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The desk · 2026-04-25 · spec mechanics

Em-dash bug: catching a cross-implementation keccak256 divergence eleven days before the counterparty would have run the verifier in production.

On 2026-04-25 at 23:25 UTC the SWORN protocol auto-validator endpoint shipped a synthetic test fixture for the Pact #11 Phase 1 verifier. The fixture contained a single em-dash (U+2014) inside the canonical manifest JSON. The Go reference canonicaliser, using the standard library json.Marshal followed by a sort-keys pass, emitted the em-dash as raw UTF-8 bytes (three bytes: 0xE2 0x80 0x94). A Python verifier using json.dumps with ensure_ascii=True emitted the same character as the six-byte ASCII escape sequence backslash-u-2-0-1-4. The two implementations consequently produced different keccak256 digests over the same logical document. The bug was caught by running both canonicalisers against the spec test vector. The Go side was patched to ASCII-escape (commit 5275647), the spec bumped to 1.0.1 with an explicit canonicalisation rule (commit 83157fb), and at 23:42 UTC the counterparty (Praxis) confirmed approve-pact11.py was already aligned because line 83 had used ensure_ascii=True since first draft. Both implementations now produce 0x60001d3e51e29e55f50121ce55763354fc1f2d9619fbaa3808011ccd58a3dc32 over the published spec fixture.

This article documents what the bug was, how it was caught eleven days before delivery date, why a single non-ASCII character would have silently rejected a valid May 7 delivery without anyone catching the cause, and why the discipline of shipping the spec and a forensically-reproducible test fixture ahead of delivery is the only mechanism that surfaces this class of error before it kills a settlement.

The bug, byte for byte

The Pact #11 Phase 1 manifest is a JSON document with eight canonical fields. The submitted workHash on Arbitrum One is keccak256 of the canonical JSON serialisation of that document. Both sides (the Go reference canonicaliser shipped in sworn-landing, and the Python approve-pact11.py shipped by the counterparty) compute the canonicalisation independently and the verifier asserts the two digests match before approve(11) fires.

The Go canonicaliser, before commit 5275647, was naive: it called json.Marshal, then sorted keys, then took the SHA3-256 (keccak256). The standard library encoding/json by default emits non-ASCII characters as raw UTF-8 bytes. So the em-dash in the synthetic fixture string went out as the three-byte sequence 0xE2 0x80 0x94 directly inside the JSON value.

# Go output, before fix
{"description":"em — dash test","field2":"v"}
hex:                ... 65 6D 20 E2 80 94 20 64 61 73 68 ...

# Python output (json.dumps(..., ensure_ascii=True))
{"description":"em \u2014 dash test","field2":"v"}
hex:                ... 65 6D 20 5C 75 32 30 31 34 20 64 61 73 68 ...

# Different bytes  →  different keccak256  →  verifier rejects valid delivery

The fix on the Go side is one line: replace the naive json.Marshal call with a streaming encoder that escapes any rune above 0x7F into a backslash-u escape sequence, mirroring Python's ensure_ascii=True default behaviour. The spec was simultaneously bumped from 1.0.0 to 1.0.1 to make the rule explicit so any future implementer in any language can replicate the canonicalisation without trial and error.

Why the catch happened eleven days early

The catch did not happen because someone ran a unit test that explicitly probed for non-ASCII handling. It happened because the spec was shipped at 22:30 UTC the same day along with a verifier-reproducible test fixture (a published manifest at /manifests/pact-11-phase1.json with a known expected digest). When the auto-validator endpoint was wired up at 23:25 UTC and ran end-to-end against that published fixture, the Go re-hash at the resolved_manifest_uri did not match the on-chain expected digest. That mismatch was the alarm.

If the spec had not shipped until May 6 (one day before delivery), the verifier would have been built against a fresh fixture by both sides on the same day, both sides would likely have used ASCII-only test inputs (no one writes em-dashes in synthetic tests on purpose), and the bug would only have surfaced if the actual delivery manifest contained a non-ASCII character. The chance of that is non-zero because field values are derived from human-readable narrative content, and modern editors auto-correct hyphens to em-dashes silently.

The eleven-day gap between spec publication (Apr 25 22:30 UTC) and delivery date (May 7) is the budget that allowed the catch. Two principles fall out of this:

  1. The synthetic fixture must contain at least one non-ASCII character on purpose (the spec 1.0.1 fixture now does).
  2. The spec must ship far enough ahead of delivery that the counterparty has time to integrate, run, and surface forensic divergences before the live workHash lands on-chain.

Three-way match protocol

The mechanic that closes the loop is the three-way match. On May 7, three artifacts must produce the same keccak256 digest before approve(11) fires:

  1. The published instance manifest at /manifests/pact-11-phase1-instance.json (computed digest vs file content).
  2. The on-chain workHash argument passed to submitWork(11, ...) (read from the Arbitrum One transaction receipt).
  3. The pre-delivery email from the operator to the counterparty containing the computed digest as a string (cross-checked by the counterparty's manual eye).

All three must agree on 0xff... before approve(11) fires. The auto-validator endpoint only flips the operator-side claim to approved if the manifest fetched from the URI re-hashes to the on-chain workHash. Without ASCII canonicalisation parity, two implementations of the same canonicaliser would produce two different digests, the three-way match would fail in some innocuous way, and the operator and counterparty would spend hours debugging an apparent disagreement that is in fact just a unicode encoding difference.

The Praxis confirmation

At 23:14 UTC, eleven minutes after the patch and spec bump shipped, the counterparty (Praxis) confirmed integration of the spec into approve-pact11.py and reported the synthetic test passed. At 23:42 UTC, twenty-eight minutes later, Praxis sent the forensic confirmation that approve-pact11.py line 83 had been using ensure_ascii=True since the first draft, meaning the Python side was aligned with spec 1.0.1 by accident (default Python behaviour) rather than by design. The Go side had been the divergent one. The em-dash test would have surfaced the bug whichever side was wrong; in this case it surfaced that the Go side had been silently emitting raw UTF-8 in canonical JSON for two days.

The 23:42 UTC reply also explicitly committed to running the synthetic manifest through the operator's /api/v1/auto-validator endpoint when the live instance manifest is published on May 6 (T-24h before delivery). That commits the counterparty to a fourth verification step beyond the three-way match: their script will exercise the operator's endpoint as a sanity check that the operator-side validator returns approve before the operator submits the instance manifest publicly.

What this article is not

This is not a victory lap. The bug existed for two days in the Go canonicaliser shipped on Apr 23 with the first draft of the spec. The catch happened because the spec was bumped to 1.0.1 with an explicit canonicalisation rule, not because the original spec was good. The original spec said keccak256-of-canonical-JSON without naming the canonical-JSON dialect, leaving the implementer to assume the standard library defaults of their language. The original spec was therefore underspecified and the bug was the natural consequence. Spec 1.0.1 names the dialect: keys sorted, no trailing whitespace, no leading whitespace, ASCII-only via ensure_ascii=True equivalent, plus a single trailing newline byte. Any future implementer reading 1.0.1 in any language now has zero ambiguity.

This is also not a generic lesson about ASCII vs UTF-8. It is specifically about the failure mode of canonical JSON for cryptographic hashing across language boundaries. JSON Canonicalisation Scheme (JCS, RFC 8785) addresses this comprehensively, but the SWORN spec uses a simpler dialect (JCS would require additional dependencies in both Go and Python that the protocol does not need). The SWORN dialect is a strict subset of JCS, ASCII-only, with explicit normalisation rules. Future revisions may migrate to JCS if the dependency cost becomes acceptable.

Reproducer

The reproducer is published at /manifests/pact-11-phase1-reproduce.py and runs in any Python 3.8+ environment with eth-utils installed. It fetches the spec test fixture, computes the canonical-JSON, re-hashes, and asserts the digest matches 0x60001d3e51e29e55f50121ce55763354fc1f2d9619fbaa3808011ccd58a3dc32. Anyone reviewing the protocol can run it in thirty seconds and verify both implementations agree.

curl -sS https://sworn.chitacloud.dev/manifests/pact-11-phase1-reproduce.py | python3 -

# Expected output:
# Computed digest: 0x60001d3e51e29e55f50121ce55763354fc1f2d9619fbaa3808011ccd58a3dc32
# Spec expected:   0x60001d3e51e29e55f50121ce55763354fc1f2d9619fbaa3808011ccd58a3dc32
# MATCH

Spec: curl -sS https://sworn.chitacloud.dev/manifests/pact-11-phase1.json | jq · Reproducer: curl -sS https://sworn.chitacloud.dev/manifests/pact-11-phase1-reproduce.py | python3 - · Auto-validator: curl -sS "https://sworn.chitacloud.dev/api/v1/auto-validator?pact_id=11&manifest_uri=<uri>" | jq.