github.com/ethereum/go-ethereum@v1.16.1/crypto/secp256k1/libsecp256k1/src/testrand_impl.h

/***********************************************************************
 * Copyright (c) 2013-2015 Pieter Wuille                               *
 * Distributed under the MIT software license, see the accompanying    *
 * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
 ***********************************************************************/

#ifndef SECP256K1_TESTRAND_IMPL_H
#define SECP256K1_TESTRAND_IMPL_H

#include <stdint.h>
#include <stdio.h>
#include <string.h>

#include "testrand.h"
#include "hash.h"
#include "util.h"

static uint64_t secp256k1_test_state[4];

SECP256K1_INLINE static void testrand_seed(const unsigned char *seed16) {
    static const unsigned char PREFIX[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', ' ', 't', 'e', 's', 't', ' ', 'i', 'n', 'i', 't'};
    unsigned char out32[32];
    secp256k1_sha256 hash;
    int i;

    /* Use SHA256(PREFIX || seed16) as initial state. */
    secp256k1_sha256_initialize(&hash);
    secp256k1_sha256_write(&hash, PREFIX, sizeof(PREFIX));
    secp256k1_sha256_write(&hash, seed16, 16);
    secp256k1_sha256_finalize(&hash, out32);
    for (i = 0; i < 4; ++i) {
        uint64_t s = 0;
        int j;
        for (j = 0; j < 8; ++j) s = (s << 8) | out32[8*i + j];
        secp256k1_test_state[i] = s;
    }
}

SECP256K1_INLINE static uint64_t rotl(const uint64_t x, int k) {
    return (x << k) | (x >> (64 - k));
}

SECP256K1_INLINE static uint64_t testrand64(void) {
    /* Test-only Xoshiro256++ RNG. See https://prng.di.unimi.it/ */
    const uint64_t result = rotl(secp256k1_test_state[0] + secp256k1_test_state[3], 23) + secp256k1_test_state[0];
    const uint64_t t = secp256k1_test_state[1] << 17;
    secp256k1_test_state[2] ^= secp256k1_test_state[0];
    secp256k1_test_state[3] ^= secp256k1_test_state[1];
    secp256k1_test_state[1] ^= secp256k1_test_state[2];
    secp256k1_test_state[0] ^= secp256k1_test_state[3];
    secp256k1_test_state[2] ^= t;
    secp256k1_test_state[3] = rotl(secp256k1_test_state[3], 45);
    return result;
}

SECP256K1_INLINE static uint64_t testrand_bits(int bits) {
    if (bits == 0) return 0;
    return testrand64() >> (64 - bits);
}

SECP256K1_INLINE static uint32_t testrand32(void) {
    return testrand64() >> 32;
}

static uint32_t testrand_int(uint32_t range) {
    uint32_t mask = 0;
    uint32_t range_copy;
    /* Reduce range by 1, changing its meaning to "maximum value". */
    VERIFY_CHECK(range != 0);
    range -= 1;
    /* Count the number of bits in range. */
    range_copy = range;
    while (range_copy) {
        mask = (mask << 1) | 1U;
        range_copy >>= 1;
    }
    /* Generation loop. */
    while (1) {
        uint32_t val = testrand64() & mask;
        if (val <= range) return val;
    }
}

static void testrand256(unsigned char *b32) {
    int i;
    for (i = 0; i < 4; ++i) {
        uint64_t val = testrand64();
        b32[0] = val;
        b32[1] = val >> 8;
        b32[2] = val >> 16;
        b32[3] = val >> 24;
        b32[4] = val >> 32;
        b32[5] = val >> 40;
        b32[6] = val >> 48;
        b32[7] = val >> 56;
        b32 += 8;
    }
}

static void testrand_bytes_test(unsigned char *bytes, size_t len) {
    size_t bits = 0;
    memset(bytes, 0, len);
    while (bits < len * 8) {
        int now;
        uint32_t val;
        now = 1 + (testrand_bits(6) * testrand_bits(5) + 16) / 31;
        val = testrand_bits(1);
        while (now > 0 && bits < len * 8) {
            bytes[bits / 8] |= val << (bits % 8);
            now--;
            bits++;
        }
    }
}

static void testrand256_test(unsigned char *b32) {
    testrand_bytes_test(b32, 32);
}

static void testrand_flip(unsigned char *b, size_t len) {
    b[testrand_int(len)] ^= (1 << testrand_bits(3));
}

static void testrand_init(const char* hexseed) {
    unsigned char seed16[16] = {0};
    if (hexseed && strlen(hexseed) != 0) {
        int pos = 0;
        while (pos < 16 && hexseed[0] != 0 && hexseed[1] != 0) {
            unsigned short sh;
            if ((sscanf(hexseed, "%2hx", &sh)) == 1) {
                seed16[pos] = sh;
            } else {
                break;
            }
            hexseed += 2;
            pos++;
        }
    } else {
        FILE *frand = fopen("/dev/urandom", "rb");
        if ((frand == NULL) || fread(&seed16, 1, sizeof(seed16), frand) != sizeof(seed16)) {
            uint64_t t = time(NULL) * (uint64_t)1337;
            fprintf(stderr, "WARNING: could not read 16 bytes from /dev/urandom; falling back to insecure PRNG\n");
            seed16[0] ^= t;
            seed16[1] ^= t >> 8;
            seed16[2] ^= t >> 16;
            seed16[3] ^= t >> 24;
            seed16[4] ^= t >> 32;
            seed16[5] ^= t >> 40;
            seed16[6] ^= t >> 48;
            seed16[7] ^= t >> 56;
        }
        if (frand) {
            fclose(frand);
        }
    }

    printf("random seed = %02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x\n", seed16[0], seed16[1], seed16[2], seed16[3], seed16[4], seed16[5], seed16[6], seed16[7], seed16[8], seed16[9], seed16[10], seed16[11], seed16[12], seed16[13], seed16[14], seed16[15]);
    testrand_seed(seed16);
}

static void testrand_finish(void) {
    unsigned char run32[32];
    testrand256(run32);
    printf("random run = %02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x\n", run32[0], run32[1], run32[2], run32[3], run32[4], run32[5], run32[6], run32[7], run32[8], run32[9], run32[10], run32[11], run32[12], run32[13], run32[14], run32[15]);
}

#endif /* SECP256K1_TESTRAND_IMPL_H */