What causes a save HMAC that signs its own tag field?

The signature field lives inside the same object that gets hashed, so the value you sign changes when you insert the tag, making the check inconsistent or skippable.

How do I catch this when it only happens for some players?

Capture it automatically from the player's device with the full stack trace, the device and OS, the build, and a breadcrumb trail of what they did. That turns a vague complaint into a specific, reproducible issue you can fix without owning the hardware it happens on.

How to Fix a Save HMAC That Signs the Tag Along With the Data

Quick answer: Separate the signed bytes from the tag: hash only the canonical payload, then append the tag outside it, and verify by stripping the tag and rehashing the remainder.

You added an HMAC to your save but stored the tag in a field inside the same JSON object you hash. Now the bytes you signed differ from the bytes you verify, so either verification always fails or you skip the field and the layout becomes ambiguous. Define a clear split between payload and tag.

How to fix it

1. Hash a fixed payload region

Serialize the game data to a canonical byte range, compute the HMAC over exactly those bytes, and never include the tag in that range. A simple layout is [length][payload][tag].

2. Verify by re-deriving the payload

On load, read the payload region and the tag separately, recompute HMAC(payload), and compare in constant time. The payload bytes must be byte-identical to what you signed.

3. Use canonical serialization

Map key ordering and whitespace can change the bytes without changing the data. Serialize deterministically (sorted keys, fixed encoding) so the same game state always hashes to the same value.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.

Most of the time the fix is small. Seeing the failure clearly is the part that actually costs you.