private_decrypt

static VALUE
ossl_rsa_private_decrypt(int argc, VALUE *argv, VALUE self)
{
    EVP_PKEY *pkey;
    int buf_len, pad;
    VALUE str, buffer, padding;

    GetPKeyRSA(self, pkey);
    if (!RSA_PRIVATE(self, pkey->pkey.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, ossl_rsa_buf_size(pkey));
    buf_len = RSA_private_decrypt(RSTRING_LEN(buffer), (unsigned char *)RSTRING_PTR(buffer),
                                  (unsigned char *)RSTRING_PTR(str), pkey->pkey.rsa,
                                  pad);
    if (buf_len < 0) ossl_raise(eRSAError, NULL);
    rb_str_set_len(str, buf_len);

    return str;
}