Logbook › Following the Wake: Proof of Where a Postcard Has Been
Following the Wake: Proof of Where a Postcard Has Been
Awaiting external review · new in Zecboat 0.7.1A shielded postcard is private, so nobody can see its history. That is the point of Zcash. But it leaves a question: how does a buyer know they are holding the real #007?
The problem
Anyone can copy a postcard's memo onto a new note of their own. The copy looks identical.
Ordinary blockchains answer this with a public history of owners. Zcash deliberately has none.
The trail Zcash already leaves
Every note in Zcash's newest shielded pools carries a hidden tag, which Zcash calls ρ (rho). When a note is spent, the note created in the same step takes the spent note's fingerprint as its ρ. That fingerprint can appear on the chain only once, so every note has exactly one successor.
That link is the wake a note leaves behind. It is hidden inside the new note, and only its owner can see it.
- Born at mint
- hop 1: proof ✓
- hop 2: proof ✓
- the note in your wallet
What Wake adds
Wake proves those links with small zero-knowledge proofs, one per transfer, back to the note born at mint, without revealing who held it, how much, or which address.
The proofs travel with the postcard, in the transfer itself: one knot per send, written to the chain, readable by the new owner. The wallet checks every one against the chain, through your own node or your provider, and says which.
- The real #007: its trail leads back to the mint
- A copy reuses the memo, not the trail: it shows No lading
- A captured lading is useless: it cannot be copied onto another note
What stays private, what is proven
| Stays private | Is proven |
|---|---|
| addresses | this note came from the mint |
| amounts | through real spends on the chain |
| keys | each step had exactly one successor |
| nothing new is published | a copied memo has no trail |
What your wallet tells you
Every postcard in the Zecboat wallet shows one of four lineage states. Each one names what was, or was not, proven.
| Badge | Meaning |
|---|---|
| ◐ Proof checks out | the trail back to the mint checks out · awaiting external review, shown in amber |
| Unverified | the trail is under 100 blocks deep: wait a little |
| No lading | the sender did not deliver the proofs: ask them to resend |
| ✗ Broken | the trail does not lead to the mint: not the real one |
Built from Zcash's own circuit
Wake’s proofs use circuits cut down from Orchard, Zcash’s own shielded circuit, by deletion rather than invention. It is built into Zecboat 0.7.1: every postcard it sends carries its proof, made before the send.
On mainnet, a real trail from mint through two transfers checks out, and our first transfer of a registered postcard, Lagos #010, arrived with its trail intact.
How a postcard is sent
- One postcard per transaction
- The fee comes from separate ordinary ZEC
- Postcards stay locked against ordinary spends
Honest limits
The Wake and Knotline specifications are published on this site, and the review repository with the release. We say the limits first because that is what makes the rest believable.
- Wake has not yet had its independent external review. Until it has, the wallet shows its result in amber.
- The new owner sees the chain of past spend tags, but not who held the postcard.
- It does not yet cross Zcash pool upgrades.
- It proves one note’s origin, not the origin of a mixed balance.