in range(blocks_nb)
+    )
+
+    matrix = [
+        vector[start:start+BLOCK_BYTES] for start in block_starts
+    ]
+
+    padding_len = len(vector)%BLOCK_BYTES
+
+    if padding_len > 0:
+        padding = vector[-padding_len:]
+        matrix.append(padding)
+
+    return matrix
+
+
+def BlockbytesMatrixToBytes(matrix):
+    return bytes(byte for block in matrix for byte in block)
+
+
+def XorState(state1, state2):
+    return [s1^s2 for (s1, s2) in zip(state1, state2)]
+
+
+def Padding10LSB(X):
+    zeroes = [0] * (BLOCK_BYTES-len(X)-1)
+    return zeroes + [0b10000000] + X
+
+
+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
+    need_padding = len(A)%BLOCK_BYTES > 0
+
+    A = ArrayToBlockbytesMatrix(A)
+
+    for i in range(0, l_a):
+        tweak = _tweakAssociatedData(t, i, padded=False)
+        enc = tbc.encrypt(tweak, key, A[i])
+        Auth = XorState(Auth, enc)
+
+    if not need_padding:
+        return Auth
+
+    tweak = _tweakAssociatedData(t, l_a, padded=True)
+    ad_padded = Padding10LSB(A[l_a])
+    enc = tbc.encrypt(tweak, key, ad_padded)
+    Auth = XorState(Auth, enc)
+
+    return Auth
+
+
+class TagValidationError(Exception):
+    def __init__(self, announced, computed):
+        msg = '\n'.join((
+            'Invalid tag:',
+            announced.hex().upper()+' (announced)',
+            computed.hex().upper()+' (computed)'
+        ))
+
+        super().__init__(msg)
+        self._announced = announced
+        self._computed = computed