1.1 Chapter 8 Encipherment Using Modern Symmetric-Key Ciphers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Presentation transcript:

1.1 Chapter 8 Encipherment Using Modern Symmetric-Key Ciphers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1.2 To review a short history of AES To show how modern standard ciphers, such as DES or AES, can be used to encipher long messages. To discuss five modes of operation designed to be used with modern block ciphers. To define which mode of operation creates stream ciphers out of the underlying block ciphers To discuss the security issues and the error propagation of different modes of operation To discuss two stream ciphers used for real-time processing of data Objectives

1.3 Modes of operation Electronic Codebook (ECB) Mode Cipher Block Chaining (CBC) Mode Cipher Feedback (CFB) Mode Output Feedback (OFB) Mode Counter (CTR) Mode Use of Modern Block Cipher

1.4 ECB mode Electronic Codebook (ECB) Mode

1.5 ECB mode Vulnerable to replay attacks Example 8.3: Assume that Eve works in a company a few hours per month (her monthly payment is very low). She knows that the seventh block is the amount of money to be deposited in the employee’s account. Eve can intercept the ciphertext sent to the bank at the end of the month, replace the block with the information about her payment with a copy of the block with the information about the payment of a full- time colleague. Electronic Codebook (ECB) Mode

1.6 ECB mode Error propagation: a single bit error in transmission can create errors in several in the corresponding block. However, the error does not have any effect on the other blocks Electronic Codebook (ECB) Mode

1.7 Ciphertext Stealing A technique called ciphertext stealing (CTS) can make it possible to use ECB mode without padding In this technique the last two plaintext blocks, P N-1 and P N, are encrypted differently and out of order, as shown below, assuming that P N-1 has n bits and P N has m bits, where m ≤ n Electronic Codebook (ECB) Mode

1.8 CBC mode each plaintext block is exclusive-ored with the previous ciphertext block before being encrypted Cipher Block Chaining (CBC) Mode

1.9 Decryption in CBC mode Initialization vector (IV) should be known by the sender and the receiver in a priori Cipher Block Chaining (CBC) Mode

1.10 CBC mode Error propagation: In CBC mode, a single bit error in ciphertext block C j during transmission may create error in most bits in plaintext block P j during decryption Ciphertext Stealing The ciphertext stealing technique described for ECB mode can also be applied to CBC mode The head function is the same as described in ECB mode; the pad function inserts 0’s Cipher Block Chaining (CBC) Mode

1.11 CFB mode Stream cipher mode of DES or AES Cipher Feedback (CFB) Mode

1.12 OFB mode Stream cipher mode of DES or AES In this mode each bit in the ciphertext is independent of the previous bit or bits  avoids error propagation Output Feedback (OFB) Mode

1.13 Counter mode The pseudo-randomness in the key stream is achieved using a counter Counter Mode

1.14 Comparison of modes