Fundamentals of InfoSecadvanced
Why can a plain hash be insufficient when an attacker can modify both a packet and its accompanying hash?
Answer
The attacker may replace the hash with a new value calculated for the modified packet.
Explanation
### Why this is right
If the checksum is not protected, an attacker who changes the data may also calculate and substitute a matching hash. A receiver then has no secret-based evidence that the hash came from an authorized party.
### The concept
A plain hash can reveal accidental or unauthorized differences when the expected value is trustworthy. A Hashed Message Authentication Code adds a secret key to the hash-based check, helping a receiver detect unauthorized modification and substitution.
### The other options
- **B. A hash automatically encrypts the packet with the sender's private key**: hashing does not encrypt, and signatures are distinct operations.
- **C. A hash guarantees that the packet arrived on time**: hashing does not establish freshness or delivery time.
- **D. A hash can be reversed only by a certificate authority**: hashes are one-way and certificate authorities do not reverse them.
### Remember
Integrity checking needs a trustworthy expected value.
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