|
@@ -0,0 +1,82 @@
|
|
|
|
|
+# DEP 0008: Transaction-local state
|
|
|
|
|
+
|
|
|
|
|
+```
|
|
|
|
|
+status: draft
|
|
|
|
|
+```
|
|
|
|
|
+
|
|
|
|
|
+## Motivation
|
|
|
|
|
+
|
|
|
|
|
+In DarkFi, spending a coin requires a Merkle inclusion proof against
|
|
|
|
|
+the global incremental Merkle tree. When a transaction contains
|
|
|
|
|
+multiple calls where one call's output is spent by a subsequent call,
|
|
|
|
|
+the intermediate coin hasn't been inserted into the global tree yet.
|
|
|
|
|
+The global tree's state at verification time depends on block ordering,
|
|
|
|
|
+so the prover and verifier cannot agree on a root.
|
|
|
|
|
+
|
|
|
|
|
+## Proposal
|
|
|
|
|
+
|
|
|
|
|
+We can build a small, deterministic Merkle tree from the intermediate
|
|
|
|
|
+output commitments produced within the transaction itself. For intra-tx
|
|
|
|
|
+spends, the burn proof demonstrates inclusion in this local tree rather
|
|
|
|
|
+than the global one. Since the transaction defines the order of calls
|
|
|
|
|
+and their outputs, both prover and verifier will always construct the
|
|
|
|
|
+same tree and arrive at the same root.
|
|
|
|
|
+
|
|
|
|
|
+### Construction
|
|
|
|
|
+
|
|
|
|
|
+Given a transaction with ordered calls `[C0, C1, C2]`:
|
|
|
|
|
+
|
|
|
|
|
+* `C0` produces output commitments `[O0, O1]`
|
|
|
|
|
+* `C1` wants to spend `O1`, so the prover builds a local tree from `{O0, O1}`
|
|
|
|
|
+* `C1` generates a standard Merkle inclusion proof for `O1` in this
|
|
|
|
|
+ local tree
|
|
|
|
|
+* If `C1` also produces `O2`, and `C2` wants to spend it, a new local
|
|
|
|
|
+ tree is built from `{O0, O1, O2}` (all outputs preceding `C2`)
|
|
|
|
|
+
|
|
|
|
|
+The local tree root becomes the public `merkle_root` input to the burn
|
|
|
|
|
+proof, replacing the global anchor.
|
|
|
|
|
+
|
|
|
|
|
+The burn circuit itself remains unchanged. The Merkle path verification
|
|
|
|
|
+logic is identical - the only difference is which root the verifier
|
|
|
|
|
+expects.
|
|
|
|
|
+
|
|
|
|
|
+We have to add a public `mode_flag` (0 = global, 1 = local) so the
|
|
|
|
|
+verifier knows which root check to perform. This would be added to
|
|
|
|
|
+the `Output` struct in the `Money` contract and the smart contract
|
|
|
|
|
+would then be able to select the correct code path for verification.
|
|
|
|
|
+
|
|
|
|
|
+## Privacy
|
|
|
|
|
+
|
|
|
|
|
+The Merkle proof hides which leaf is being proven. If the local tree
|
|
|
|
|
+contains multiple leaves, an observer knows "this spend consumed one
|
|
|
|
|
+of the outputs from previous calls" but not which one. If there's only
|
|
|
|
|
+a single preceding output, the privacy set is 1 and the link is
|
|
|
|
|
+trivially visible, but in that case the transaction structure already
|
|
|
|
|
+reveals it.
|
|
|
|
|
+
|
|
|
|
|
+## Nullifier Handling
|
|
|
|
|
+
|
|
|
|
|
+Intermediate coins must still emit nullifiers. The verifier checks:
|
|
|
|
|
+
|
|
|
|
|
+* No two nullifiers within the same transaction collide (prevents
|
|
|
|
|
+ intra-tx double spend)
|
|
|
|
|
+* No nullifier collides with any previously spent nullifier in the
|
|
|
|
|
+ global state.
|
|
|
|
|
+
|
|
|
|
|
+## Global Tree Updates
|
|
|
|
|
+
|
|
|
|
|
+After a transaction is finalized, only the surviving output commitments
|
|
|
|
|
+(those not consumed by an intra-tx spend) need to be inserted into the
|
|
|
|
|
+global Merkle tree. Intermediate coins that were both created and
|
|
|
|
|
+destroyed within the transaction can be omitted from the global tree
|
|
|
|
|
+to save space. The nullifiers still need to be recorded to prevent
|
|
|
|
|
+replay.
|
|
|
|
|
+
|
|
|
|
|
+## Transaction Validity Rules
|
|
|
|
|
+
|
|
|
|
|
+* A call at index `K` may only spend outputs from calls at indices `0..K-1`.
|
|
|
|
|
+ Circular dependencies are impossible by construction.
|
|
|
|
|
+* Proofs must be generated sequentially: call 0 first (to produce outputs),
|
|
|
|
|
+ then call 1 (which may reference those outputs), and so on.
|
|
|
|
|
+* If any call in the transaction fails verification, the entire transaction
|
|
|
|
|
+ is rejected atomically.
|
