bbdcb54f_public_key_cryptography

A wants to send a message to B. A wants to ensure that only B can read the message and that it is not intercepted or altered during transmission to B.

Think of B as being in possession of a letterbox. The letterbox contains messages that only he can read. Anyone can put letters in it for B to read but B alone can read them.

This is achieved by encyrpting the messages with two keys: a public key and a private key. The public key can be shared with others and anyone in possession of the public key can encrypt a message for B. Once encrypted, only B can decrypt it using his private key. Once receiving the message, B uses his private secret key - that he doesn’t share with anyone else - to decrypt. This is the equivalent to opening up the letterbox and taking out the letters.

Another useful analogy is to think of invisible ink. Anyone can write a letter using the invisible ink but only the recipient has the liquid that can render it visible again. Anyone intercepting the message (without the liquid) will see nothing, equivalent to the encrypted data.

This is asymmetric encryption because the two communicating parties do not have equal access to the messages exchanged. A can only send messages - he can’t decrypt them, only B can. So A is unequal relative to B. With symmetric encryption A and B would both share a single private key which they would each use to encrypt and decrypt their messages.