Block Cipher Operation CBC CFB OFB CSS441: Security and - - PowerPoint PPT Presentation

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Block Cipher Operation CBC CFB OFB CSS441: Security and - - PowerPoint PPT Presentation

Aug 24, 2023 530 likes •875 views

CSS441 Block Cipher Operation Modes ECB Block Cipher Operation CBC CFB OFB CSS441: Security and Cryptography CTR Feedback Sirindhorn International Institute of Technology XTS-AES Thammasat University Prepared by Steven Gordon on 20

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Block Cipher Operation

CSS441: Security and Cryptography

Sirindhorn International Institute of Technology Thammasat University

Prepared by Steven Gordon on 20 December 2015 css441y15s2l04, Steve/Courses/2015/s2/css441/lectures/modes-of-operation.tex, r4295

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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Modes of Operation

◮ Block cipher: operates on fixed length b-bit input to

produce b-bit ciphertext

◮ What about encrypting plaintext longer than b bits? ◮ Break plaintext into b-bit blocks (padding if necessary)

and apply cipher on each block

◮ Security issues arise: different modes of operation have

been developed

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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ECB Encryption

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ECB Decryption

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Summary

◮ Each block of 64 plaintext bits is encoded

independently using same key

◮ Typical applications: secure transmission of single

values (e.g. encryption key)

◮ Problem: with long message, repetition in plaintext may

cause repetition in ciphertext

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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CBC Encryption

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CBC Decryption

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CBC Summary

◮ Input to encryption algorithm is XOR of next 64-bits

plaintext and preceding 64-bits ciphertext

◮ Typical applications: General-purpose block-oriented

transmission; authentication

◮ Initialisation Vector (IV) must be known by

sender/receiver, but secret from attacker

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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CFB Encryption

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CFB Decryption

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CFB Summary

◮ Converts block cipher into stream cipher

◮ No need to pad message to integral number of blocks ◮ Operate in real-time: each character encrypted and

transmitted immediately

◮ Input processed s bits at a time ◮ Preceding ciphertext used as input to cipher to produce

pseudo-random output

◮ XOR output with plaintext to produce ciphertext ◮ Typical applications: General-purpose stream-oriented

transmission; authentication

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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OFB Encryption

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OFB Decryption

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OFB Summary

◮ Converts block cipher into stream cipher ◮ Similar to CFB, except input to encryption algorithm is

preceding encryption output

◮ Typical applications: stream-oriented transmission over

noisy channels (e.g. satellite communications)

◮ Advantage compared to OFB: bit errors do not

propagate

◮ Disadvantage: more vulnerable to message stream

modification attack

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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CTR Encryption

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CTR Decryption

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CTR Summary

◮ Converts block cipher into stream cipher ◮ Each block of plaintext XORed with encrypted counter ◮ Typical applications: General-purpose block-oriented

transmission; useful for high speed requirements

◮ Efficient hardware and software implementations ◮ Simple and secure

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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Feedback: CBC and CFB

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Feedback: OFB and CTR

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Contents

Modes of Operation Electronic Code Book Cipher Block Chaining Mode Cipher Feedback Mode Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES

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XTS-AES Encryption of Single Block

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XTS-AES Decryption of Single Block

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XTS-AES Encryption

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XTS-AES Decryption

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Encryption for Stored Data

◮ XTS-AES designed for encrypting stored data (as

  • pposed to transmitted data)

◮ See Chapter 6.7 for details and differences to

transmitted data encryption