Format Preserving Encryption ======================================== .. versionadded:: 1.9.17 Format preserving encryption (FPE) refers to a set of techniques for encrypting data such that the ciphertext has the same format as the plaintext. For instance, you can use FPE to encrypt credit card numbers with valid checksums such that the ciphertext is also an credit card number with a valid checksum, or similiarly for bank account numbers, US Social Security numbers, or even more general mappings like English words onto other English words. The scheme currently implemented in botan is called FE1, and described in the paper `Format Preserving Encryption <http://eprint.iacr.org/2009/251>`_ by Mihir Bellare, Thomas Ristenpart, Phillip Rogaway, and Till Stegers. FPE is an area of ongoing standardization and it is likely that other schemes will be included in the future. To use FE1, use these functions, from ``fpe_fe1.h``: .. cpp:function:: BigInt FPE::fe1_encrypt(const BigInt& n, const BigInt& X, \ const SymmetricKey& key, const MemoryRegion<byte>& tweak) Encrypts the value *X* modulo the value *n* using the *key* and *tweak* specified. Returns an integer less than *n*. The *tweak* is a value that does not need to be secret that parameterizes the encryption function. For instance, if you were encrypting a database column with a single key, you could use a per-row-unique integer index value as the tweak. To encrypt an arbitrary value using FE1, you need to use a ranking method. Basically, the idea is to assign an integer to every value you might encrypt. For instance, a 16 digit credit card number consists of a 15 digit code plus a 1 digit checksum. So to encrypt a credit card number, you first remove the checksum, encrypt the 15 digit value modulo 10\ :sup:`15`, and then calculate what the checksum is for the new (ciphertext) number. .. cpp:function:: BigInt FPE::fe1_decrypt(const BigInt& n, const BigInt& X, \ const SymmetricKey& key, const MemoryRegion<byte>& tweak) Decrypts an FE1 ciphertext produced by :cpp:func:`fe1_encrypt`; the *n*, *key* and *tweak* should be the same as that provided to the encryption function. Returns the plaintext. Note that there is not any implicit authentication or checking of data, so if you provide an incorrect key or tweak the result is simply a random integer. This example encrypts a credit card number with a valid `Luhn checksum <http://en.wikipedia.org/wiki/Luhn_algorithm>`_ to another number with the same format, including a correct checksum. .. literalinclude:: examples/fpe.cpp