SMART CONTRACTS, DEFI & WEB3 SECURITY - Chapter 1, Exercise 2 Solution
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Correcting "I Marked It Private So Nobody Can See the Secret Value"

PROBLEM
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A colleague says: "I marked this function private so nobody else can
see the secret value it stores." Using this chapter's own warning
about what private actually restricts, explain what's wrong with this
reasoning.

SOLUTION
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This confuses two genuinely different things: restricting who can call
a function or access a variable from OTHER SOLIDITY CODE, versus
actually hiding data from anyone examining the blockchain directly.
The private keyword only does the first of these.

As this chapter explains, private means only code inside this exact
contract can call the function or read the variable directly - not even
an inheriting contract can reach it. But that restriction only applies
to how other CONTRACTS interact with it programmatically. It says
nothing about whether the underlying value is visible to someone
simply reading the blockchain's own data.

Every state variable's value, every transaction, and everything a
contract does is stored on a public, replicated ledger - a genuinely
core property of how blockchains work, established back in Course 1
(every node holds a full, verifiable copy of the chain's own state).
Anyone with the right tools (a blockchain explorer, or even directly
reading the contract's own storage slots) can inspect a "private" state
variable's actual stored value, completely independent of what
visibility keyword the Solidity source code used. The keyword affects
compiled contract-to-contract call permissions, not on-chain data
confidentiality.

So if the colleague's "secret value" is meant to be genuinely hidden
from public view, marking the variable or function private does
nothing to achieve that goal - the value is still sitting in plain,
readable form on a public ledger, exactly where anyone could look at
it directly.

ANSWER: The colleague is wrong - private only restricts which other
Solidity code can call a function or read a variable directly; it does
not hide the underlying data from anyone examining the blockchain
itself, since all contract state lives on a public, replicated ledger
regardless of Solidity-level visibility keywords.

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WHY THIS WORKS AS AN ANSWER

This separates code-level access control (what private actually does)
from data-level confidentiality (what the colleague mistakenly believes
it does), grounding the correction in the blockchain's own established
public-ledger property from earlier in the course rather than treating
it as an isolated Solidity quirk.