Block Cipher Modes of Operation

How to use a block cipher?

• Block Ciphers encrypt fixed size blocks
• e.g. DES encrypts 64bit blocks
• We need some way to encrypt a message of arbitary length
• e.g. a message of 1000 bytes
• NIST defines several ways to do it
• called modes of operation

Encrypting a Large Message

1. Electronic Code Book (ECB)
2. Cipher Block Chaining (CBC)
3. Output Feedback Mode (OFB)
4. Cipher Feedback Mode (CFB)
5. Counter Mode (CTR)