Recent cryptographic research has shown that, "the security of this cipher is ... close to zero". Crypto1 is a stream cipher very similar in its structure to its successor, Hitag2. Crypto1 consists of
It can operate as an NLFSR and as an LFSR, depending on its input parameters. Outputs of one or both linear and nonlinear functions can be fed back into the cipher state or used as its output filters. The usual operation of Crypto1 and Hitag2 ciphers uses nonlinear feedback only during the initialization/authentication stage, switching to operation as LFSR with a nonlinear output filter for encrypting the tag's communications in both directions.
^Garcia, Flavio D.; Gerhard de Koning Gans; Ruben Muijrers; Peter van Rossum, Roel Verdult; Ronny Wichers Schreur; Bart Jacobs (2008-10-04). "Dismantling MIFARE Classic"(PDF). 13th European Symposium on Research in Computer Security (ESORICS 2008), LNCS, Springer.