Crypto1 is a
stream cipher very similar in its structure to its successor, Hitag2. Crypto1 consists of • a 48-bit
linear feedback shift register for the state of the cipher, • a two-layer 20-to-1 nonlinear function used to generate the keystream, and • a 16-bit LFSR which is used during the authentication phase as a
pseudo random number generator The usual operation of Crypto1 and Hitag2 ciphers uses nonlinear feedback only during the initialization/authentication stage, switching to operation as a LFSR with a nonlinear output filter (filter generator) for the rest of the communications. ==See also==