std::vector<RTSS_Share>
RTSS_Share::split(byte M, byte N,
const byte S[], u16bit S_len,
const byte identifier[16],
RandomNumberGenerator& rng)
{
if(M == 0 || N == 0 || M > N)
throw Encoding_Error("RTSS_Share::split: M == 0 or N == 0 or M > N");
SHA_256 hash; // always use SHA-256 when generating shares
std::vector<RTSS_Share> shares(N);
// Create RTSS header in each share
for(byte i = 0; i != N; ++i)
{
shares[i].m_contents += std::make_pair(identifier, 16);
shares[i].m_contents += rtss_hash_id(hash.name());
shares[i].m_contents += M;
shares[i].m_contents += get_byte(0, S_len);
shares[i].m_contents += get_byte(1, S_len);
}
// Choose sequential values for X starting from 1
for(byte i = 0; i != N; ++i)
shares[i].m_contents.push_back(i+1);
// secret = S || H(S)
secure_vector<byte> secret(S, S + S_len);
secret += hash.process(S, S_len);
for(size_t i = 0; i != secret.size(); ++i)
{
std::vector<byte> coefficients(M-1);
rng.randomize(coefficients.data(), coefficients.size());
for(byte j = 0; j != N; ++j)
{
const byte X = j + 1;
byte sum = secret[i];
byte X_i = X;
for(size_t k = 0; k != coefficients.size(); ++k)
{
sum ^= gfp_mul(X_i, coefficients[k]);
X_i = gfp_mul(X_i, X);
}
shares[j].m_contents.push_back(sum);
}
}
return shares;
}
std::vector<RTSS_Share>
RTSS_Share::split(uint8_t M, uint8_t N,
const uint8_t S[], uint16_t S_len,
const std::vector<uint8_t>& identifier,
const std::string& hash_fn,
RandomNumberGenerator& rng)
{
if(M <= 1 || N <= 1 || M > N || N >= 255)
throw Invalid_Argument("RTSS_Share::split: Invalid N or M");
if(identifier.size() > 16)
throw Invalid_Argument("RTSS_Share::split Invalid identifier size");
const uint8_t hash_id = rtss_hash_id(hash_fn);
std::unique_ptr<HashFunction> hash;
if(hash_id > 0)
hash = HashFunction::create_or_throw(hash_fn);
// secret = S || H(S)
secure_vector<uint8_t> secret(S, S + S_len);
if(hash)
secret += hash->process(S, S_len);
if(secret.size() >= 0xFFFE)
throw Encoding_Error("RTSS_Share::split secret too large for TSS format");
// +1 byte for the share ID
const uint16_t share_len = static_cast<uint16_t>(secret.size() + 1);
secure_vector<uint8_t> share_header(RTSS_HEADER_SIZE);
copy_mem(&share_header[0], identifier.data(), identifier.size());
share_header[16] = hash_id;
share_header[17] = M;
share_header[18] = get_byte(0, share_len);
share_header[19] = get_byte(1, share_len);
// Create RTSS header in each share
std::vector<RTSS_Share> shares(N);
for(uint8_t i = 0; i != N; ++i)
{
shares[i].m_contents.reserve(share_header.size() + share_len);
shares[i].m_contents = share_header;
}
// Choose sequential values for X starting from 1
for(uint8_t i = 0; i != N; ++i)
shares[i].m_contents.push_back(i+1);
for(size_t i = 0; i != secret.size(); ++i)
{
std::vector<uint8_t> coefficients(M-1);
rng.randomize(coefficients.data(), coefficients.size());
for(uint8_t j = 0; j != N; ++j)
{
const uint8_t X = j + 1;
uint8_t sum = secret[i];
uint8_t X_i = X;
for(size_t k = 0; k != coefficients.size(); ++k)
{
sum ^= gfp_mul(X_i, coefficients[k]);
X_i = gfp_mul(X_i, X);
}
shares[j].m_contents.push_back(sum);
}
}
return shares;
}
