IPA IP 2026 Question 91

The same input always gives the same hash value — a hash function is deterministic and one-way.

A hash function such as SHA-256 maps data of any length to a value of fixed length (256 bits for SHA-256, 512 bits for SHA-512). Three properties matter here:

• •Deterministic: the same function applied to the same data always gives the same value. This is what lets a receiver recompute the digest of a signed message, or of the previous block in a blockchain, and compare it.
• •One-way: there is no practical way to get the data back from the value; feeding the value into the function again just gives another hash value.
• •Specific to the function: different functions give different values, even of different lengths.

Answer (c)

Why not others:

• •(a) — hashing a hash value gives a new hash value, not the original data; a hash function is not a cipher that undoes itself
• •(b) — SHA-256 and SHA-512 give values of different lengths (256 and 512 bits) for the same data, so their results cannot be the same
• •(d) — many inputs share each value, and no inverse function exists; being irreversible is the whole point of a cryptographic hash

Key rule: A cryptographic hash is deterministic, fixed-length and one-way: same input, same digest, and no way back from the digest to the data.

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