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diff --git a/comm/third_party/botan/doc/api_ref/fpe.rst b/comm/third_party/botan/doc/api_ref/fpe.rst new file mode 100644 index 0000000000..9d77a40864 --- /dev/null +++ b/comm/third_party/botan/doc/api_ref/fpe.rst @@ -0,0 +1,98 @@ +Format Preserving Encryption +======================================== + +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 similarly 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 +<https://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 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. Or, if you were encrypting words in a +dictionary, you could rank the words by their lexicographical order, +and choose the modulus to be the number of words in the dictionary. + +The interfaces for FE1 are defined in the header ``fpe_fe1.h``: + +.. versionadded:: 2.5.0 + +.. cpp:class:: FPE_FE1 + + .. cpp:function:: FPE_FE1(const BigInt& n, size_t rounds = 5, \ + bool compat_mode = false, \ + std::string mac_algo = "HMAC(SHA-256)") + + Initialize an FPE operation to encrypt/decrypt integers less + than *n*. It is expected that *n* is trivially factorable into + small integers. Common usage would be n to be a power of 10. + + Note that the default parameters to this constructor are + **incompatible** with the ``fe1_encrypt`` and ``fe1_decrypt`` + function originally added in 1.9.17. For compatibility, use + 3 rounds and set ``compat_mode`` to true. + + .. cpp:function:: BigInt encrypt(const BigInt& x, const uint8_t tweak[], size_t tweak_len) const + + 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. The same + tweak value must be used during decryption. + + .. cpp:function:: BigInt decrypt(const BigInt& x, const uint8_t tweak[], size_t tweak_len) const + + Decrypts an FE1 ciphertext. The *tweak* must be the same as that provided + to the encryption function. Returns the plaintext integer. + + Note that there is not any implicit authentication or checking of data in + FE1, so if you provide an incorrect key or tweak the result is simply a + random integer. + + .. cpp:function:: BigInt encrypt(const BigInt& x, uint64_t tweak) + + Convenience version of encrypt taking an integer tweak. + + .. cpp:function:: BigInt decrypt(const BigInt& x, uint64_t tweak) + + Convenience version of decrypt taking an integer tweak. + +There are two functions that handle the entire FE1 encrypt/decrypt operation. +These are the original interface to FE1, first added in 1.9.17. However because +they do the entire setup cost for each operation, they are significantly slower +than the class-based API presented above. + +.. warning:: These functions are hardcoded to use 3 rounds, which may be + insufficient depending on the chosen modulus. + +.. cpp:function:: BigInt FPE::fe1_encrypt(const BigInt& n, const BigInt& X, \ + const SymmetricKey& key, const std::vector<uint8_t>& tweak) + + This creates an FPE_FE1 object, sets the key, and encrypts *X* using + the provided tweak. + +.. cpp:function:: BigInt FPE::fe1_decrypt(const BigInt& n, const BigInt& X, \ + const SymmetricKey& key, const std::vector<uint8_t>& tweak) + + This creates an FPE_FE1 object, sets the key, and decrypts *X* using + the provided tweak. + +This example encrypts a credit card number with a valid `Luhn checksum +<https://en.wikipedia.org/wiki/Luhn_algorithm>`_ to another number with the same +format, including a correct checksum. + +.. literalinclude:: ../../src/cli/cc_enc.cpp |