secure_vector<byte>
RTSS_Share::reconstruct(const std::vector<RTSS_Share>& shares)
{
const size_t RTSS_HEADER_SIZE = 20;
for(size_t i = 0; i != shares.size(); ++i)
{
if(shares[i].size() != shares[0].size())
throw Decoding_Error("Different sized RTSS shares detected");
if(shares[i].share_id() == 0)
throw Decoding_Error("Invalid (id = 0) RTSS share detected");
if(shares[i].size() < RTSS_HEADER_SIZE)
throw Decoding_Error("Missing or malformed RTSS header");
if(!same_mem(&shares[0].m_contents[0],
&shares[i].m_contents[0], RTSS_HEADER_SIZE))
throw Decoding_Error("Different RTSS headers detected");
}
if(shares.size() < shares[0].m_contents[17])
throw Decoding_Error("Insufficient shares to do TSS reconstruction");
u16bit secret_len = make_u16bit(shares[0].m_contents[18],
shares[0].m_contents[19]);
byte hash_id = shares[0].m_contents[16];
std::unique_ptr<HashFunction> hash(get_rtss_hash_by_id(hash_id));
if(shares[0].size() != secret_len + hash->output_length() + RTSS_HEADER_SIZE + 1)
throw Decoding_Error("Bad RTSS length field in header");
std::vector<byte> V(shares.size());
secure_vector<byte> secret;
for(size_t i = RTSS_HEADER_SIZE + 1; i != shares[0].size(); ++i)
{
for(size_t j = 0; j != V.size(); ++j)
V[j] = shares[j].m_contents[i];
byte r = 0;
for(size_t k = 0; k != shares.size(); ++k)
{
// L_i function:
byte r2 = 1;
for(size_t l = 0; l != shares.size(); ++l)
{
if(k == l)
continue;
byte share_k = shares[k].share_id();
byte share_l = shares[l].share_id();
if(share_k == share_l)
throw Decoding_Error("Duplicate shares found in RTSS recovery");
byte div = RTSS_EXP[(255 +
RTSS_LOG[share_l] -
RTSS_LOG[share_k ^ share_l]) % 255];
r2 = gfp_mul(r2, div);
}
r ^= gfp_mul(V[k], r2);
}
secret.push_back(r);
}
if(secret.size() != secret_len + hash->output_length())
throw Decoding_Error("Bad length in RTSS output");
hash->update(secret.data(), secret_len);
secure_vector<byte> hash_check = hash->final();
if(!same_mem(hash_check.data(),
&secret[secret_len], hash->output_length()))
throw Decoding_Error("RTSS hash check failed");
return secure_vector<byte>(secret.cbegin(), secret.cbegin() + secret_len);
}
secure_vector<uint8_t>
RTSS_Share::reconstruct(const std::vector<RTSS_Share>& shares)
{
if(shares.size() <= 1)
throw Decoding_Error("Insufficient shares to do TSS reconstruction");
for(size_t i = 0; i != shares.size(); ++i)
{
if(shares[i].size() < RTSS_HEADER_SIZE + 1)
throw Decoding_Error("Missing or malformed RTSS header");
if(shares[i].share_id() == 0)
throw Decoding_Error("Invalid (id = 0) RTSS share detected");
if(i > 0)
{
if(shares[i].size() != shares[0].size())
throw Decoding_Error("Different sized RTSS shares detected");
if(!same_mem(&shares[0].m_contents[0],
&shares[i].m_contents[0], RTSS_HEADER_SIZE))
throw Decoding_Error("Different RTSS headers detected");
}
}
const uint8_t N = shares[0].m_contents[17];
if(shares.size() < N)
throw Decoding_Error("Insufficient shares to do TSS reconstruction");
const uint16_t share_len = make_uint16(shares[0].m_contents[18],
shares[0].m_contents[19]);
const uint8_t hash_id = shares[0].m_contents[16];
std::unique_ptr<HashFunction> hash(get_rtss_hash_by_id(hash_id));
const size_t hash_len = (hash ? hash->output_length() : 0);
if(shares[0].size() != RTSS_HEADER_SIZE + share_len)
{
/*
* This second (laxer) check accomodates a bug in TSS that was
* fixed in 2.9.0 - previous versions used the length of the
* *secret* here, instead of the length of the *share*, which is
* precisely 1 + hash_len longer.
*/
if(shares[0].size() <= RTSS_HEADER_SIZE + 1 + hash_len)
throw Decoding_Error("Bad RTSS length field in header");
}
std::vector<uint8_t> V(shares.size());
secure_vector<uint8_t> recovered;
for(size_t i = RTSS_HEADER_SIZE + 1; i != shares[0].size(); ++i)
{
for(size_t j = 0; j != V.size(); ++j)
V[j] = shares[j].m_contents[i];
uint8_t r = 0;
for(size_t k = 0; k != shares.size(); ++k)
{
// L_i function:
uint8_t r2 = 1;
for(size_t l = 0; l != shares.size(); ++l)
{
if(k == l)
continue;
uint8_t share_k = shares[k].share_id();
uint8_t share_l = shares[l].share_id();
if(share_k == share_l)
throw Decoding_Error("Duplicate shares found in RTSS recovery");
uint8_t div = RTSS_EXP[(255 +
RTSS_LOG[share_l] -
RTSS_LOG[share_k ^ share_l]) % 255];
r2 = gfp_mul(r2, div);
}
r ^= gfp_mul(V[k], r2);
}
recovered.push_back(r);
}
if(hash)
{
if(recovered.size() < hash->output_length())
throw Decoding_Error("RTSS recovered value too short to be valid");
const size_t secret_len = recovered.size() - hash->output_length();
hash->update(recovered.data(), secret_len);
secure_vector<uint8_t> hash_check = hash->final();
if(!constant_time_compare(hash_check.data(),
&recovered[secret_len],
hash->output_length()))
{
throw Decoding_Error("RTSS hash check failed");
}
// remove the trailing hash value
recovered.resize(secret_len);
}
return recovered;
}
