Which Encryption Process Encrypts on Either Side?

Making stream ciphers from AES: CFB Mode

Introduction

Okay… the cryptographers have been keeping a secret. You think that your encrypted content is perfectly safe, but it often suffers from many problems:

• Encryption works great, until it doesn’t.
• Encryption works great, as long as no one makes a mistake.
• Encryption works great, unless something goes wrong.
• Encryption works great, as long as everything works right.

Apart from using a password to generate an encryption key, which completely decimates the key space, we have the problem of the algorithm used to process the plain text. If this is ECB (Electronic Code Book) we have repeating cipher blocks for the same plain text.

If I take “eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee” and encrypt with 3-DES and a key of “bill12345” we get:

1122900B30BF1183 1122900B30BF1183 1122900B30BF11831 
122900B30BF1183 1122900B30BF1183 1122900B30BF1183 
7591F6A1D8B4FC8A

where we can see that the “e..e” values are always coded with the same cipher text. As 3-DES has message blocks of 64-bits, then 8 ‘e’ values will fill each block.