# Quick Stream Cipher

Quick Stream Cipher ( QUISCI ) is a software-optimized symmetrical stream cipher that was developed in 2001 by Stefan Müller ( FGAN -FHR).

## features

The algorithm is very compact and faster than the well-known stream ciphers RC4 or SEAL . It can be operated in both a plain text-independent mode and a plain-text dependent mode. An Opteron - processor such needs. B. in the plaintext-independent mode 1 clock cycle to encrypt a byte . The encryption performance of the method increases linearly with the bit width of the processor. In contrast to the SEAL algorithm, initialization takes little time. The key length is variable.

## Core of the algorithm

Let i , j , k integers and S [] be a field of integers that is filled depending on a key. Then an existing code segment C is encrypted or decrypted in the following steps:

step calculation
1. i  : = i + S [ j ]
2. if i equals 0 otherwise
3. i  : = not ( S [ j ]) k  : = i xor S [ j ]
4th S [ j ]: = k S [ j ]: = i
5. j  : = i
6th C  : = C xor i C  : = C xor k

Depending on the size of the S [] field, the indexing is adjusted by shifting it to the right.

## safety

In order to achieve the necessary quality of the pseudo random number generator (PRNG) for secure encryption, the key field S [] must be at least 2048 bytes long and the whole numbers at least 11 bits wide. The PRNG can therefore assume at least 2 ^ 16406 states. A complete cryptanalysis of the method is not yet available, but implementations that have been published on the Internet have been examined for weaknesses.

• In February 2006, Paul Crowley found a miserable equal distribution of the PRNG in a sample implementation of the method. The reason for this result was the field length of 128 bytes that was selected too small.
• In March 2009, Nils Reimers set his sights on the initialization of a newer implementation, which was published on the Internet at the time. A weak key expansion made it possible to reconstruct the key with a few known plain text blocks at the beginning of the encryption.