int PKCS5_PBKDF2_HMAC(const char *pass, int passlen,
                      const unsigned char *salt, int saltlen, int iter,
                      const EVP_MD *digest, int keylen, unsigned char *out)
{
    unsigned char digtmp[EVP_MAX_MD_SIZE], *p, itmp[4];
    int cplen, j, k, tkeylen, mdlen;
    unsigned long i = 1;
    HMAC_CTX *hctx_tpl = NULL, *hctx = NULL;

    mdlen = EVP_MD_size(digest);
    if (mdlen < 0)
        return 0;

    hctx_tpl = HMAC_CTX_new();
    if (hctx_tpl == NULL)
        return 0;
    p = out;
    tkeylen = keylen;
    if (!pass)
        passlen = 0;
    else if (passlen == -1)
        passlen = strlen(pass);
    if (!HMAC_Init_ex(hctx_tpl, pass, passlen, digest, NULL)) {
        HMAC_CTX_free(hctx_tpl);
        return 0;
    }
    hctx = HMAC_CTX_new();
    if (hctx == NULL) {
        HMAC_CTX_free(hctx_tpl);
        return 0;
    }
    while (tkeylen) {
        if (tkeylen > mdlen)
            cplen = mdlen;
        else
            cplen = tkeylen;
        /*
         * We are unlikely to ever use more than 256 blocks (5120 bits!) but
         * just in case...
         */
        itmp[0] = (unsigned char)((i >> 24) & 0xff);
        itmp[1] = (unsigned char)((i >> 16) & 0xff);
        itmp[2] = (unsigned char)((i >> 8) & 0xff);
        itmp[3] = (unsigned char)(i & 0xff);
        if (!HMAC_CTX_copy(hctx, hctx_tpl)) {
            HMAC_CTX_free(hctx);
            HMAC_CTX_free(hctx_tpl);
            return 0;
        }
        if (!HMAC_Update(hctx, salt, saltlen)
            || !HMAC_Update(hctx, itmp, 4)
            || !HMAC_Final(hctx, digtmp, NULL)) {
            HMAC_CTX_free(hctx);
            HMAC_CTX_free(hctx_tpl);
            return 0;
        }
        HMAC_CTX_reset(hctx);
        memcpy(p, digtmp, cplen);
        for (j = 1; j < iter; j++) {
            if (!HMAC_CTX_copy(hctx, hctx_tpl)) {
                HMAC_CTX_free(hctx);
                HMAC_CTX_free(hctx_tpl);
                return 0;
            }
            if (!HMAC_Update(hctx, digtmp, mdlen)
                || !HMAC_Final(hctx, digtmp, NULL)) {
                HMAC_CTX_free(hctx);
                HMAC_CTX_free(hctx_tpl);
                return 0;
            }
            HMAC_CTX_reset(hctx);
            for (k = 0; k < cplen; k++)
                p[k] ^= digtmp[k];
        }
        tkeylen -= cplen;
        i++;
        p += cplen;
    }
    HMAC_CTX_free(hctx);
    HMAC_CTX_free(hctx_tpl);
# ifdef OPENSSL_DEBUG_PKCS5V2
    fprintf(stderr, "Password:\n");
    h__dump(pass, passlen);
    fprintf(stderr, "Salt:\n");
    h__dump(salt, saltlen);
    fprintf(stderr, "Iteration count %d\n", iter);
    fprintf(stderr, "Key:\n");
    h__dump(out, keylen);
# endif
    return 1;
}