Skip to main content

​
Manual Proof Submission (Fallback Path)

What to do if the Writz relayer is unavailable and you need to submit an SPV proof yourself. bitcoin-spv::verify_transaction (see SPV Verification) takes raw_tx as the non-witness serialization of a Bitcoin transaction - the txid is SHA256d(raw_tx), and Bitcoin’s block Merkle tree is built from non-witness txids. The Writz relayer strips SegWit witness data automatically before submitting a proof. This document explains that step so a technical user can reproduce it manually if the relayer is down. This is a liveness concern only: if the raw transaction bytes are stripped incorrectly, verify_transaction simply fails with MerkleProofInvalid and your deposit is rejected - it can be retried with correct bytes. No funds are ever at risk from a stripping mistake.

​
Why stripping is needed

A SegWit transaction (P2WPKH, P2WSH, P2TR) has two serializations:
  • Full serialization (wtxid form) - includes a 2-byte marker+flag (0x00 0x01) right after the version field, and a witness section (signatures/scripts) after the outputs.
  • Non-witness serialization (txid form) - the marker+flag and witness section are removed entirely.
Bitcoin’s block Merkle tree is built from txids, not wtxids. If you pass bitcoin-spv the full serialization, it computes SHA256d of the wrong bytes, gets the wrong txid, and the Merkle proof will not reconstruct the block’s Merkle root. Legacy (pre-SegWit) transactions have no witness data - the two serializations are identical, and no stripping is needed.

​
Algorithm

This mirrors relayer/src/bitcoin/tx.ts’s stripWitness exactly:
  1. Check for a SegWit marker: is rawTx[4] == 0x00 and rawTx[5] == 0x01? If not, the transaction is legacy - use it unchanged.
  2. Copy the 4-byte version field.
  3. Skip the 2-byte marker+flag.
  4. Copy the input count (varint) and every input unchanged (each input is prevout (36 bytes) + scriptSig (varint-prefixed) + sequence (4 bytes) - none of this is witness data).
  5. Copy the output count (varint) and every output unchanged (value (8 bytes) + scriptPubKey (varint-prefixed)).
  6. Skip the witness section: for each input (in order), read one varint (stack item count), then for each stack item read a varint (item length) and skip that many bytes.
  7. Copy the final 4-byte locktime.
The result is the non-witness serialization - pass this as raw_tx to verify_transaction.

​
Reference implementation

If you cannot run the relayer’s TypeScript directly, implement the 7 steps above in any language - the varint format is standard Bitcoin CompactSize encoding (single byte if < 0xfd; 0xfd + 2-byte LE if it fits in u16; 0xfe + 4-byte LE for u32; 0xff + 8-byte LE for larger).

​
Verifying you stripped correctly

Compute SHA256d(nonWitnessHex) and compare it against the txid your Bitcoin node reports for the transaction (e.g. bitcoin-cli getrawtransaction <txid> 1, or a block explorer’s txid field - not its “wtxid” field, if shown separately). They must match exactly.