OpenSSL::PKey::RSA#private_decrypt

static VALUE
ossl_rsa_private_decrypt(int argc, VALUE *argv, VALUE self)
{
    RSA *rsa;
    const BIGNUM *rsa_n;
    int buf_len, pad;
    VALUE str, buffer, padding;

    GetRSA(self, rsa);
    RSA_get0_key(rsa, &rsa_n, NULL, NULL);
    if (!rsa_n)
        ossl_raise(eRSAError, "incomplete RSA");
    if (!RSA_PRIVATE(self, rsa))
        ossl_raise(eRSAError, "private key needed.");
    rb_scan_args(argc, argv, "11", &buffer, &padding);
    pad = (argc == 1) ? RSA_PKCS1_PADDING : NUM2INT(padding);
    StringValue(buffer);
    str = rb_str_new(0, RSA_size(rsa));
    buf_len = RSA_private_decrypt(RSTRING_LENINT(buffer), (unsigned char *)RSTRING_PTR(buffer),
                                  (unsigned char *)RSTRING_PTR(str), rsa, pad);
    if (buf_len < 0) ossl_raise(eRSAError, NULL);
    rb_str_set_len(str, buf_len);

    return str;
}