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// Copyright (c) 2026 Bitflash developers
// Distributed under the MIT/X11 software license.
//
// See compactblock.h.
#include "headers.h"
#include "compactblock.h"
#include <sodium.h>
#include <map>
uint256 CCmpctBlock::HeaderHash() const
{
// Same technique CBlock::GetHash() uses: the header fields are laid out
// contiguously and in the same order here as in CBlock, so hashing the
// raw member range gives the identical block-identity hash.
return Hash(BEGIN(nVersion), END(nNonce));
}
uint64 ComputeShortTxId(const uint256& hashPrevBlock, uint64 shortIdNonce, const uint256& txid)
{
// Derive a 16-byte SipHash key from (hashPrevBlock, shortIdNonce), so
// short IDs are specific to this block: they can't be precomputed
// before the block (and its freshly-chosen nonce) exist, and the same
// transaction gets a different short ID in every block it appears in
// (relevant across reorgs/re-announcements).
unsigned char keyMaterial[32 + sizeof(uint64)];
memcpy(keyMaterial, BEGIN(hashPrevBlock), 32);
memcpy(keyMaterial + 32, &shortIdNonce, sizeof(uint64));
uint256 keyHash = Hash((const char*)keyMaterial, (const char*)keyMaterial + sizeof(keyMaterial));
unsigned char out[crypto_shorthash_BYTES]; // 8 bytes
crypto_shorthash(out, (const unsigned char*)BEGIN(txid), 32, (const unsigned char*)BEGIN(keyHash));
uint64 id = 0;
memcpy(&id, out, sizeof(uint64));
return id & 0xFFFFFFFFFFFFULL; // low 48 bits -- matches the wire field width
}
bool BuildCompactBlock(const CBlock& block, CCmpctBlock& out)
{
if (block.vtx.empty())
return false; // every real block has at least the coinbase
out.nVersion = block.nVersion;
out.hashPrevBlock = block.hashPrevBlock;
out.hashMerkleRoot = block.hashMerkleRoot;
out.nTime = block.nTime;
out.nBits = block.nBits;
out.nNonce = block.nNonce;
// Upstream ignored the RAND_bytes return value here. On failure nonceBytes
// is left uninitialised and an indeterminate shortIdNonce goes out on the
// wire -- which is precisely the value this file relies on being
// unpredictable to stop short IDs being precomputed or ground against.
// Fail closed instead, matching every other RAND_bytes call site in this
// tree (bip32.cpp, db.cpp): the caller falls back to full-block relay,
// which is always correct.
unsigned char nonceBytes[sizeof(uint64)];
if (RAND_bytes(nonceBytes, sizeof(nonceBytes)) != 1)
return false;
memcpy(&out.shortIdNonce, nonceBytes, sizeof(uint64));
out.vPrefilledIndex.clear();
out.vPrefilledTx.clear();
out.vShortTxId.clear();
// Only the coinbase is prefilled (index 0) -- see file comment in
// compactblock.h for why that's enough for this chain's scale.
out.vPrefilledIndex.push_back(0);
out.vPrefilledTx.push_back(block.vtx[0]);
for (size_t i = 1; i < block.vtx.size(); i++)
out.vShortTxId.push_back(ComputeShortTxId(out.hashPrevBlock, out.shortIdNonce, block.vtx[i].GetHash()));
return true;
}
bool TryReconstructBlock(const CCmpctBlock& cmpct, CBlock& outBlock, std::vector<unsigned int>& vMissingIndex)
{
vMissingIndex.clear();
size_t nTotal = cmpct.vPrefilledIndex.size() + cmpct.vShortTxId.size();
if (cmpct.vPrefilledIndex.size() != cmpct.vPrefilledTx.size() || nTotal == 0)
return false; // malformed
// nTotal comes straight off the wire once this is wired into the P2P
// layer, and serialize.h only bounds a vector length by MAX_SIZE
// (0x02000000), so an unbounded assign() below would let one 60-odd-byte
// message ask this node for ~33M default-constructed CTransactions. The
// smallest a transaction can serialise to is 10 bytes (nVersion, empty
// vin, empty vout, nLockTime), so no block within MAX_BLOCK_SIZE can hold
// more than MAX_BLOCK_SIZE/10 of them; anything above that is not a block
// we could ever accept. Rejecting here just falls back to full-block
// relay, which is always correct.
if (nTotal > MAX_BLOCK_SIZE / 10)
return false; // implausible tx count -- refuse to size a vector from it
outBlock.SetNull();
outBlock.nVersion = cmpct.nVersion;
outBlock.hashPrevBlock = cmpct.hashPrevBlock;
outBlock.hashMerkleRoot = cmpct.hashMerkleRoot;
outBlock.nTime = cmpct.nTime;
outBlock.nBits = cmpct.nBits;
outBlock.nNonce = cmpct.nNonce;
outBlock.vtx.assign(nTotal, CTransaction());
std::vector<bool> vFilled(nTotal, false);
for (size_t i = 0; i < cmpct.vPrefilledIndex.size(); i++)
{
unsigned int idx = cmpct.vPrefilledIndex[i];
if (idx >= nTotal)
return false; // malformed
outBlock.vtx[idx] = cmpct.vPrefilledTx[i];
vFilled[idx] = true;
}
// Index every mempool tx by its short ID under this block's keying, so
// resolving each block position is a map lookup rather than a linear
// scan of the mempool per short ID.
std::map<uint64, const CTransaction*> mapShortIdToTx;
for (std::map<uint256, CTransaction>::const_iterator mi = mapTransactions.begin();
mi != mapTransactions.end(); ++mi)
{
uint64 sid = ComputeShortTxId(cmpct.hashPrevBlock, cmpct.shortIdNonce, mi->first);
// A short-ID collision between two distinct mempool transactions is
// possible (48 bits, birthday bound ~2^24 mempool entries) though
// very unlikely at this chain's scale; on collision, prefer keeping
// neither mapped rather than risk resolving to the wrong tx --
// that position just falls through to getblocktxn like a genuine
// miss, which is always correct, only occasionally not optimal.
if (mapShortIdToTx.count(sid))
mapShortIdToTx[sid] = NULL;
else
mapShortIdToTx[sid] = &mi->second;
}
size_t shortIdPos = 0;
for (size_t idx = 0; idx < nTotal; idx++)
{
if (vFilled[idx])
continue;
if (shortIdPos >= cmpct.vShortTxId.size())
return false; // malformed: more unfilled positions than short IDs
uint64 sid = cmpct.vShortTxId[shortIdPos++];
std::map<uint64, const CTransaction*>::const_iterator found = mapShortIdToTx.find(sid);
if (found != mapShortIdToTx.end() && found->second != NULL)
{
outBlock.vtx[idx] = *found->second;
vFilled[idx] = true;
}
else
{
vMissingIndex.push_back(idx);
}
}
return vMissingIndex.empty();
}
bool FillMissingAndFinish(CBlock& outBlock, const std::vector<unsigned int>& vMissingIndex,
const std::vector<CTransaction>& vTxn)
{
if (vMissingIndex.size() != vTxn.size())
return false; // peer didn't answer what was asked
for (size_t i = 0; i < vMissingIndex.size(); i++)
{
unsigned int idx = vMissingIndex[i];
if (idx >= outBlock.vtx.size())
return false; // malformed
outBlock.vtx[idx] = vTxn[i];
}
return true;
}
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