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

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 December 2015 css441y15s2l04, Steve/Courses/2015/s2/css441/lectures/modes-of-operation.tex, r4295 1/32

CSS441 Contents Block Cipher Operation Modes of Operation Modes ECB Electronic Code Book CBC CFB Cipher Block Chaining Mode OFB CTR Cipher Feedback Mode Feedback XTS-AES Output Feedback Mode Counter Mode Feedback Characteristics of Modes XTS-AES 2/32

CSS441 Modes of Operation Block Cipher Operation ◮ Block cipher: operates on fixed length b -bit input to produce b -bit ciphertext Modes ECB ◮ What about encrypting plaintext longer than b bits? CBC ◮ Break plaintext into b -bit blocks (padding if necessary) CFB and apply cipher on each block OFB ◮ Security issues arise: different modes of operation have CTR been developed Feedback XTS-AES 3/32

CSS441 ECB Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 5/32

CSS441 ECB Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 6/32

CSS441 Summary Block Cipher Operation ◮ Each block of 64 plaintext bits is encoded independently using same key Modes ECB ◮ Typical applications: secure transmission of single CBC values (e.g. encryption key) CFB ◮ Problem: with long message, repetition in plaintext may OFB cause repetition in ciphertext CTR Feedback XTS-AES 7/32

CSS441 CBC Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 9/32

CSS441 CBC Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 10/32

CSS441 CBC Summary Block Cipher Operation ◮ Input to encryption algorithm is XOR of next 64-bits plaintext and preceding 64-bits ciphertext Modes ECB ◮ Typical applications: General-purpose block-oriented CBC transmission; authentication CFB ◮ Initialisation Vector (IV) must be known by OFB sender/receiver, but secret from attacker CTR Feedback XTS-AES 11/32

CSS441 CFB Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 13/32

CSS441 CFB Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 14/32

CSS441 CFB Summary Block Cipher ◮ Converts block cipher into stream cipher Operation ◮ No need to pad message to integral number of blocks Modes ◮ Operate in real-time: each character encrypted and ECB transmitted immediately CBC ◮ Input processed s bits at a time CFB OFB ◮ Preceding ciphertext used as input to cipher to produce CTR pseudo-random output Feedback ◮ XOR output with plaintext to produce ciphertext XTS-AES ◮ Typical applications: General-purpose stream-oriented transmission; authentication 15/32

CSS441 OFB Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 17/32

CSS441 OFB Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 18/32

CSS441 OFB Summary Block Cipher Operation ◮ Converts block cipher into stream cipher Modes ◮ Similar to CFB, except input to encryption algorithm is ECB preceding encryption output CBC ◮ Typical applications: stream-oriented transmission over CFB noisy channels (e.g. satellite communications) OFB ◮ Advantage compared to OFB: bit errors do not CTR propagate Feedback XTS-AES ◮ Disadvantage: more vulnerable to message stream modification attack 19/32

CSS441 CTR Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 21/32

CSS441 CTR Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 22/32

CSS441 CTR Summary Block Cipher Operation ◮ Converts block cipher into stream cipher Modes ◮ Each block of plaintext XORed with encrypted counter ECB ◮ Typical applications: General-purpose block-oriented CBC transmission; useful for high speed requirements CFB ◮ Efficient hardware and software implementations OFB CTR ◮ Simple and secure Feedback XTS-AES 23/32

CSS441 Feedback: CBC and CFB Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 25/32

CSS441 Feedback: OFB and CTR Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 26/32

CSS441 XTS-AES Encryption of Single Block Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 28/32

CSS441 XTS-AES Decryption of Single Block Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 29/32

CSS441 XTS-AES Encryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 30/32

CSS441 XTS-AES Decryption Block Cipher Operation Modes ECB CBC CFB OFB CTR Feedback XTS-AES 31/32

CSS441 Encryption for Stored Data Block Cipher Operation ◮ XTS-AES designed for encrypting stored data (as opposed to transmitted data) Modes ECB ◮ See Chapter 6.7 for details and differences to CBC transmitted data encryption CFB OFB CTR Feedback XTS-AES 32/32

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

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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