Which property is described by the term preimage resistance?

• A. Preimage resistance
• B. Collision resistance
• C. Second preimage resistance
• D. Pseudorandomness

Answer: (A)

In cryptography, preimage resistance means that given a hash value h, it is computationally infeasible to find any input x such that H(x) = h. This property prevents someone from reversing a hash to recover the original data, which is crucial for keeping the original message hidden when only the hash is stored or transmitted. The security level grows with the hash length, so for a 256-bit hash, attacking it by brute force would require around 2^256 attempts on average. This distinguishes it from other properties: collision resistance is about finding two inputs that produce the same hash, second preimage resistance is about given one input, finding a different input that hashes to the same value, and pseudorandomness concerns the output appearing random and unpredictable rather than being easily invertible.