Which of the following is NOT a property of Secure Hash Algorithm (SHA)?

A.  Irreversible
B.  Fixed-length Output
C.  Variable-length Output
D.  Collision Resistance

Answer: C
Explanation:

A. Irreversible (One-Way Function)

A primary requirement of a cryptographic hash is that it must be a "one-way" function.

• The Concept: It should be computationally easy to generate a hash from a piece of data, but it should be impossible to take the hash and "reverse" it to find the original data.  

•  Why it matters: This is why we store "hashes" of passwords instead of the passwords themselves; if the database is stolen, the attacker cannot simply read the passwords.   

B. Fixed-length Output

Regardless of whether you hash a single word or an entire library of books, the resulting hash will always be the exact same size.  

• The Mechanism: For example, SHA-256 always produces a 256-bit (64-character hex) string.

• Why it's a property: This makes hashes predictable for storage in databases and for use in digital signatures.

C. Variable-length Output

This is the correct answer because SHA does not do this.

• Why it's the Answer: If a hash function produced variable-length outputs, it would leak information about the size of the input data, potentially weakening its security. Furthermore, it would make it impossible to use the hash in fixed-size data structures.

• The Nuance: While there are modern functions called "Extendable-Output Functions" (XOFs), like SHAKE, standard SHA algorithms are strictly fixed-length.

D. Collision Resistance

A "collision" occurs when two different inputs produce the exact same hash output 

• The Requirement: In a secure algorithm like SHA-256 or SHA-3, it should be computationally infeasible to find two different inputs that result in the same hash  

• The Risk: If an algorithm is not collision-resistant (like the now-obsolete MD5 or SHA-1), an attacker could substitute a malicious file for a legitimate one without changing the file's hash.

Key Takeaway for the CISSP Exam

To be considered a "Cryptographic Hash," the algorithm must satisfy five main properties:

1. Deterministic: The same input always results in the same output.

2. Quick to Compute: The process should be efficient.

3. Pre-image Resistance: Given a hash, it’s hard to find the input (One-way).  

4.  Avalanche Effect: A tiny change in the input (like changing one bit) results in a drastically different hash   

5. Collision Resistance: It’s hard to find two different inputs with the same hash.