rng.tcc

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/** @file
 *****************************************************************************
 
 Implementation of functions for generating randomness.
 
 See rng.hpp .
 
 *****************************************************************************
 * @author     This file is part of libff, developed by SCIPR Lab
 *             and contributors (see AUTHORS).
 * @copyright  MIT license (see LICENSE file)
 *****************************************************************************/
 
#ifndef RNG_TCC_
#define RNG_TCC_
 
#include <gmp.h>
#include <libff/algebra/fields/bigint.hpp>
#include <libff/common/rng.hpp>
#include <libff/common/utils.hpp>
#include <openssl/sha.h>
 
namespace libff
{
 
template<typename FieldT> FieldT SHA512_rng(const uint64_t idx)
{
    // current Python code cannot handle larger
    // values, so testing here for some assumptions.
    assert(GMP_NUMB_BITS == 64);
    assert(is_little_endian());
 
    assert(FieldT::size_in_bits() <= SHA512_DIGEST_LENGTH * 8);
 
    bigint<FieldT::num_limbs> rval;
    uint64_t iter = 0;
    do {
        mp_limb_t hash
            [((SHA512_DIGEST_LENGTH * 8) + GMP_NUMB_BITS - 1) / GMP_NUMB_BITS];
 
        SHA512_CTX sha512;
        SHA512_Init(&sha512);
        SHA512_Update(&sha512, &idx, sizeof(idx));
        SHA512_Update(&sha512, &iter, sizeof(iter));
        SHA512_Final((unsigned char *)hash, &sha512);
 
        for (mp_size_t i = 0; i < FieldT::num_limbs; ++i) {
            rval.data[i] = hash[i];
        }
 
        /* clear all bits higher than MSB of modulus */
        size_t bitno = GMP_NUMB_BITS * FieldT::num_limbs - 1;
 
        /* mod is non-zero so the loop will always terminate */
        while (FieldT::mod.test_bit(bitno) == false) {
            const std::size_t part = bitno / GMP_NUMB_BITS;
            const std::size_t bit = bitno - (GMP_NUMB_BITS * part);
 
            rval.data[part] &= ~(1ul << bit);
 
            bitno--;
        }
 
        ++iter;
    }
 
    /* if r.data is still >= modulus -- repeat (rejection sampling) */
    while (mpn_cmp(rval.data, FieldT::mod.data, FieldT::num_limbs) >= 0);
 
    return FieldT(rval);
}
 
} // namespace libff
 
#endif // RNG_TCC_