#hash-chain

Question 1: Single choice

An agent signs a receipt for every action with its private key, and each receipt stores the hash of the previous receipt (previous_receipt_hash). Receipts 1–4 form a chain. What would an attacker have to do to delete receipt 2 without the deletion being detected?

Nothing more than deleting it, because every receipt's signature is verified on its own1
Fix receipt 3's link and re-sign receipts 3 and 4, which needs the agent's private key2
Only change receipt 3's previous-hash field so that it points at receipt 1 instead3
Delete receipt 1 as well, so that the chain simply starts at receipt 34

Timed test