Counter Mode, Output Feedback Mode

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Presentation transcript:

Counter Mode, Output Feedback Mode Network Security

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.

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

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

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.

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

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.

Counter Mode, Output Feedback Mode

Counter Mode, Output Feedback Mode

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.

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.

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).

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.

Counter Mode, Output Feedback Mode

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.

Counter Mode, Output Feedback Mode

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.

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.

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.

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