pickparams.c

#include <stdio.h>
#include <inttypes.h>
#include <math.h>

// Function, which calculates N, p and r from opslimit and memslimit copied from
// libsodium/crypto_pwhash/scryptsalsa208sha256/pwhash_scryptsalsa208sha256.c
int pickparams(unsigned long long opslimit, const size_t memlimit,
        uint32_t *const N_log2, uint32_t *const p, uint32_t *const r) {
    unsigned long long maxN;
    unsigned long long maxrp;

    if (opslimit < 32768) {
        opslimit = 32768;
    }
    *r = 8;
    if (opslimit < memlimit / 32) {
        *p = 1;
        maxN = opslimit / (*r * 4);
        for (*N_log2 = 1; *N_log2 < 63; *N_log2 += 1) {
            if ((uint64_t)(1) << *N_log2 > maxN / 2) {
                break;
            }
        }
    } else {
        maxN = memlimit / ((size_t) *r * 128);
        for (*N_log2 = 1; *N_log2 < 63; *N_log2 += 1) {
            if ((uint64_t)(1) << *N_log2 > maxN / 2) {
                break;
            }
        }
        maxrp = (opslimit / 4) / ((uint64_t)(1) << *N_log2);
        /* LCOV_EXCL_START */
        if (maxrp > 0x3fffffff) {
            maxrp = 0x3fffffff;
        }
        /* LCOV_EXCL_STOP */
        *p = (uint32_t)(maxrp) / *r;
    }
    return 0;
}

int main() {
    unsigned long long opslimit = pow(2, 20);
    const size_t memlimit = pow(2, 24);

    uint32_t N_log2;
    uint32_t p;
    uint32_t r;
    pickparams(opslimit, memlimit, &N_log2, &p, &r);

    printf("N: %" PRIu64 "\n", (uint64_t)(1) << N_log2);
    printf("r: %" PRIu32 "\n", r);
    printf("p: %" PRIu32 "\n", p);

    return 0;
}