Official BFD56 Discussion

Official discussion thread for BFD56. Please do not post any spoilers or big hints.

Super nice challenge. Challenging in all the right ways. Would love to talk to other people about their solutions. Mine took around 5h to compute but I’m sure it could be better.
The hint in the description of the challenge is really useful.

any hint on how to recover the flag. so far i think i know what the encryption is and have reversed the encryption process. it works on my own generated cip****xet and using my own k**, i think we should probably find some way to “guess” the k**? any pointers on what i should do next?

I found a C program code in the web. With modification for this challenge the program found the key in approx 12 seconds on a Intel i7 3.3GHz (single thread code).


the description says: [At least Delastelle didn’t have to worry about bit flipping]
so, i guess this bit flipping thing has to be something related to bit flipping attacks who works with " [Stream ciphers], such as [RC4] "
the other hints is delastelle, who made the delatelle cipher or bifid and trifid cipher.

is this trifid cipher some kind of stream cipher?

the challenge is called bfd56, is bfd called after bifid ?
is the key 56 ?
or something related to base56 ?

the encrypt file takes a file “secret” that has 3 variables pt,key,block_length.

i guess i have to generate those 3 in order to get a encoded message that has to be equal to the text in the file ciphertext

The encrypted text is given. The plaintext must be found. The flag is inside the plaintext.

A Python program code is also given. The Python code can be analysed. There you can read the encryption steps. You can run the Python code with a bit preparation.

You have mentioned some encryption methods. When reading the code, you will see which of these methods are used.

I could not find or guess the key in the challenge. I found how to compute/break the key by a web search.