fastcrypto.md

Fast-Crypto

We have a .wav, which is XOR'ed with some generated byte-stream. The stream depends on a power (2-16) and a seed (0-2**16). That leaves about 1M possibilities, which is few enough for brute force. However, encryption is very slow, so we have to speed it up.

• get_next should use modular exponentiation pow(a, power, N)
• The loop for iterated power can be turned into one: pow(a, power**31337, N)
• We can factor N to compute phi(N) and use Euler's Theorem: pow(a, (power**31337) % phi(N), N)
• A .wav begins with RIFF as signature, so we just check the first 4 Byte. If they do not match, try the next combination.

Running brute force yields the decoded file (in my case about 20 minutes). Then just listen to the flag.