In cryptography

Cryptography

Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

, Nimbus is a block cipher

Block cipher

In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...

 invented by Alexis Machado in 2000. It was submitted to the NESSIE

NESSIE

NESSIE was a European research project funded from 2000–2003 to identify secure cryptographic primitives. The project was comparable to the NIST AES process and the Japanese Government-sponsored CRYPTREC project, but with notable differences from both...

 project, but was not selected.

The algorithm uses a 128-bit key. It operates on blocks of 64 bits and consists of 5 rounds of
encryption. The round function is exceedingly simple. In each round the block is XORed with a subkey, the order of its bits is reversed, and then it is multiplied mod 2⁶⁴ by another subkey, which is forced to be odd.

Nimbus was broken by Vladimir Furman; he found a differential attack

Differential cryptanalysis

Differential cryptanalysis is a general form of cryptanalysis applicable primarily to block ciphers, but also to stream ciphers and cryptographic hash functions. In the broadest sense, it is the study of how differences in an input can affect the resultant difference at the output...

using only 256 chosen plaintexts.