Exercise 1: "Harvest Now, Decrypt Later" — Possible Solution
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"Harvest now, decrypt later" describes an attack strategy where an
adversary doesn't try to break encryption at the moment it's used at
all. Instead, they simply INTERCEPT AND STORE encrypted traffic today
-- traffic they currently have no way to decrypt -- and wait. If a
sufficiently powerful quantum computer capable of running Shor's
algorithm (Chapter 9) is ever built in the future, the adversary can
go back and decrypt everything they stockpiled years or even decades
earlier, using the same RSA/DH/ECC-protected traffic captured long
before that quantum computer existed.

Why this makes migration urgent NOW, despite no threatening quantum
computer existing yet:

The attack doesn't require the adversary to have quantum capability
TODAY -- it only requires them to capture ciphertext today and have
quantum capability AT SOME POINT IN THE FUTURE, however far off that
point turns out to be. This means the relevant question for any given
piece of data isn't "is quantum computing a threat right now," it's
"will this specific data still need to be confidential by the time a
threatening quantum computer might exist."

For data with a short shelf life, this is a minor concern -- by the
time decryption becomes possible, the data may no longer matter. But
for data that needs to stay confidential for YEARS OR DECADES --
government secrets, long-term medical records, some financial
records, per this chapter's own examples -- the "future decryption"
timeline can easily fall within that data's required confidentiality
window, even if that timeline is currently uncertain and possibly
distant. Waiting to migrate until a threatening quantum computer
actually exists would already be too late for any long-lived
ciphertext an adversary captured in the meantime -- the exposure
happened at the moment of INTERCEPTION, not at the moment of
decryption.

WHY THIS WORKS AS AN ANSWER
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This reuses the chapter's own framing directly and makes explicit the
specific reasoning step the chapter states more briefly: the relevant
clock isn't "when will quantum computers exist," it's "will today's
ciphertext still need protecting by then" -- which is precisely why
the chapter treats post-quantum migration as a present-day action item
rather than a future one to defer.