Format-preserving cipher

A method of format-preserving ciphering, comprising the steps of:

• a) providing predetermined criteria for selecting portions of data to be ciphered;
• b) providing a forward mapping from N allowable ciphertext values to set {0 . . . N−1}, and a corresponding reverse mapping;
• c) iteratively accessing each said portion in increments of size S bits, where S is set according to an application of said format-preserving ciphering;
• d) providing a key-stream with a block-cipher operating in a Counter (CTR) mode and iteratively accessing it in increments of said size S bits;
• e) determining a limiter value L as the difference of 2S and (2S modulo N);
• f) ignoring any said accessed key-stream value in said ciphering, if said key-stream value is greater than or equal to said limiter value L;
• g) allowing zero or more combinations of said S bits in each said portion to be skipped from said ciphering;
• h) encrypting each said portion in an encryption module by taking the sum of said forward mapping of said accessed portion and said accessed key-stream value, taking modulo 10 of said sum to produce a final sum, and taking said forward mapping of said final sum;
• and
• i) decrypting each said portion in a decryption module by taking the difference of said forward mapping of said accessed portion and said accessed key-stream value, taking modulo 10 of said difference, if said difference is less than 0 then adding N to said difference to produce a final difference, and taking said reverse mapping of said final difference.