Does applying encryption multiple times always increase security?

• A. Yes, always
• B. It doubles the key length
• C. It makes decryption impossible
• D. No, it can be unnecessary or even reduce security

Answer: (D)

Stacking encryption layers doesn't automatically make things more secure. In fact, it can introduce new weaknesses. When you encrypt data twice with a cipher, an attacker can sometimes exploit the cascade with specialized attacks, such as meet-in-the-middle, which test inner keys and outer keys in a way that cuts the effort far shorter than trying all combinations. For example, with two layers and two keys, the effective security can end up far less than the sum of the two keys because the attacker can separate the problem into two smaller searches. So, additional encryption can be unnecessary and, in some cases, can actually weaken security by providing a practical attack path or by encouraging weaker key choices or misconfigurations. In practice, using a single, strong cipher with a large enough key (or using authenticated encryption) is usually a better security choice than layering encryptions.