Exercise 2: Why a Shared Key Makes a Session Log Meaningless — Possible Solution
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WHAT THIS CHAPTER SAYS
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Per this chapter's finding-box, "a session log is only as trustworthy
as the identity behind each connection... with proper per-person keys,
'who connected' in the log is real, reliable evidence. With a shared
key, the log entry is present, but it tells you nothing meaningful
about which actual person was behind it."

THE CONNECTION TO CHAPTER 2's ORIGINAL REASONING
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Chapter 2 warned that a shared private key "destroys per-user
accountability entirely - a log showing 'the shared key connected'
tells you nothing about which actual person was at the keyboard." This
chapter's point is the direct, specific application of that exact
warning to session logs themselves: the log system faithfully records
that a particular key connected at a particular time, but if that key
is shared among multiple people, the recorded identity doesn't
actually correspond to one specific person.

WHY THE LOG IS "PRESENT BUT MEANINGLESS," NOT SIMPLY MISSING
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The log entry itself isn't absent or broken - a connection event with
a timestamp genuinely exists. The problem is specifically that the
identity field in that entry, which is supposed to answer "who," only
narrows things down to "someone with access to the shared key" rather
than a specific individual - the log looks complete and useful on the
surface while actually failing at its core purpose (identifying who
did something) the moment the key behind it is shared.

WHY THIS IS DESCRIBED AS THE "FINAL, CLEAREST PAYOFF"
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Chapter 2 introduced the accountability problem in the abstract; this
chapter shows its most concrete, practical consequence - an entire
category of evidence (session logs) that this course has spent a
chapter establishing as valuable becomes worthless the moment the
underlying key discipline from Chapter 2 isn't followed.

WHY THIS WORKS AS AN ANSWER
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It states this chapter's own explicit claim, traces it back precisely
to Chapter 2's original wording about shared keys, and explains
specifically why the log entry existing isn't the same as the log
entry being useful once the key behind it is shared.