Block Cipher Modes of Operation Electronic Code Book Cipher Block - PDF document

Cryptography Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Block Cipher Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cryptography Cipher Feedback Mode School of Engineering and Technology Output Feedback CQUniversity Australia Mode Counter Mode Prepared by Steven Gordon on 09 Mar 2020, Feedback modes.tex, r1818 Characteristics of Modes XTS-AES 1

Cryptography Contents Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Block Ciphers with Multiple Blocks Electronic Code Book Electronic Code Book Cipher Block Chaining Mode Cipher Block Chaining Mode Cipher Feedback Cipher Feedback Mode Mode Output Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Counter Mode Modes XTS-AES Feedback Characteristics of Modes XTS-AES 2

Cryptography How Do Block Ciphers Encrypt Arbitrary Length Block Cipher Plaintext? Modes of Operation ◮ Block cipher: operates on fixed length b -bit input to Block Ciphers with Multiple Blocks produce b -bit ciphertext Electronic Code Book ◮ What about encrypting plaintext longer than b bits? Cipher Block ◮ Naive approach: Break plaintext into b -bit blocks Chaining Mode (padding if necessary) and apply cipher on each block Cipher Feedback Mode independently Output Feedback ◮ ECB Mode ◮ Security issues arise: Counter Mode Feedback ◮ Repetitions of input plaintext blocks produces Characteristics of Modes repetitions of output ciphertext blocks ◮ Repetitions (patterns) in ciphertext are bad! XTS-AES ◮ Different modes of operation have been developed ◮ Tradeoffs between security, performance, error handling and additional features (e.g. include authentication) 3 We will not cover each mode of operation in detail, but rather present them so you are aware of some of the common modes.

Cryptography ECB Summary Block Cipher Modes of Operation ◮ Each block of 64 plaintext bits is encoded independently using same key Block Ciphers with Multiple Blocks ◮ Typical applications: secure transmission of single Electronic Code Book values (e.g. encryption key) Cipher Block ◮ Problem: with long message, repetition in plaintext may Chaining Mode cause repetition in ciphertext Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES 5

Cryptography ECB Encryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:ECB_encryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 6

Cryptography ECB Decryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:ECB_decryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 7

Cryptography CBC Summary Block Cipher Modes of Operation ◮ Input to encryption algorithm is XOR of next 64-bits plaintext and preceding 64-bits ciphertext Block Ciphers with Multiple Blocks ◮ Typical applications: General-purpose block-oriented Electronic Code Book transmission; authentication Cipher Block ◮ Initialisation Vector (IV) must be known by Chaining Mode sender/receiver, but secret from attacker Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES 9

Cryptography CBC Encryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:CBC_encryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 10

Cryptography CBC Decryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:CBC_decryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 11

Cryptography CFB Summary Block Cipher Modes of ◮ Converts block cipher into stream cipher Operation ◮ No need to pad message to integral number of blocks Block Ciphers with ◮ Operate in real-time: each character encrypted and Multiple Blocks Electronic Code transmitted immediately Book ◮ Input processed s bits at a time Cipher Block Chaining Mode ◮ Preceding ciphertext used as input to cipher to produce Cipher Feedback Mode pseudo-random output Output Feedback ◮ XOR output with plaintext to produce ciphertext Mode ◮ Typical applications: General-purpose stream-oriented Counter Mode Feedback transmission; authentication Characteristics of Modes XTS-AES 13

Cryptography CFB Encryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Credit: Wikimedia https://commons.wikimedia.org/wiki/File:CFB_encryption.svg , public domain Feedback Characteristics of Modes XTS-AES 14

Cryptography CFB Decryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Credit: Wikimedia https://commons.wikimedia.org/wiki/File:CFB_decryption.svg , public domain Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES 15

Cryptography OFB Summary Block Cipher Modes of Operation ◮ Converts block cipher into stream cipher ◮ Similar to CFB, except input to encryption algorithm is Block Ciphers with Multiple Blocks preceding encryption output Electronic Code Book ◮ Typical applications: stream-oriented transmission over Cipher Block noisy channels (e.g. satellite communications) Chaining Mode ◮ Advantage compared to OFB: bit errors do not Cipher Feedback Mode propagate Output Feedback Mode ◮ Disadvantage: more vulnerable to message stream Counter Mode modification attack Feedback Characteristics of Modes XTS-AES 17

Cryptography OFB Encryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:OFB_encryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 18

Cryptography OFB Decryption Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Credit: Wikimedia https://commons.wikimedia.org/wiki/File:OFB_decryption.svg , public domain Mode Counter Mode Feedback Characteristics of Modes XTS-AES 19

Block Cipher Modes of Operation Electronic Code Book Cipher Block - PDF document

Cryptography Block Cipher Modes of Operation Block Ciphers with Multiple Blocks Block Cipher Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cryptography Cipher Feedback Mode School of Engineering and Technology

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