I was giving a presentation to my colleagues about cryptography basics in which I explained about asymmetric algorithm and its use. One of the common question from the audience about asymmetric algorithm encryption/decryption is, why can't we decrypt the cipher data using the same key(e.g. public key) that we've used to encrypt(like symmetric algorithm). I know it is the mathematical property that prevents this but I really don't know how to explain in plain english. The question is more like if we do "10 + 2"(assume 10 as plaintext and 2 is key) then why can't we do "12 - 2"(12 is ciphertext and 2 is encryption key) to get the original data.

Can anyone help me to explain the principle of asymmetric algorithm in plain english?

It's like one of these:



Say you want to secure something in a box. Anyone can close the lock (public key). This means anyone will be able to put something into the box and lock the box (they won't be able to open the lock once it's locked (you just pinch these closed)). The key to open the lock is something only you have (private key). You are the only one that will be able to open the lock and see what's inside the box.

I suggest you buy on of these to demonstrate how they work.
