Encryption consists of applying an algorithm that scrambles the data, and then a key can be used that returns the scrambled data to its original state. ROT13 is one of the simplest examples of a substitution cipher. It basically replaces each letter with one 13 places away in the alphabet:

Blumira is awesome = Oyhzven vf njrfbzr

ROT13 is obviously quite a weak cipher, but it is useful to illustrate the key point here: encrypted data is reversible to anyone who knows the key; in this case it is that the shift size is 13. It is like that by design.

There is no point encrypting a secret message if the person at the other end is unable to decipher it. Therefore, it is useful in situations such as VPNs, HTTPS traffic, and many other forms of communication. Other common encryption algorithms include Triple DES, AES, RSA, and Blowfish.

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