string hexify(const vector<unsigned char>& v) {
    string s(2 * v.size(), 'x');

    string::iterator k = s.begin();

    for (auto i = v.begin(); i != v.end(); ++i) {
        *k++ = "0123456789ABCDEF"[*i >> 4];
        *k++ = "0123456789ABCDEF"[*i & 0x0F];
    }

    return s;
}