AES-ABC

aes_abc/sol.py

@@ -0,0 +1,40 @@
+#!python3
+import math
+
+BLOCK_SIZE = 16
+UMAX = int(math.pow(256, BLOCK_SIZE))
+
+def remove_line(s):
+    # returns the header line, and the rest of the file
+    return s[:s.index(b'\n') + 1], s[s.index(b'\n')+1:]
+
+def parse_header_ppm(f):
+    data = f.read()
+
+    header = b""
+
+    for i in range(3):
+        header_i, data = remove_line(data)
+        header += header_i
+
+    return header, data
+ 
+with open("body.enc.ppm", 'rb') as f:
+    header, data = parse_header_ppm(f)
+
+blocks = [data[i * BLOCK_SIZE:(i+1) * BLOCK_SIZE] for i in range(len(data) // BLOCK_SIZE)]
+
+for i in reversed(range(len(blocks) - 1)):
+    prev_blk = int.from_bytes(blocks[i], 'big')
+    curr_blk = int.from_bytes(blocks[i+1], 'big')
+
+    n_curr_blk = (curr_blk - prev_blk) % UMAX
+    blocks[i+1] = n_curr_blk.to_bytes(16, 'big')
+
+blocks = blocks[1:]
+
+with open("body.ppm", 'wb') as f:
+    f.write(header)
+    for block in blocks:
+        f.write(block)
+