Implement ZIP 244 txid logic for v5 transactions

[str4d]

Support for parsing v5 transactions (as specified in ZIP 225) was added in #367. However, the txid computation for v5 transactions (as specified in ZIP 244) was not implemented. This means that at present, lightwalletd returns incorrect txids for v5 transactions in compact blocks:

lightwalletd/common/common.go

Lines 261 to 274 in ab4c0fe

	block := parser.NewBlock()
	rest, err := block.ParseFromSlice(blockData)
	if err != nil {
	return nil, errors.Wrap(err, "error parsing block")
	}
	if len(rest) != 0 {
	return nil, errors.New("received overlong message")
	}
	if block.GetHeight() != height {
	return nil, errors.New("received unexpected height block")
	}
	return block.ToCompact(), nil

lightwalletd/parser/block.go

Lines 108 to 127 in ab4c0fe

	// ToCompact returns the compact representation of the full block.
	func (b *Block) ToCompact() *walletrpc.CompactBlock {
	compactBlock := &walletrpc.CompactBlock{
	//TODO ProtoVersion: 1,
	Height: uint64(b.GetHeight()),
	PrevHash: b.hdr.HashPrevBlock,
	Hash: b.GetEncodableHash(),
	Time: b.hdr.Time,
	}
	// Only Sapling transactions have a meaningful compact encoding
	saplingTxns := make([]*walletrpc.CompactTx, 0, len(b.vtx))
	for idx, tx := range b.vtx {
	if tx.HasShieldedElements() {
	saplingTxns = append(saplingTxns, tx.ToCompact(idx))
	}
	}
	compactBlock.Vtx = saplingTxns
	return compactBlock
	}

lightwalletd/parser/transaction.go

Lines 379 to 383 in ab4c0fe

	// ToCompact converts the given (full) transaction to compact format.
	func (tx *Transaction) ToCompact(index int) *walletrpc.CompactTx {
	ctx := &walletrpc.CompactTx{
	Index: uint64(index), // index is contextual
	Hash: tx.GetEncodableHash(),

lightwalletd/parser/transaction.go

Lines 346 to 365 in ab4c0fe

	// GetDisplayHash returns the transaction hash in big-endian display order.
	func (tx *Transaction) GetDisplayHash() []byte {
	if tx.cachedTxID != nil {
	return tx.cachedTxID
	}
	// SHA256d
	digest := sha256.Sum256(tx.rawBytes)
	digest = sha256.Sum256(digest[:])
	// Convert to big-endian
	tx.cachedTxID = Reverse(digest[:])
	return tx.cachedTxID
	}
	// GetEncodableHash returns the transaction hash in little-endian wire format order.
	func (tx *Transaction) GetEncodableHash() []byte {
	digest := sha256.Sum256(tx.rawBytes)
	digest = sha256.Sum256(digest[:])
	return digest[:]
	}

[str4d]

#367 had test vectors that include txids; these passed because the test conditional was backwards, and checking that the derived txid didn't match the test vector.

lightwalletd/parser/transaction_test.go

Lines 88 to 90 in ab4c0fe

	if bytes.Equal(tx.cachedTxID, []byte(txtestdata.Txid)) {
	t.Fatal("txid")
	}

[str4d]

Reopening because this shouldn't have been closed by #393 (which only partially fixed the problem, the internal code still derives the wrong txid and needs fixing).