Chapter 4. Site-to-Site VPNs

Cryptographic Services

This section covers the key topics of cryptography. You should understand these principles before studying VPN technologies.


Cryptology is the science of making and breaking secret codes. A cipher is an algorithm for performing encryption and decryption. The Vigenère cipher is a polyalphabetic cipher that encrypts text by using a series of different Caesar ciphers based on the letters of a keyword.

Cryptanalysis is the practice of breaking codes to obtain the meaning of encrypted data. Following are examples of attacks:

Brute-force attack: The attacker tries every possible key with the decryption algorithm.

A cipher-text-only attack: The attacker has the cipher text of several messages ...

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