179 - Code Breaking

[deadangelx]

I give 2 hints.

the first hint:
If n is not multiple of k
you can add any the same character after plaintext and cyphertext, like:
k = 5
"abcdabcd"
"bcdabcda"

after add '?'
"abcdabcd??"
"bcdabcda??"

then you can check k = 5

the second hint:
If one string is "abcdeee"
it has possible 3! permutation.
so "abcdeeefghiii"
it has possible 3! * 3! permutation.
when string having 80 characters, the factorial may be very big.
you can set a threshold to limit it (ex. 10000 permutation).
It needn't to check all possible string or u would get TLE.

[metaphysis]

Thanks for your hints, I got AC.