hackyoutoo ctf crypto 300 writeup

Crypto 300 - Everybody Lies

  

 download algorithm.py 

Most xor operations cancel out each other especially the token[0-6] except token[7]. As 'token' is reset in the last line of the encoding loop with the 7th character of the sha1 hash of the previous char, we can decrypt all but the first character of the cipher text starting from the end.

We can combine all xor operations

0xfe ^ 0xc3 ^ 0x42 ^ 0x21 ^ 0xc2 ^ 0xf3 ^ 0x27 ^ 0x4c ^ 0x21 ^ 0xfe ^ 0xa3 ^ 0xf0 ^ 0x11 ^ 0x54 ^ 0xca ^ 0x3c ^ 0x20 ^ 0xd1 ^ 0xf2 == 0x38

Here is a ruby script to accomplish the same.

$ ruby everybody_lies.rb
_lied_no_flag_for_you!

With a little guessing game the first character turns out to be 'i'
so the flag is 'i_lied_no_flag_for_you!'