biweekly can you crack this? thread

Hey prog, I wrote a simple xor cipher that takes the exclusive or of the plain text with the key, but it also takes some state from the previous calculation.


cipher[i] = plain[i] ^ key[i] ^ prev_state[i - 1]

prev_state[i] = (cipher[i] + plain[i]) mod 256

The first value of prev_state is always zero. That is, prev_state[-1] = 0.


The cipher text and cipher implementation is below, cut among many posts to avoid long post detection.

"Those who don't know RC4 are doomed to reinvent it poorly."

The key is a single ascii english word, terminated with a newline

while read W ; do
 echo $W > key
 if cipher key plaintxt | grep -q "[\x80-\xFF]" ; then
  continue
 fi
 echo key is `cat key`
 break
done < /usr/share/dict/words