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