prac/cdpf.tcc

// Templated method implementations for cdpf.hpp

// I/O for CDPFs

// Read the DPF from the output stream.  You can use this to read DPFs
// from files or from the network.
template <typename T>
T& operator>>(T &is, CDPF &cdpf)
{
    is.read((char *)&cdpf.seed, sizeof(cdpf.seed));
    cdpf.whichhalf = get_lsb(cdpf.seed);
    uint8_t depth = VALUE_BITS - 7;
    cdpf.cw.clear();
    for (uint8_t i=0; i<depth; ++i) {
        DPFnode cw;
        is.read((char *)&cw, sizeof(cw));
        cdpf.cw.push_back(cw);
    }
    value_t cfbits = 0;
    is.read((char *)&cfbits, BITBYTES(depth));
    cdpf.cfbits = cfbits;
    is.read((char *)&cdpf.leaf_cwr, sizeof(cdpf.leaf_cwr));
    is.read((char *)&cdpf.as_target, sizeof(cdpf.as_target));
    is.read((char *)&cdpf.xs_target, sizeof(cdpf.xs_target));

    return is;
}

// Write the DPF to the output stream.  You can use this to write DPFs
// to files or over the network.
template <typename T>
T& operator<<(T &os, const CDPF &cdpf)
{
    os.write((const char *)&cdpf.seed, sizeof(cdpf.seed));
    uint8_t depth = VALUE_BITS - 7;
    for (uint8_t i=0; i<depth; ++i) {
        os.write((const char *)&cdpf.cw[i], sizeof(cdpf.cw[i]));
    }
    os.write((const char *)&cdpf.cfbits, BITBYTES(depth));
    os.write((const char *)&cdpf.leaf_cwr, sizeof(cdpf.leaf_cwr));
    os.write((const char *)&cdpf.as_target, sizeof(cdpf.as_target));
    os.write((const char *)&cdpf.xs_target, sizeof(cdpf.xs_target));

    return os;
}

// Determine whether the given additively or XOR shared element is 0.
// The output is a bit share, which is a share of 1 iff the passed
// element is a share of 0.  Note also that you can compare two RegAS or
// RegXS values A and B for equality by passing A-B here.
//
// Cost:
// 1 word sent in 1 message
// VALUE_BITS - 7 = 57 local AES operations
template <typename T>
RegBS CDPF::is_zero(MPCTIO &tio, yield_t &yield,
    const T &x, size_t &aes_ops)
{
    // Reconstruct S = target-x
    // The server does nothing in this protocol
    if (tio.player() < 2) {
        T S_share;
        get_target(S_share);
        S_share -= x;
        tio.iostream_peer() << S_share;
        yield();
        T peer_S_share;
        tio.iostream_peer() >> peer_S_share;
        S_share += peer_S_share;
        value_t S = S_share.share();

        // After that one single-word exchange, the rest of this
        // algorithm is entirely a local computation.
        return is_zero(S, aes_ops);
    } else {
        yield();
    }
    // The server gets a share of 0
    RegBS eq;
    return eq;
}