Data Protection and Security

I

Secret Key Cryptography

I.I

Symmetric Encryption

There are six tools that comprise the toolbox of cryptography consumers:

• Symmetric encryption
• Hash functions
• Message authentication codes
• Random number generators
• Public-key encryption
• Digital signatures

We will see the first four of them in this chapter and the last two in the next.

Symmetric Encryption

As opposed to public-key encryption, the key used for both encryption and decryption is the same in symmetric encryption. There are basically two ways to make a symmetric cipher:

• Stream cipher: The encryption rule depends on the plaintext symbol’s position in the stream of plaintext symbols. e.g.: Vigenere, RC4, A5.
• Block cipher: Encrypt several plaintext symbols at once in a block. e.g.: DES, AES, Twofish, RC6.

We now introduce DES and AES in short as examples of block ciphers.