btc_b58check fails on DE0 Nano NIOS II

[jmaldon1]

I am trying to run libbtc on a DE0-Nano running NIOS II, I have been successful so far in implementing it but when I try to derive a child key the code below fails:

int btc_b58check(const void* bin, size_t binsz, const char* base58str)
{
    uint256 buf;

    const uint8_t* binc = bin;

    unsigned i;

    if (binsz < 4) {
        return -4;
    }
    sha256_Raw(bin, binsz - 4, buf);
    sha256_Raw(buf, sizeof(buf), buf);

    if (memcmp(&binc[binsz - 4], buf, 4)) {
        return -1;
    }
    ...
}

the if (memcmp(&binc[binsz - 4], buf, 4)) conditional fails. Would anyone be able to tell me why this happens?
I have tried commenting out this conditional and libbtc is able to produce results but the p2pkh address is not a valid testnet bitcoin address.