The Constant-Time Compare That Compounded The Opposite Debugging Lab

The Constant-Time Compare That Compounded The OppositeSecrets & AuthHardPreparing workspaceAutosaved

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Incident Dossier

The Constant-Time Compare That Compounded The Opposite

A webhook handler verifies HMAC-SHA256 signatures on every incoming message. A genuine message with the correct signature was bounced; a forged message with a wrong signature passed the check. The compare helper computed the right per-byte XOR accumulation but returned the wrong boolean. The cryptographic primitives are fine; the bug is in the boolean at the end.

Initial failing signalverify('hello', correctSig, 'secret') returns false instead of trueExpected verify('hello', correctSig, 'secret') === true · received verify('hello', correctSig, 'secret') === false

Progress

1Reproduce the bugRun checks

2Edit the fixOpen

3Pass checksOpen

4Submit fixOpen

Reproduction Steps

Run npm test.
Inspect the verify function in src/signature.js.
Note that the per-byte loop accumulates XOR differences into `diff` — the constant-time trick.
Look at the final boolean. Is the equality check using the right relationship between `diff` and what it represents?

Focus Mode

Start

src/signature.js

Likely area

src/signature.js

Progressive hints

Hints reveal broad direction first, then narrow toward the suspect path without showing hidden tests or exact solution code.

TopicSecrets & Auth

DifficultyHard

Est. time24 min

Workspacecreating

Explorer

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Evidence Map

Failing signal loaded from challenge API
Workspace files loaded from workspace API
Results rendered only after API run/submit

Autosaved

workspace

Outputread-onlyready$ workspace boot

Workspace booting...