I had the same issue as in this (very) old unanswered thread below. It took
me quite a while to figure out why. In case it might help someone else
puzzled by the same, the root cause is insufficient buffer size.
Specifically, the "Shares[i]" should be made larger.
Based on the original poster's code, the "last byte or two" problem can be
shown by:
const Bytes secret = {0x01, 0x02, 0x03, 0x04, 0x05};
const auto shares = SecretShareBytes(secret, 3, 7);
std::vector<Bytes> partial;
partial.push_back(shares[6]);
partial.push_back(shares[3]);
partial.push_back(shares[5]);
const auto recovered = SecretRecoverBytes(partial, partial.size());
std::cout << "Secret -> " << std::dec << secret.size() << " bytes: ";
for (const auto & uch : secret)
std::cout << std::hex << std::setfill('0') << std::setw(2)
<< static_cast<unsigned>(uch) << " ";
std::cout << std::endl;
std::cout << "Recovered -> " << std::dec << recovered.size() << "
bytes: ";
for (const auto & uch : recovered)
std::cout << std::hex << std::setfill('0') << std::setw(2)
<< static_cast<unsigned>(uch) << " ";
std::cout << std::endl;
The output will be something like:
Secret -> 5 bytes: 01 02 03 04 05
Recovered -> 5 bytes: 01 02 03 04 7a
What I found is that if the output is to FileSink (as
in https://github.com/weidai11/cryptopp/blob/master/test.cpp#L657), the
share/recovery works perfectly. Yet strangely ArraySink always gives
trouble. The culprit seems to be in:
shares[i] = Bytes( secret.size() + sizeof(int) );
This should be given a larger size. Otherwise the outputs of SecretSharing
are chopped off. With the incomplete shares, re-construction can't be
successful.
There might be a way to precisely decide the correct buffer size for the
shares. However, I find it easier to use ostringstream to manage the
buffers. Counter-intuitively, ArraySink will have to be replaced by
FileSink, due to the fact that ArraySink only accepts fixed length buffers.
With this change, the original poster's code would look like:
std::vector<Bytes> SecretShareBytes(const Bytes& secret, int threshold, int
nShares)
{
CryptoPP::AutoSeededRandomPool rng;
CryptoPP::ChannelSwitch *channelSwitch;
CryptoPP::ArraySource source( secret.data(), secret.size(), false,new
CryptoPP::SecretSharing( rng, threshold, nShares, channelSwitch = new
CryptoPP::ChannelSwitch) );
std::vector<std::ostringstream> shares( nShares );
CryptoPP::vector_member_ptrs<CryptoPP::FileSink> sinks( nShares );
std::string channel;
for (int i = 0; i < nShares; i++)
{
sinks[i].reset( new CryptoPP::FileSink(shares[i]));
channel = CryptoPP::WordToString<word32>(i);
sinks[i]->Put( (byte *)channel.data(), 4 );
channelSwitch->AddRoute( channel,*sinks[i], DEFAULT_CHANNEL);
}
source.PumpAll();
std::vector<Bytes> ret;
for (const auto &share : shares)
{
const auto & piece = share.str();
ret.push_back(Bytes(piece.begin(), piece.begin() + piece.size()));
}
return move(ret);
}
Bytes SecretRecoverBytes(std::vector<Bytes>& shares, int threshold)
{
std::ostringstream out;
CryptoPP::SecretRecovery recovery( threshold, new
CryptoPP::FileSink(out) );
CryptoPP::SecByteBlock channel(4);
for (int i = 0; i < threshold; i++)
{
CryptoPP::ArraySource arraySource(shares[i].data(),
shares[i].size(), false);
arraySource.Pump(4);
arraySource.Get( channel, 4 );
arraySource.Attach( new CryptoPP::ChannelSwitch( recovery,
std::string( (char *)channel.begin(), 4) ) );
arraySource.PumpAll();
}
const auto & secret = out.str();
return Bytes(secret.begin(), secret.begin() + secret.size());
}
And the print out will be correct.
Secret -> 5 bytes: 01 02 03 04 05
Recovered -> 5 bytes: 01 02 03 04 05
On Friday, 11 December 2015 10:34:42 UTC, Whou Lee wrote:
>
> Hi, all!
> I am trying to make CryptoPP::SecretSharing to work with in memory byte
> objects.
> My current solution partly works. I can reconstruct the original secret
> except for the last byte or two, depends on the length of original secret.
> Could you please suggest me how to fix this.
>
> The code:
> std::vector<Bytes> SecretShareBytes(const Bytes& secret, int threshold,
> int nShares)
> {
> CryptoPP::AutoSeededRandomPool rng;
>
> CryptoPP::ChannelSwitch *channelSwitch;
> CryptoPP::ArraySource source( secret.data(), secret.size(), false,new
> CryptoPP::SecretSharing( rng, threshold, nShares, channelSwitch = new
> CryptoPP::ChannelSwitch) );
>
> std::vector<Bytes> shares( nShares );
> CryptoPP::vector_member_ptrs<CryptoPP::ArraySink> arraySinks( nShares );
> std::string channel;
> for (int i = 0; i < nShares; i++)
> {
> shares[i] = Bytes( secret.size() + sizeof(int) );
> arraySinks[i].reset( new CryptoPP::ArraySink((byte*)shares[i].data(),
> shares[i].size()) );
>
> channel = CryptoPP::WordToString<word32>(i);
> arraySinks[i]->Put( (byte *)channel.data(), 4 );
> channelSwitch->AddRoute(
> channel,*arraySinks[i],CryptoPP::BufferedTransformation::NULL_CHANNEL );
> }
>
> source.PumpAll();
> return shares;
> }
>
>
> Bytes SecretRecoverBytes(std::vector<Bytes>& shares, int threshold)
> {
> Bytes bytes( shares[0].size() - sizeof( int ) );
> CryptoPP::SecretRecovery recovery( threshold, new
> CryptoPP::ArraySink(bytes.data(), bytes.size()) );
>
> CryptoPP::SecByteBlock channel(4);
> for (int i = 0; i < threshold; i++)
> {
> CryptoPP::ArraySource arraySource(shares[i].data(), shares[i].size(),
> false);
>
> arraySource.Pump(4);
> arraySource.Get( channel, 4 );
> arraySource.Attach( new CryptoPP::ChannelSwitch( recovery, std::string(
> (char *)channel.begin(), 4) ) );
>
> arraySource.PumpAll();
> }
>
> return bytes;
> }
>
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