BS ISO/IEC 18033-4:2005+A1:2009

This part of ISO/IEC 18033 specifies stream cipher algorithms. A stream cipher is an encryption mechanism that uses a keystream to encrypt a plaintext in bitwise or block-wise manner. There are two types of stream cipher: a synchronous stream cipher, in which the keystream is only generated from the secret key (and an initialization vector) and a self-synchronizing stream cipher, in which the keystream is generated from the secret key and some past ciphertexts (and an initialization vector). Typically the encryption operation is the additive bitwise XOR operation between a keystream and the message. This part of ISO/IEC 18033 describes both pseudorandom number generators for producing keystream, and output functions for stream ciphers.

The algorithms specified in this part of ISO/IEC 18033 have been assigned object identifiers in accordance with ISO/IEC 9834. The list of assigned object identifiers is given in Annex C. Any change to the specification of the algorithms resulting in a change of functional behaviour will result in a change of the object identifier assigned to the algorithm.

NOTE 1 - In applications where a combination of algorithms is being used, or when an algorithm is parameterized by the choice of a combination of other algorithms, the combination may be specified as a sequence of object identifiers. Alternatively, the object identifiers of lower layer algorithms may be included in the parameter field of the higher layer algorithm’s identifier structure. For example, the object identifier of a block cipher could be included as a parameter in the algorithm identifier structure for a keystream generator. The algorithm identifier structure is defined in ISO/IEC 9594-8.

NOTE 2 - The encoding of object identifiers is application dependent.