Checksum Crown

A vault token combines an encoded claim with an integrity value. Determine whether that value depends on anything unavailable to the token holder.

Objective

Reproduce a client-visible checksum over a modified claim.
Distinguish error detection from keyed authentication.

Before you begin

Open /challenge/hard/checksum. Preserve pctf_session; work only with the separate vault token.

Concepts

An unkeyed checksum can detect accidental changes, but anyone who edits the message can recompute it. A MAC binds data to a secret held by the verifier.

A comparison distinguishes reversible encoding, accidental-error checksums, cryptographic hashes, shared-secret MACs, and private-key signatures with public-key verification.

Encoding is reversible. Checksums detect accidental errors. Hashes fingerprint bytes but do not authenticate a source. A MAC provides integrity and origin within a group that shares a secret; any member can create tags. A signature binds bytes to a private-key holder when verification and public-key trust are correct. None of these alone encrypts, proves freshness, or grants authorization.

Suggested tools

Storage and source inspectors: Separate the token and read its loaded JavaScript. Python can reproduce the small checksum locally.

Tool practice

Rehearse the worked tool exercise before using challenge data.

Print the decoded payload with repr and compare its byte length before calculating. The checksum script encodes that decoded text as UTF-8; it does not sum the characters of the Base64 wrapper. Reproduce the original checksum first, then consider a changed claim.

Investigation

Observe

Split the token at its separator, decode the payload, and inspect the script's integrity algorithm.

Form a hypothesis

Can you replace the ordinary role with the privileged role and calculate a matching integrity value without a server secret?

Test carefully

Preserve the token's two-part format and change only the authorization claim.

Progressive hints

Hint 1

Decode the first component and open the loaded script.

Hint 2

The script sums the decoded payload's UTF-8 bytes modulo 65,536, not the Base64 wrapper's characters.

Hint 3

Encode the privileged payload and format its new checksum as four hex digits.

Troubleshooting and reset

Use lowercase, zero-padded hexadecimal and retain Base64 padding. Delete only vault_token to receive a fresh ordinary token.