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WASM browser demo - build, prove, verify

The companion to the read-only csmt-verify demo: this page cross-compiles the write path (csmt-write-wasm) to WebAssembly too, so everything - building the tree, generating proofs, and re-verifying them against the root - happens inside sandboxed WASM.

Try it

Open the standalone demo

What the demo ships

  • csmt-write.wasm - the write entry point produced by wasm32-wasi-cabal via nix build .#csmt-write-wasm. It takes a prior InMemoryDB blob, a batch of inserts/deletes, and a query key; it returns the updated blob, the post-mutation root, and either an inclusion proof or an exclusion proof.
  • csmt-verify.wasm - the existing verifier, reused by the page to independently re-check each proof it produces, exported via nix build .#csmt-verify-wasm.
  • index.html + write.js - the static page that drives both modules under @bjorn3/browser_wasi_shim.

State is persisted to IndexedDB so anything you build survives a page reload. A capped undo/redo stack is persisted alongside it, and the page supports Cmd/Ctrl-Z shortcuts.

Workflow

  1. Insert key/value pairs and watch the 32-byte Blake2b-256 root update.
  2. Prove a key to get an inclusion proof if present or an exclusion proof if absent.
  3. Verify the returned proof against the root via csmt-verify.wasm.
  4. Tamper and verify to demonstrate rejection of corrupted proof bytes.
  5. Delete a key, then prove it again to observe the exclusion witness.

Stdin protocol

csmt-write-wasm consumes the entire stdin as one envelope. All 4-byte lengths are big-endian:

Field Payload
slen state prior InMemoryDB blob (empty to start)
n number of ops
n x op opcode-tagged: 0 klen key vlen value for insert, 1 klen key for delete
qlen queryKey key to prove or disprove

The stdout response is:

Field Payload
slen state updated InMemoryDB blob
root 32-byte post-mutation root hash
vlen value queried key's value, empty if absent
ptype 1 byte: 0 inclusion, 1 exclusion, 0xff none
plen proof CBOR-encoded proof of the declared type

The browser forwards ptype straight through as the verifier opcode, so no host-side translation is needed.

Build it yourself

nix build .#csmt-write-wasm
nix build .#csmt-verify-wasm
nix build .#csmt-wasm-write-demo
PORT=8001 nix run .#csmt-wasm-write-demo

The output is a plain tree of static files suitable for any static host. The docs site here is built from the same flake output via nix build .#docs.