derive BIP 32

gistfile1.txt

void derive(cx_curve_t curve, unsigned int *path, unsigned int pathLength,
            unsigned char *privateKey, unsigned char *chain) {
    unsigned char tmpPrivateChain[64];
    unsigned int i;
    cx_ecfp_private_key_t tmpEcdsaPrivate;
    cx_curve_domain_t *domain = cx_ecfp_get_domain(curve);

    os_memmove(tmpPrivateChain, privateKey, 32);
    os_memmove(tmpPrivateChain + 32, chain, 32);

    for (i = 0; i < pathLength; i++) {
        unsigned char tmp[65];

        if (((path[i] >> 24) & 0x80) != 0) {
            tmp[0] = 0;
            os_memmove(tmp + 1, tmpPrivateChain, 32);
        } else {
            if (curve == CX_CURVE_Ed25519) {
                THROW(EXCEPTION);
            }
            cx_ecfp_public_key_t tmpEcdsaPublic;
            cx_ecdsa_init_private_key(curve, tmpPrivateChain, 32,
                                      &tmpEcdsaPrivate);
            cx_ecfp_generate_pair(curve, &tmpEcdsaPublic, &tmpEcdsaPrivate, 1);
            tmpEcdsaPublic.W[0] = ((tmpEcdsaPublic.W[64] & 1) ? 0x03 : 0x02);
            os_memmove(tmp, tmpEcdsaPublic.W, 33);
        }

        for (;;) {
            unsigned char failed = 0;

            tmp[33] = ((path[i] >> 24) & 0xff);
            tmp[34] = ((path[i] >> 16) & 0xff);
            tmp[35] = ((path[i] >> 8) & 0xff);
            tmp[36] = (path[i] & 0xff);

            cx_hmac_sha512(tmpPrivateChain + 32, 32, tmp, 37, tmp);

            if (curve != CX_CURVE_Ed25519) {
                if (cx_math_cmp(tmp, domain->n, 32) >= 0) {
                    failed = 1;
                } else {
                    cx_math_addm(tmp, tmp, tmpPrivateChain, domain->n, 32);
                    failed = cx_math_is_zero(tmp, 32);
                }
            }
            if (!failed) {
                break;
            }
            tmp[0] = 1;
            os_memmove(tmp + 1, tmp + 32, 32);
        }

        os_memmove(tmpPrivateChain, tmp, 32);
        os_memmove(tmpPrivateChain + 32, tmp + 32, 32);
    }

    os_memmove(privateKey, tmpPrivateChain, 32);
    os_memmove(chain, tmpPrivateChain + 32, 32);
}