Which of the following is not a common cryptographic vulnerability?

The Security+ exam outline expects test takers to be familiar with three types of attacks: downgrade attacks that request weaker encryption types; collision attacks, which focus on finding collisions in hashes, allowing arbitrary changes to files that would normally be verified with hashes; and birthday attacks, which allow nonlinear guesses to be more likely to find a solution than simply progressing through key space. Magic key attacks were made up for this question.