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CalcForge

JWT Decoder

Inspect JWT header and payload claims locally—no token sent to any server.

Read JWT structure safely

JSON Web Tokens pack header and claims into Base64url segments. CalcForge splits the token, decodes each part, and pretty-prints the JSON—helpful for debugging auth flows. Decoding is not verification; signature checks still belong in your backend.

Good fits

  • Checking `exp`, `aud`, and `iss` claims during OAuth integrations
  • Inspecting identity tokens from Auth0, Cognito, or custom IdPs
  • Teaching JWT anatomy without command-line Base64 tricks
  • Confirming which scopes made it into an access token
Token parts:
header.payload.signature

Decoded payload:
{"sub":"user-123","exp":1710000000,"scope":"read:items"}

Practical notes

  • Never paste production secrets into shared screens—decode locally and close the tab.
  • Expired tokens still decode; check `exp` against current time in your debugger.
  • Use JSON Base64 for non-JWT Base64 blobs; JWT uses URL-safe alphabet.

Frequently asked questions

Does CalcForge verify the signature?

No. This tool decodes only. Validate signatures with your library and public keys on the server.

Is it safe to paste live tokens here?

Processing is local, but anyone shoulder-surfing sees the token. Prefer expired or redacted tokens when demoing.

Why does decoding fail?

Tokens must have three dot-separated segments using Base64url encoding. Copy the full string without Bearer prefix unless the tool strips it.

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