What is the main idea of the passage?

A checksum is a compact value recalculated after transfer to help detect whether a file's data changed. (The passage explains calculation at both ends, comparison, useful evidence, and the method's limits.)

Why must sender and receiver use the same rule?

Only values calculated in the same way can be meaningfully compared for consistency. (The process works by recalculating the sender's test under identical conditions and comparing results.)

What does “compact” mean in the final paragraph?

small while containing useful checking information (The checksum is a short value rather than a second full file, so it is compact.)

What indicates that something is wrong?

different calculated values (The second paragraph directly contrasts matching values with different ones that signal a problem.)

The Checksum at the End: Level 2 Reading Comprehension Questions

A file can change while it is stored or sent. A damaged cable, failing device, or incomplete transfer may alter data without leaving a visible tear. A checksum helps reveal that change.

A program follows a fixed rule to calculate a short value from the file's contents. The sender records that value. After transfer, the receiver applies the same rule again. Matching values provide evidence that the data arrived unchanged; different values show that something is wrong.

A simple checksum cannot identify every possible change, and two different files can sometimes produce the same value. Strong cryptographic hash functions make accidental matches far less likely, though their purpose and security properties vary.

The checksum is not a spare copy of the file. It is a compact test. Like a total written at the bottom of a list, it lets the receiver check consistency without comparing every item by eye.

Comprehension questions