Data Protection and Security

I

Secret Key Cryptography

I.I

Symmetric Encryption

Remedy of the Short-Key Problem

The short-key problem of DES has been known for a long time. This problem has different proposed solutions:

• Triple-DES (3DES)
• DESX
• Independent subkeys
• Slow-key schedule
• Key dependent S-boxes

We will explain 3DES, the mostly implemented solution, here. As seen from the figure to the right, 3DES uses triple encryptions in the order of (Encrypt-Decrypt-Encrypt) with two different keys K₁ and K₂.

The reason of using two keys instead of three is because 112 bits of key is considered sufficient against brute-force attack. One reason why K₂ is used in decryption mode is that with K₁ = K₂ is equivalent to simple DES, so that such a system can interoperate with a simple DES system.

Figure I.I-II

3DES encryption order. [click to enlarge]

I.I.Q