The problems with repeated keystream in stream ciphers Repeated keystream can sometimes be devastating when using stream ciphers. The Capture the Flag event co-organized by Debricked at Lund University included examples of this problem. Stream ciphers try to mimic the One Time Pad (OTP), but without the inherent drawbacks of a cipher that requires a key the size of the plaintext. Instead, the stream cipher expands a short key (80-256 bits) to a long sequence through the use of a keystream generator. The keystream generator outputs keystream bits (or words) based on the value of an internal state, a key and an initialization vector (IV). The exact definition of this function varies between stream ciphers and often the key and IV is only used to initialize the internal state. The output then only depends on the current internal state. The goal of an attack could be to either compute the…