BLOCKCHAIN & WEB3 FUNDAMENTALS - Chapter 4, Exercise 2 Solution
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Correcting "A 51% Attack Is Basically Impossible"

PROBLEM
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A friend says: "A 51% attack is basically impossible, so proof-of-work
is perfectly secure." Using this chapter's own real cost estimate and
its own honest framing, correct this overstatement into a more
accurate claim.

SOLUTION
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This overstates the case in two separate ways.

First, "basically impossible" isn't accurate. A 51% attack is not
theoretically impossible at all - the chapter describes exactly how it
would work: control more than half the network's mining power, and you
can out-mine the honest chain and eventually overtake it. It's real
and mechanically straightforward to describe. What actually deters it
isn't impossibility, it's cost.

Second, "perfectly secure" also overstates things. The chapter gives a
real, concrete number: a 2025 estimate put the cost of a week-long
51% attack on Bitcoin at around $6 billion at October 2025 prices.
That's a real, achievable amount of money for a sufficiently
well-resourced attacker (a hostile government, for instance) - it's
just enormous relative to any realistic payoff, and still under 1% of
Bitcoin's own total market value, meaning the attack would also be
economically self-destructive since it would likely crash the value of
the very asset being attacked.

A more accurate version of the claim would be: a 51% attack on Bitcoin
is technically possible and well understood, but it remains
economically impractical against an established network specifically
because of its enormous real-world cost relative to any likely benefit
- not because the attack itself is somehow unachievable.

ANSWER: A 51% attack is real and possible, not impossible - it's
deterred by cost (a real, multi-billion-dollar estimate for Bitcoin
specifically), not by any inherent impossibility, and "perfectly
secure" overstates what a cost-based deterrent actually guarantees.

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WHY THIS WORKS AS AN ANSWER

This separates the two distinct overstated claims in the friend's
sentence (impossibility vs. perfect security) and corrects each one
using the chapter's own specific, real cost figure rather than a vague
restatement of "it's expensive."