BLOCKCHAIN & WEB3 FUNDAMENTALS - Chapter 10, Exercise 2 Solution
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Why Wait for More Than One Confirmation?

PROBLEM
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A colleague asks: "If Alice and Bob's payment is confirmed after Step
7, why would anyone ever wait for more than one confirmation?" Using
this chapter's own reasoning (and Chapter 4's own 51%-attack cost
economics), write an accurate answer.

SOLUTION
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"Confirmed" isn't really a single, all-or-nothing state - it's better
understood as a spectrum of increasing security, and one confirmation
sits at the weak end of that spectrum, not the strong end.

Step 7 of this chapter's own flow explains that a transaction becomes
"progressively harder to reverse" as more blocks are mined on top of
it, because reversing it would mean an attacker out-mining the entire
honest network's own accumulated proof-of-work since that point.
With only one confirmation, an attacker only has to out-mine the
network past a single block's worth of work to attempt a rewrite -
still real, genuine effort, but the least amount of accumulated work
protecting the transaction that it will ever have.

Chapter 4's own real cost estimates for a 51% attack (around $6 billion
for a sustained week-long attack on Bitcoin, per a real 2025 estimate)
apply more convincingly the deeper a transaction is buried. A
one-confirmation transaction is a comparatively easier, cheaper target
than a transaction buried under, say, six confirmations, which would
require an attacker to redo six full blocks' worth of proof-of-work
while also racing the honest network's own continued progress the
entire time.

This is exactly why, in practice, larger or higher-stakes payments
(a large purchase, an exchange deposit) are typically treated as safe
only after multiple confirmations, while smaller, lower-stakes payments
might reasonably be accepted after just one, or even before any
confirmation at all, if the risk of a reversal is judged small enough
to accept.

ANSWER: One confirmation is real, but it's the weakest point on a
security spectrum that only gets stronger - the amount of proof-of-work
protecting a transaction, and therefore the real cost of reversing it
(per Chapter 4's own economics), increases with every additional block
mined on top of it, so waiting for more confirmations is a real, direct
way to reduce the remaining risk for a higher-value or higher-stakes
payment.

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WHY THIS WORKS AS AN ANSWER

This corrects the implicit binary framing in the question ("confirmed"
vs. "not confirmed") into the graduated security model the chapter
actually describes, and ties the practical recommendation (more
confirmations for higher-value payments) directly back to Chapter 4's
own real, quantified attack-cost reasoning.