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Random number generation Random numbers are used in a plethora of cryptographic applications. A random number generator (RNG) is a device that generates a sequence of numbers such that they can not be predicted better than guessing. There are two different types of random number generators — pseudo-random number generators (PRNGs) and true random number generators (TRNGs). A PRNG is a deterministic algorithm that produces seemingly random numbers. It needs a seed as an initial value, and will produce the same “random” sequence for a fixed seed. Applications such as games, simulations, and cryptography use such generators. A TRNG is a device that generates truly random numbers. In contrast to a deterministic algorithm, a TRNG utilizes physical processes, such as thermal noise (utilized in the RPG100 circuit), quantum phenomena, and so on. A PRNG is much faster than a TRNG, hence it is common to generate a seed using a…

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…