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# lec5 - Modern Cryptography Lecture 5 Yongdae Kim Admin...

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Modern Cryptography Lecture 5 Yongdae Kim

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2 Admin Stuff E-mail Subject should have [5471] in front, e.g. “[5471] Project proposal” CC TA and PostDoc: [email protected], [email protected] Office hours Me: T 1:30 ~ 2:30, Th 10:00 ~ 11:00 (and by appointment) TA: M 1:15 PM ~ 2:15 PM Work on projects Full proposal due: Feb 23 (Firm Deadline: Read instruction) Meeting with groups: This week and next week 3rd assignment is due: 3/2 9:00 AM. Programming, Test vector, Your name Study Guide Come and talk to me and TA as much as possible. (Google chat is good!) Check Calendar
3 Recap Math… Proof techniques Direct/Indirect proof, Proof by contradiction, Proof by cases, Existential/Universal Proof, Forward/backward reasoning Divisibility: a divides b (a|b) if c such that b = ac GCD, LCM, relatively prime, existence of GCD Eucledean Algorithm d = gcd (a, b) x, y such that d = a x + b y. gcd(a, b) = gcd(a, b + ka) Modular Arithmetic a b (mod m ) iff m | a-b iff a = b + mk for some k a b (mod m ), c d (mod m ) a+c ( b+d ) (mod m ), ac bd (mod m ) gcd(a, n) =1 a has an arithmetic inverse modulo n. Counting, probability, cardinality, …

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4 Recap (cnt) Security Symmetric Key vs. Public Key, Hash function, MAC, Digital signature, Key management through SKE and PKE, certificate Block Cipher Modes of operation and their properties: ECB, OFB, CFB, CBC, CTR Meet-in-the-middle attack and the Double (triple) DES Feistal Cipher and DES
5 New Era! DES broken DES III Challenge by RSA Idle CPU time of around 100,000 computers In 22 hours Triple DES? Original DES was designed for H/W implementation 64 bit block size too small for security and efficiency Now what?

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6 Advanced Encryption Standard In 1997, NIST issued a call for proposal Block length = 128 bit Key size = 128, 192, 256 bits In the first round, 15 algorithms were accepted Second round, 5 algorithms were selected In November 2001, final standard was published Rijndel, FIPS PUB 197 http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf Joan Daemen and Vincent Rijmen
7 AES Evaluation Criteria Security Actual security: compared with other submissions Randomness: output is indistinguishable from random Soundness: of mathematical basis Other security factors: raised by security community Cost No licensing: World-wide, non-exclusive, royalty-free Computation efficiency: both S/W and H/W Memory requirements Algorithm and Implementation characteristics Flexibility: key-/block-size, wide variety of platforms Simplicity

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8 Stream Cipher Definition encrypt individual characters of plaintext message one at a time, using encryption transformation which varies with time.
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