2.3 Encryption and Decryption

Zounds! EGOP hid a secret message! How did they do that?

• The process of hiding information is called encryption or encoding.
• The process of ``unhiding'' information is called decryption or decoding.

Both processes are required! Encryption provides privacy and protection for information, like messages proclaiming intentions to take over the world. But, what good is the ability to hide something if you can't find (``unhide'') it, and what good is the ability to unhide if nothing can be hidden?

To hide or unhide information, something must change the information. The set of rules for performing this transformation is called a key:

• Encryption transforms text using an encryption key.
• Decryption transforms text using an decryption key.

A cryptosystem consists of both the rules for encryption and the rules for decryption. That is, a cryptosystem is an encryption key together with the corresponding decryption key:

• To encrypt text, transform unencrypted text with an encryption key.
• To decrypt text, transform encrypted text with a decryption key.

You might sometimes hear or see the term cracking, which means figuring out the unencrypted text from encrypted text without knowing the decryption key.¹ Note that it suffices to find the decryption key: if you can figure out the decryption key, then you can use it to decrypt the encrypted text and thereby extract the unencrypted text.