WeChall - Training - Crypto - Caesar II

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Challenge

I guess you are done with Caesar I, aren't you? The big problem with caesar is that it does not allow digits or other characters. I have fixed this, and now I can use any ascii character in the plaintext. The keyspace has increased from 26 to 128 too. /

Enjoy!

Caesar cipher with full ASCII range (0-127). The keyspace has increased from 26 to 128.

Solution

The cipher bytes are displayed as hex values. Decode by trying all 128 shifts:

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hex_text="""53 7B 7B 70 20 76 7B 6E 38 20 05 7B 01 20 7F 7B
78 02 71 70 20 7B 7A 71 20 79 7B 7E 71 20 6F 74
6D 78 78 71 7A 73 71 20 75 7A 20 05 7B 01 7E 20
76 7B 01 7E 7A 71 05 3A 20 60 74 75 7F 20 7B 7A
71 20 03 6D 7F 20 72 6D 75 7E 78 05 20 71 6D 7F
05 20 00 7B 20 6F 7E 6D 6F 77 3A 20 63 6D 7F 7A
33 00 20 75 00 4B 20 3D 3E 44 20 77 71 05 7F 20
75 7F 20 6D 20 7D 01 75 00 71 20 7F 79 6D 78 78
20 77 71 05 7F 7C 6D 6F 71 38 20 7F 7B 20 75 00
20 7F 74 7B 01 78 70 7A 33 00 20 74 6D 02 71 20
00 6D 77 71 7A 20 05 7B 01 20 00 7B 7B 20 78 7B
7A 73 20 00 7B 20 70 71 6F 7E 05 7C 00 20 00 74
75 7F 20 79 71 7F 7F 6D 73 71 3A 20 63 71 78 78
20 70 7B 7A 71 38 20 05 7B 01 7E 20 7F 7B 78 01
00 75 7B 7A 20 75 7F 20 6F 7E 73 6D 6E 7C 7F 7B
6D 7C 73 79 3A"""
ct_bytes = [int(x, 16) for x in hex_text.split()]
for K in range(128):
    decoded = ''.join(chr((b - K) % 128) for b in ct_bytes)
    if 'is' in decoded:
        print(f"K={K}: {decoded}")

The space character (0x20) is part of the shift and will appear as a different character after decoding. Replace the most frequent non-alpha character with space to read the message.

crgabpsoapgm