pigweed / third_party / github / h2o / picotls / 3e61e3178740855bbc0a3a6dfab6cdb19bd9c4b7 / . / include / picotls / ffx.h

/* | |

* Copyright (c) 2019 Christian Huitema <huitema@huitema.net> | |

* | |

* Permission to use, copy, modify, and distribute this software for any | |

* purpose with or without fee is hereby granted, provided that the above | |

* copyright notice and this permission notice appear in all copies. | |

* | |

* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES | |

* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF | |

* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR | |

* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES | |

* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN | |

* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF | |

* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. | |

*/ | |

#ifndef PTLS_FFX_H | |

#define PTLS_FFX_H | |

/* | |

* Format preserving encryption using the FFX algorithm. | |

* | |

* We demonstrate here a simple encryption process derived | |

* from the FFX algorithms, which is effectively a specific | |

* mode of running a verified encryption code. The | |

* algorithm is Feistel cipher in which the S-boxes are | |

* defined by a symmetric encryption algorithm such as | |

* AES or ChaCha20. | |

* See "Ciphers with Arbitrary Finite Domains" by | |

* John Black and Phillip Rogaway, 2001 -- | |

* http://web.cs.ucdavis.edu/~rogaway/papers/subset.pdf | |

* | |

* An instantiation of the algorithm is defined by a | |

* series of parameters: | |

* - the context of the symmetric crypto algorithm, | |

* - key used for the symmetric algorithm, | |

* - number of rounds, | |

* - length of the block in bits | |

* | |

* We consider just two symmetric algorithms for now, | |

* ChaCha20 and AES128CTR. In theory, any symmetric algorithm | |

* operating on a 128 bit block would work, and crytographic | |

* hashes producing at least 128 bits of output could also | |

* be used. In practice, ChaCha20 and AES128 cover most of | |

* the use cases. | |

* | |

* The implementation will produce a result for any block | |

* length lower than 256, although values lower than 32 would | |

* not be recommended. | |

* | |

* The block to be encrypted is passed as a byte array of size | |

* (block_length + 7)/8. When the block_length is not a | |

* multiple of 8, the algorithm guarantees that the extra bits | |

* in the last byte are left untouched. For example, if the | |

* block length is 39, the least significant bit of the | |

* fifth byte will be copied from input to output. | |

* | |

* The number of rounds is left as a configuration parameter, | |

* which is constrained to be even by our implementation. The | |

* required number of passes varies with the application's | |

* constraints. The practical minimum is 4 passes. Demanding | |

* applications can use 8 passes, and the practical conservative | |

* value is 10, as specified by NIST for the FF1 variant of | |

* the same algorithm. This choice between 4, 8 or 10 is | |

* based on "Luby-Rackoff: 7 Rounds are Enough | |

* for 2^n(1-epsilon) Security" by Jacques Patarin, 2003 -- | |

* https://www.iacr.org/archive/crypto2003/27290510/27290510.pdf | |

* | |

* Encrypting short numbers, by nature, produces a codebook | |

* of limited size. In many applications, the short number is | |

* part of a larger object that is passed in clear text. In that | |

* case, NIST recommends using as much as possible of that clear | |

* text as an initialization vector, used as "tweak" in the | |

* FFX algorithm. See: | |

* https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-38G.pdf | |

*/ | |

typedef struct st_ptls_ffx_context_t { | |

ptls_cipher_context_t super; | |

ptls_cipher_context_t *enc_ctx; | |

int nb_rounds; | |

int is_enc; | |

size_t byte_length; | |

size_t nb_left; | |

size_t nb_right; | |

uint8_t mask_last_byte; | |

uint8_t tweaks[16]; | |

} ptls_ffx_context_t; | |

/** | |

* The PTLS_FFX_CIPHER_ALGO macro will define a variant of the FFX algorithm by specifying | |

* the base algorithm (vraiable name of type ptls_cipher_algorithm_t), the bit length | |

* of the block, the selected number of blocks and the key size of the base algorithm, | |

* in bytes. | |

* | |

* The macro will automatically generate an algorithm name, of the form: | |

* ptls_ffx_<base algorithm name>_b<bit length>_r<number of rounds> | |

* For example, selecting the algorithm "ptls_minicrypto_chacha20" with a block | |

* size of 53 bits and 8 rounds will generate the name: | |

* ptls_ffx_ptls_minicrypto_chacha20_b53_r8 | |

* This name is declared as a static variable. | |

* | |

* Once the FFX variant is defined, the name can be used to create a | |

* cipher context using ptls_cipher_new. The context can then be used | |

* through the function ptls_cipher_init, ptls_cipher_encrypt, and | |

* can be freed by calling ptls_cipher_free. | |

* | |

* The ptls_cipher_encrypt will encrypt a code word of the specified | |

* bit length, or decrypt it if the context was created with the | |

* option "is_enc = 0". The code word is represented as an array of | |

* bytes. If the bit length is not a multiple of 8, the remaining | |

* low level bits in the last byte will be left unchanged. | |

*/ | |

#define PTLS_FFX_CIPHER_ALGO_NAME(base, bitlength, nbrounds) #base "-ffx-b" #bitlength "-r" #nbrounds | |

#define PTLS_FFX_CIPHER_ALGO(base, bitlength, nbrounds, keysize) \ | |

static int ptls_ffx_##base##_b##bitlength##_r##nbrounds##_setup(ptls_cipher_context_t *ctx, int is_enc, const void *key) \ | |

{ \ | |

return ptls_ffx_setup_crypto(ctx, &base, is_enc, nbrounds, bitlength, key); \ | |

} \ | |

static ptls_cipher_algorithm_t ptls_ffx_##base##_b##bitlength##_r##nbrounds = { \ | |

PTLS_FFX_CIPHER_ALGO_NAME(base, bitlength, nbrounds), keysize, (bitlength + 7) / 8, 16, sizeof(ptls_ffx_context_t), \ | |

ptls_ffx_##base##_b##bitlength##_r##nbrounds##_setup}; | |

/* | |

* The function ptls_ffx_new creates a cipher context for a specific FFX variant. | |

* It is equivalent to defining the variant with the PTLS_FFX_CIPHER_ALGO macro, | |

* then creating the context using ptls_cipher_new. | |

*/ | |

ptls_cipher_context_t *ptls_ffx_new(ptls_cipher_algorithm_t *algo, int is_enc, int nb_rounds, size_t bit_length, const void *key); | |

/** | |

* The function ptls_ffx_setup_crypto is called by ptls_cipher_new or | |

* ptls_ffx_new when initializing an FFX variant. It should not be | |

* called directly. | |

*/ | |

int ptls_ffx_setup_crypto(ptls_cipher_context_t *_ctx, ptls_cipher_algorithm_t *algo, int is_enc, int nb_rounds, size_t bit_length, | |

const void *key); | |

#endif /* PTLS_FFX_H */ |