Quick Stream Cipher
Quick Stream Cipher ( QUISCI ) is a software-optimized symmetrical stream cipher that was developed in 2001 by Stefan Müller ( FGAN -FHR).
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:
|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.
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.
- ^ Paul Crowley: Easy cryptanalysis challenge , Feb. 17, 2006.
- ↑ Nils Reimers: Key Recovery Attack on QuiSci (PDF; 115 kB), October 11, 2009.