Exercise 1: What Encryption Alone Does and Doesn't Guarantee — Possible Solution
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Encryption on its own only provides CONFIDENTIALITY — it stops anyone
without the key from reading the plaintext. It says nothing at all
about the other three goals from this chapter: integrity (was the
data altered?), authenticity (did it really come from who it claims?),
or non-repudiation (can the sender later deny sending it?).

Concrete scenario where encrypted data is still successfully attacked:

An attacker intercepts an encrypted message travelling between two
systems. They cannot read it (confidentiality holds), but if the
encryption scheme provides no integrity protection, the attacker can
still flip bits in the ciphertext before forwarding it on. Depending
on the cipher and mode in use, this can cause predictable, controlled
changes in the decrypted plaintext at the receiving end — the
recipient decrypts something different from what was actually sent,
with no indication anything was tampered with. The data was never
"read," so confidentiality was never broken, but the system was still
successfully attacked.

This is exactly why Chapter 8 (Message Authentication) exists as its
own chapter rather than being folded into encryption — confidentiality
and integrity are separate problems requiring separate tools (MACs,
HMAC, or an AEAD cipher that provides both at once), and "we encrypted
it" answers only one of the four questions this chapter opened with.