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CS 285 Network Security Block Cipher Modes of Operation

CS 285 Network Security Block Cipher Modes of Operation. Fall 2008. Introduction. How to encrypt a message with variable lengths Decompose the message into blocks, padding if necessary. How should the encryption/decryption process of each individual block interact with each other?

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CS 285 Network Security Block Cipher Modes of Operation

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  1. CS 285 Network SecurityBlock Cipher Modes of Operation Fall 2008

  2. Introduction • How to encrypt a message with variable lengths • Decompose the message into blocks, padding if necessary. • How should the encryption/decryption process of each individual block interact with each other? • Modes of operation

  3. ECB (Electronic Codebook)

  4. CBC (Cipher Block Chaining)

  5. CFB (Cipher Feedback) E

  6. CFB vs. OFB CFB OFB

  7. OFB (Output Feedback Mode) E

  8. CTR (Counter)

  9. Confidentiality and Integrity Protection • ECB • Same plaintext blocks produce same ciphertext blocks. This means that the data pattern is revealed. For example, ECB mode will reveal the image pattern if used to encrypt image files. • Rearranging the blocks is undetectable. • CBC • Random IV gurantees that even if the same message is repeated, the ciphertext is different. • Modifying ciphertext blocks and rearranging ciphertext blocks undetected are still possible. • CFB • No integrity protection; Better in detecting alterations than OFB • OFB • Able to make controlled changes to recovered plaintext. No integrity protection; not as good as CFB • CTR • Same as OFB

  10. ECB Block oriented transmission Not suitable for long messages or highly structured messages. Good for single values (e.g. keys) CBC Block-oriented transmission General-purpose encryption message authentication code design CTR Block-oriented transmission Able to preprocess to generate one-time pad; Random access; High performance requirement; IPsec CFB Stream-oriented transmission, no need for padding; ciphertext has the same length of message; pipeline is possible for encryption, thus good for low-latency real-time transmission encryption. OFB Stream-oriented transmission transmission over noisy channel Able to preprocess to generate one-time pad Application

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