Which statement about rainbow tables is true?

• A. They are used to generate random numbers
• B. They are precomputed tables used to reverse cryptographic hash functions
• C. They improve password hashing speed
• D. They are used to generate new cryptographic keys

Answer: (B)

Rainbow tables rely on the one-way nature of hash functions by using precomputed mappings from candidate inputs to their hash outputs. By building a large table in advance, an attacker can take a hash value and look up (or trace through chained reductions to) the original input that produced that hash. This makes it possible to reverse hashes for common passwords without brute-forcing every possibility each time. The approach is a storage-time tradeoff: you precompute and store results to speed up lookups later, which is why rainbow tables are a risk to unsalted hash databases. Defenses include adding a unique salt to each password and using slow, memory-hard hash algorithms (like bcrypt, scrypt, or Argon2) so precomputed tables become impractical. The statement provided—that rainbow tables are precomputed tables used to reverse cryptographic hash functions—is accurate. They aren’t about generating random numbers, speeding up password hashing, or creating new cryptographic keys.