77 lines
1.9 KiB
Python
Executable File
77 lines
1.9 KiB
Python
Executable File
#!python3
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from Crypto.Cipher import AES
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import os
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import math
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BLOCK_SIZE = 16
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UMAX = int(math.pow(256, BLOCK_SIZE))
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def remove_line(s):
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# returns the header line, and the rest of the file
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return s[:s.index(b'\n') + 1], s[s.index(b'\n')+1:]
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def parse_header_ppm(f):
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data = f.read()
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header = b""
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for i in range(3):
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header_i, data = remove_line(data)
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header += header_i
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return header, data
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def pad(pt):
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padding = BLOCK_SIZE - len(pt) % BLOCK_SIZE
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return pt + (bytes([padding]) * padding)
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def aes_abc_encrypt(pt):
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KEY = b"0123456789abcdef"
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cipher = AES.new(KEY, AES.MODE_ECB)
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ct = cipher.encrypt(pad(pt))
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blocks = [ct[i * BLOCK_SIZE:(i+1) * BLOCK_SIZE] for i in range(len(ct) // BLOCK_SIZE)]
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iv = b"0123456789abcdef"
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blocks.insert(0, iv)
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for i in range(len(blocks) - 1):
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prev_blk = int(blocks[i].hex(), 16)
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curr_blk = int(blocks[i+1].hex(), 16)
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n_curr_blk = (prev_blk + curr_blk) % UMAX
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blocks[i+1] = n_curr_blk.to_bytes(BLOCK_SIZE, 'big')
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ct_abc = b"".join(blocks)
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_blocks = [ct_abc[i * BLOCK_SIZE:(i+1) * BLOCK_SIZE] for i in range(len(ct_abc) // BLOCK_SIZE)]
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assert len(blocks) == len(_blocks)
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assert blocks == _blocks
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for i in reversed(range(len(_blocks)-1)):
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prev_blk = int.from_bytes(_blocks[i], 'big')
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curr_blk = int.from_bytes(_blocks[i+1], 'big')
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n_curr_blk = (curr_blk - prev_blk)%UMAX
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_blocks[i+1] = n_curr_blk.to_bytes(BLOCK_SIZE, 'big')
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_blocks = _blocks[1:]
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assert len(blocks) == len(_blocks)+1
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_ct = b"".join(_blocks)
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assert len(_ct) == len(ct)
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assert _ct == ct
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return iv, ct_abc, ct
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if __name__=="__main__":
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with open('flag.ppm', 'rb') as f:
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header, data = parse_header_ppm(f)
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iv, c_img, ct = aes_abc_encrypt(data)
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with open('test.enc.ppm', 'wb') as fw:
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fw.write(header)
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fw.write(c_img)
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