Base 64 is too mainstream but we could not manage to decide on the final encoding scheme… https://base65536.ctf.insecurity-insa.fr
We got access to the White House’s registration page which allows to recover nuclear bomb codes. Will you be able to recover the nuclear codes for
Bill Clinton? Please note that it’s easy to get a nuclear code that looks like a flag for any username, but only
Bill Clinton’s code will be valid (and you guessed it, the system won’t let you get that one too easily). The White House’s server is available at
nc whitehouse.ctf.insecurity-insa.fr 18470
You’re almost done with this, try harder! Once you have all the needed information from previous step, go have a look here (https://gcorp-stage-4.ctf.insecurity-insa.fr/) Note: you should validate stage 3 to have more information on stage 4.
As crypto expert we designed our own streamcipher that combines two linear elements into a secure design. It works as follows. The secret key of NONSENSE consists of two invertible matrices K 51 , K 52 ∈ Z 64×64 To encrypt a plaintext M of l bits, our algorithm takes a 64-bit IV, generates an l-bit key stream k and computes the ciphertext C = M ⊕ k. The keystream is generated in 64-bit blocks as implemented in our open source file. To enforce a bit more the security, we decided to include IV into the secret key as well, it is incremented after every encryption query by 1, i.e. IV = (int(IV) + 1 mod 2^64 i) with limited 64 bits. You can find attached our implementation and here is our incrackable test : BXkOb8rYcnNpR3db/Ly5cD+EyBJnm8sorjHZTx/yAhUi
You did a very good job until now. To be sure no one is going to be trapped again, could you find a way to protect the whole company?
To do so, you can use the binary located on the website of part 2, our expert told us that is was safe to run.
Note: you need to solve part 2 before attempting this part
Edit: Microsoft flagged the old binary as a malware, please redownload the new version (the old version won’t give you the correct flag)