To be considered strong today, a symmetric encryption key must be at least how many bits long?

• A. 6
• B. 8
• C. 100
• D. 1,000

Answer: (C)

Strength in symmetric encryption comes from the size of the key space. To break it, an attacker would try every possible key, and the number of possibilities grows exponentially with the key length. With a key length of about a hundred bits, there are roughly 2^100 possible keys, which is far beyond what can be brute-forced with current hardware in any practical timeframe, so it's considered strong today. Shorter keys are trivially crackable, while extremely long keys add marginal security and can complicate systems without providing a proportional real-world gain. In current practice, 128-bit keys have become the standard baseline, but the threshold described here—about a hundred bits—reflects the general idea of what constitutes strong today.