A double change is a limit, not an event

Lesson map

Lesson 18. Map, example, EX064.

flowchart TD
  A["A match means the bytes equal the published file"] --> B["Same-site replacement of both would still match"]
  B --> C{"Did this book record that attack?"}
  C -->|No| D["Keep the limit and the supervised match"]
  C -->|Yes| E["No attack and no project signature are filed"]

The limit explains a weakness of the method. It does not report an attack.

Logical drawing

a match
    file equals the published fingerprint's file
    does not prove the software is safe
limit
    attacker changes file and fingerprint on the same site
not recorded
    that attack at this project
    a signature on this release
    the project's private key

Learning outcomes

After this chapter, you should be able to:

1. Say what a match proves.
2. Keep the same-site limit as a limit.
3. Leave this release unsigned in the file.

18.1 The warning is conditional.

A match means the downloaded bytes are the bytes whose fingerprint was published. Chapter 4 adds two limits. The software may still be unsafe or buggy, which is why the install starts on one laptop. And if an attacker replaced both the file and the fingerprint on the same website, the check would still match. Some projects also sign releases, because forging a signature needs the project's private key. The file uses "some projects" and "if". It does not say this catalogue project was attacked, and it does not say this release was signed. The supervised download matched. A classmate who writes "the site was altered" or "the signature checked out" adds an event and a control the chapter did not record. Leo keeps the conditional limit and does not invent a signer.

18.2 Worked example

Lesson table; scroll horizontally to read all columns →

Synthetic case. No attacker and no project signature are added to the summary.

Errors and limits

A limit that did not fire is still worth teaching. Teaching it is not a claim that it fired.

Apply

• EX064 — Was the site altered? A match shows the file equals the published one. The same check can be fooled if both the file and the fingerprint on one site are replaced. Some projects sign releases. What do you write about this catalogue download?
  Show marking criteria

  Rubric: 3 = the supervised download matched; the same-site case stays a limit, not a recorded attack; this file does not say the project signed this release; do not invent a key or an attacker;; 2 = the limit stays conditional;; 1 = the match only;; 0 = none.