Write-up Codegate 2010 #17 - Crypto, Linear Congruential Generators and Vernam Cipher, the power of XOR

Challenge #17 was crypto, based on Linear Congruential Generators (LCG), a well known pseudorandom number generator (PRNG), and the Vernam Cipher which is basically a XOR cipher relying on exclusive OR . Sadly, we did not succeed this challenge in time. However my friend Ivan found it afterwards thanks to Julianor (a staff member).

Basically, there is a TCP server listening to incoming connections. We simply use netcat (or telnet) to connect to it:

$ nc ctf3.codegate.org 10909
a?

c?

m?

Bad values