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An analogy of a cryptosystem is a deadbolt lock. A deadbolt lock can be easily identified, and its inner working mechanisms aren’t closely guarded state secrets. What makes a deadbolt lock effective is the individual key that controls a specific lock on a specific door. However, if the key is weak (imagine only one or two notches on a flat key) or not well protected (left under your doormat), the lock won’t protect your belongings. Similarly, if an attacker is able to determine what cryptographic algorithm (lock) was used to encrypt a message, it should still be protected because you’re using a strong key (128 bit) that you have kept secret rather than a 6-character password written on a scrap of paper left under your mousepad.
Peter H. Gregory 