aes_ctr_prf.tcc

Go to the documentation of this file.

/** @file
 *****************************************************************************
 
 AES-Based PRF for ADSNARK.
 
 *****************************************************************************
 * @author     This file is part of libsnark, developed by SCIPR Lab
 *             and contributors (see AUTHORS).
 * @copyright  MIT license (see LICENSE file)
 *****************************************************************************/
 
#include "depends/libsnark-supercop/include/crypto_core_aes128encrypt.h"
#include "depends/libsnark-supercop/include/randombytes.h"
#include "gmp.h"
 
#include <libsnark/common/default_types/r1cs_ppzkadsnark_pp.hpp>
 
namespace libsnark
{
 
template<> aesPrfKeyT prfGen<default_r1cs_ppzkadsnark_pp>()
{
    aesPrfKeyT key;
    randombytes(key.key_bytes, 32);
    return key;
}
 
template<>
libff::Fr<snark_pp<default_r1cs_ppzkadsnark_pp>> prfCompute<
    default_r1cs_ppzkadsnark_pp>(const aesPrfKeyT &key, const labelT &label)
{
    unsigned char seed_bytes[16];
    mpz_t aux, Fr_mod;
    unsigned char random_bytes[16 * 3];
    size_t exp_len;
 
    mpz_init(aux);
    mpz_init(Fr_mod);
 
    // compute random seed using AES as PRF
    crypto_core_aes128encrypt_openssl(
        seed_bytes, label.label_bytes, key.key_bytes, NULL);
 
    // use first 128 bits of output to seed AES-CTR
    // PRG to expand to 3*128 bits
    crypto_core_aes128encrypt_openssl(
        random_bytes, seed_bytes, key.key_bytes + 16, NULL);
 
    mpz_import(aux, 16, 0, 1, 0, 0, seed_bytes);
    mpz_add_ui(aux, aux, 1);
    mpz_export(seed_bytes, &exp_len, 0, 1, 0, 0, aux);
    while (exp_len < 16)
        seed_bytes[exp_len++] = 0;
 
    crypto_core_aes128encrypt_openssl(
        random_bytes + 16, seed_bytes, key.key_bytes + 16, NULL);
 
    mpz_add_ui(aux, aux, 1);
    mpz_export(seed_bytes, &exp_len, 0, 1, 0, 0, aux);
    while (exp_len < 16)
        seed_bytes[exp_len++] = 0;
 
    crypto_core_aes128encrypt_openssl(
        random_bytes + 32, seed_bytes, key.key_bytes + 16, NULL);
 
    // see output as integer and reduce modulo r
    mpz_import(aux, 16 * 3, 0, 1, 0, 0, random_bytes);
    libff::Fr<snark_pp<default_r1cs_ppzkadsnark_pp>>::mod.to_mpz(Fr_mod);
    mpz_mod(aux, aux, Fr_mod);
 
    return libff::Fr<snark_pp<default_r1cs_ppzkadsnark_pp>>(
        libff::bigint<
            libff::Fr<snark_pp<default_r1cs_ppzkadsnark_pp>>::num_limbs>(aux));
}
 
} // namespace libsnark