Twenty-four digits are not the whole fingerprint | Rapid course Cryptography and trust services

Open, free learning for anyone who wants to protect people and technology. No account is required to read this text or its worked answers.

Lesson map

Lesson 06. Map, example, EX052.

flowchart TD
  A["Script slices 24 hex digits"] --> B["Print adds three dots"]
  B --> C{"May a note fill the rest?"}
  C -->|No| D["Quote the prefix only"]
  C -->|Yes| E["That would be a new computation"]

The dots mark digits the embedded output never shows.

Logical drawing

published = full SHA-256 of the synthetic bytes
print uses published[:24] plus "..."
embedded line
    published: 215e60edb672063786f8bb92...
a note that writes further hex
    is not this output

Learning outcomes

After this chapter, you should be able to:

Read the printed prefix as a slice.
Leave the unseen digits unwritten.
Refuse a note that completes the fingerprint.

6.1 The slice is the evidence.

lab_checkfile.py computes a SHA-256 value and then prints only published[:24] followed by three dots. The embedded line is exactly published: 215e60edb672063786f8bb92.... FIPS 180-4 defines a longer digest than those twenty-four digits. Nothing in the worked results, the staff summary, or the script's print statement reveals the rest. A classmate who "finishes" the hex is running a calculation the chapter did not publish. Leo may copy the prefix and the dots. He may not replace the dots with digits this book does not contain.

6.2 Worked example

Lesson table; scroll horizontally to read all columns →

What someone cites	What the file holds	Decision
`published[:24]`	The script's slice	Keep the slice
`215e60edb672063786f8bb92...`	The embedded line	Quote it with the dots
A completed 64-digit string	Not printed	Refuse
"The dots are optional decoration."	The print adds them	Refuse

Synthetic case. No one recomputes the hidden digits in this lesson.

Errors and limits

Quoting the prefix does not claim the digest was shorter than SHA-256. It claims only that the page shows twenty-four digits and dots.

Apply

EX052 — Finish the hex? A note replaces the three dots after 215e60edb672063786f8bb92 with the rest of a SHA-256 digest. Is that the embedded output?
Show marking criteria
Rubric: 3 = no; the script prints published[:24] and dots; the embedded line stays published: 215e60edb672063786f8bb92...; do not invent the remaining digits; do not edit lab_checkfile.py;; 2 = the dots stay;; 1 = the prefix only;; 0 = none.