Format-preserving cipher
A format-preserving cipher including an encryption and a decryption scheme supporting non-linear access to input data by allowing the selection of portions of data from a potentially larger dataset to be encrypted, thus avoiding a necessarily sequential access into the input plaintext data. The cipher first defines a forward mapping from the allowable ciphertext values to an integer set of the number of such allowable ciphertext values, and a corresponding reverse mapping. It also supports exclusion of a certain set of characters from the ciphering process. Further, the encryption algorithm can encrypt the input plaintext data while preserving its original format and length and a corresponding decryption algorithm. The cipher advantageously embodies the encryption and decryption of multi-byte values or strings of data, thus fitting a variety of industrial needs.
1. 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.
2. The method of claim 1 , where said format-preserving ciphering preserves an alpha-numeric format of said portions.
3. The method of claim 1 , where S=8, representing said increments of said size S to be bytes.
4. The method of claim 1 , where said combinations of said S bits in step 1(g) represent characters selected from the group consisting of binary characters, unprintable characters, dashes and delimiting characters.
5. The method of claim 1 , where said allowable ciphertext values in said forward mapping and said reverse mapping are non-contiguous.
6. The method of claim 1 , where said set {0 . . . N−1} in said forward mapping and said reverse mapping is in no particular order.
7. The method of claim 1 , where said key-stream is derived using block-cipher in Counter (CTR) mode.
8. The method of claim 1 , where said key-stream is an RC4 key-stream.
9. The method of claim 1 , where said forward mapping maps said allowable ciphertext values as multi-byte values to said set {0 . . . N−1}, and said corresponding reverse mapping maps said set {0 . . . N−1} to corresponding said multi-byte allowable ciphertext values, and said size S divided by 8 equals to the number of bytes in each said multi-byte value.
10. The method of claim 1 , where said format-preserving ciphering is used to implement a stream-cipher.