How to use Shamir Secret Sharing Class in Crypto++

Question

I tried to use the SecretSharing Class in Crypto++, but I couldn't make it work.

Here is my code:

using namespace CryptoPP;

void secretSharing(){
    AutoSeededRandomPool rng;
    SecretSharing shamir(rng, 4, 6); 
    byte test[] = {'a', 'b', 'c', 'd'};
    shamir.Put(test, 4); 
    //shamir.MessageEnd();

    //cout << shamir.TotalBytesRetrievable() <<endl;
}

After compile and run, I will get:

./main 
terminate called after throwing an instance of 'CryptoPP::BufferedTransformation::NoChannelSupport'
  what():  unknown: this object doesn't support multiple channels
[1]    3597 abort (core dumped)  ./main

The declaration of SecretSharing::SecretSharing() is:

SecretSharing (RandomNumberGenerator &rng, int threshold, int nShares, BufferedTransformation *attachment=NULL, bool addPadding=true)

Should I give it a BufferedTransformation*, but exactly which class should I use?

Are there any Secret Sharing example code in Crypto++?

Answer 1

Building on Fraser's answer, here's the code.

void SecretShareFile(int threshold, int nShares, const char *filename, const char *seed)
{
    RandomPool rng;
    rng.IncorporateEntropy((byte *)seed, strlen(seed));

    ChannelSwitch *channelSwitch;
    FileSource source(filename, false, new SecretSharing(rng,
        threshold, nShares, channelSwitch = new ChannelSwitch));

    vector_member_ptrs<FileSink> fileSinks(nShares);
    string channel;
    for (int i=0; i<nShares; i++)
    {
        char extension[5] = ".000";
        extension[1]='0'+byte(i/100);
        extension[2]='0'+byte((i/10)%10);
        extension[3]='0'+byte(i%10);
        fileSinks[i].reset(new FileSink((string(filename)+extension).c_str()));

        channel = WordToString<word32>(i);
        fileSinks[i]->Put((byte *)channel.data(), 4);
        channelSwitch->AddRoute(channel, *fileSinks[i], DEFAULT_CHANNEL);
    }

    source.PumpAll();
}

In the code above, there's a named ChannelSwitch variable created with new. The FileSource source owns it and will delete it. But he needed a named variable (and not an anonymous or temporary) because he later calls channelSwitch->AddRoute

Wei could have done it with a Redirector to allow a stack allocation (stay out of the memory manager) and to ensure the FileSource filter does not delete it:

ChannelSwitch channelSwitch;
FileSource source(filename, false, new SecretSharing(rng,
    threshold, nShares, new Redirector(channelSwitch));
...
channelSwitch.AddRoute(channel, *fileSinks[i], DEFAULT_CHANNEL);

And the recovery code:

void SecretRecoverFile(int threshold, const char *outFilename, char *const *inFilenames)
{
    SecretRecovery recovery(threshold, new FileSink(outFilename));

    vector_member_ptrs<FileSource> fileSources(threshold);
    SecByteBlock channel(4);
    int i;
    for (i=0; i<threshold; i++)
    {
        fileSources[i].reset(new FileSource(inFilenames[i], false));
        fileSources[i]->Pump(4);
        fileSources[i]->Get(channel, 4);
        fileSources[i]->Attach(new ChannelSwitch(recovery, string((char *)channel.begin(), 4)));
    }

    while (fileSources[0]->Pump(256))
        for (i=1; i<threshold; i++)
            fileSources[i]->Pump(256);

    for (i=0; i<threshold; i++)
        fileSources[i]->PumpAll();
}