Garcia, Flavio D.; Peter van Rossum; Roel Verdult; Ronny Wichers Schreur (2009-03-17). "Wirelessly Pickpocketing a Mifare Classic Card"

Crypto1 is a proprietary encryption algorithm created by NXP Semiconductors specifically for Mifare RFID tags, including Oyster card, CharlieCard and OV-chipkaart.

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

  • one 48-bit feedback shift register for the main secret state of the cipher,
  • a linear function,
  • a two-layer 20-to-1 nonlinear function and
  • a 16-bit LFSR which is used during the authentication phase (which also serves as the pseudo random number generator on some card implementations).

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.