1. Counter Mode, Output Feedback Mode
Network Security

2. Counter Mode, Output Feedback Mode
Objectives of the Topic
After completing this topic, a student will be able to
explain counter and output feedback modes.

3. Counter Mode, Output Feedback Mode
Figures and material in this topic have been adapted from
“Network Security Essentials”, 2014, by William Stallings.
W. Stallings, “Crypto. and Network Security Principles and Practice”, Pearson Education, 2014

4. Counter Mode, Output Feedback Mode
Electronic Codebook Mode (ECB)
Cipher Block Chaining Mode (CBC)
Cipher Feedback Mode (CFB)
Output Feedback (OFB)
Counter Mode (CTR)

5. Counter Mode, Output Feedback Mode
Similar in structure to that of CFB.
For OFB, the output of encryption function is fed back to become the input for encrypting the next block of plaintext.

6. Counter Mode, Output Feedback Mode
The OFB mode operates on full blocks of plaintext and ciphertext, whereas CFB operates on an s-bit subset.

7. Counter Mode, Output Feedback Mode
The OFB mode requires a nonce; an initialization vector, unique to each execution of the encryption operation.
Sequence of encryption output blocks depends only on the key and the IV and not on plaintext.

8. Counter Mode, Output Feedback Mode

9. Counter Mode, Output Feedback Mode

10. Counter Mode, Output Feedback Mode
An advantage of the OFB method is that bit errors in transmission do not propagate.
The disadvantage of OFB is that it is more vulnerable to a message stream modification attack than is CFB.

11. Counter Mode, Output Feedback Mode
Employed in applications to ATM (asynchronous transfer mode), network security and IPSec (IP security).
A counter equal to the plaintext block size is used in this mode.

12. Counter Mode, Output Feedback Mode
The counter value must be different for each plaintext block that is encrypted.
Typically, the counter is initialized to some value and then incremented by 1 for each subsequent block (modulo 2b, where b is block size).

13. Counter Mode, Output Feedback Mode
For encryption, the counter is encrypted and then XORed with the plaintext block to produce the ciphertext block.
There is no chaining.

14. Counter Mode, Output Feedback Mode

15. Counter Mode, Output Feedback Mode
For decryption, the same sequence of counter values is used, with each encrypted counter XORed with a ciphertext block to recover the corresponding plaintext block.

16. Counter Mode, Output Feedback Mode

17. Counter Mode, Output Feedback Mode
Advantages of Counter Mode:
Preprocessing
when the plaintext or ciphertext input is presented, the only computation is a series of XORs, greatly enhancing throughput.

18. Counter Mode, Output Feedback Mode
Random access
The ith block of plaintext or ciphertext can be processed in random-access fashion
Provable security
CTR can be shown to be at least as secure as the other modes.

19. Counter Mode, Output Feedback Mode
Hardware efficiency
Encryption/decryption can be done in parallel on multiple blocks of plaintext or ciphertext
Software efficiency
Processors that support parallel features can be effectively utilized.

20. Counter Mode, Output Feedback Mode
Simplicity
Requires only the implementation of the encryption algorithm and not the decryption algorithm
End