Understanding_Cryptography_Chptr_2---Stream_Ciphers

Understanding_Cryptography_Chptr_2---Stream_Ciphers -...

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Understanding Cryptography – A Textbook for Students and Practitioners by Christof Paar and Jan Pelzl www.crypto-textbook.com Chapter 2 – Stream Ciphers ver. October 29, 2009 These slides were prepared by Thomas Eisenbarth, Christof Paar and Jan Pelzl
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl Some legal stuff (sorry): Terms of Use The slides can used free of charge. All copyrights for the slides remain with the authors. The title of the accompanying book “Understanding Cryptography” by Springer and the author’s names must remain on each slide. If the slides are modified, appropriate credits to the book authors and the book title must remain within the slides. It is not permitted to reproduce parts or all of the slides in printed form whatsoever without written consent by the authors. 2/27
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl Content of this Chapter Intro to stream ciphers Random number generators (RNGs) One-Time Pad (OTP) Linear feedback shift registers (LFSRs) Trivium: a modern stream cipher 3/27
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl Content of this Chapter Intro to stream ciphers Random number generators (RNGs) One-Time Pad (OTP) Linear feedback shift registers (LFSRs) Trivium: a modern stream cipher 4/27
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl U Stream Ciphers in the Field of Cryptology Cryptology Cryptography Cryptanalysis Symmetric Ciphers Asymmetric Ciphers Protocols Block Ciphers Stream Ciphers Stream Ciphers were invented in 1917 by Gilbert Vernam 5/27
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl U Stream Cipher vs. Block Cipher Stream Ciphers Encrypt bits individually Usually small and fast b common in embedded devices (e.g., A5/1 for GSM phones) Block Ciphers: Always encrypt a full block (several bits) Are common for Internet applications 6/27
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U Encryption and Decryption with Stream Ciphers Encryption and decryption are simple additions modulo 2 (aka XOR) Encryption and decryption are the same functions Encryption: y i = e si ( x i ) = x i + s i mod 2 x i , y i , s i {0,1} Decryption: x i = e si ( y i ) = y i + s i mod 2 Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl Plaintext x i , ciphertext y i and key stream s i consist of individual bits 7/27
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Chapter 2 of Understanding Cryptography by Christof Paar and Jan Pelzl U Synchronous vs. Asynchronous Stream Cipher Security of stream cipher depends entirely on the key stream s i : Should be random , i.e., Pr( s i = 0) = Pr( s i = 1) = 0.5 Must be reproducible by sender and receiver Synchronous Stream Cipher Key stream depend only on the key (and possibly an initialization vector IV)
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This note was uploaded on 04/01/2011 for the course MA C taught by Professor Anario during the Spring '11 term at Universitat Politècnica de Catalunya.

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Understanding_Cryptography_Chptr_2---Stream_Ciphers -...

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