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What is cryptography? Dan Boneh Crypto core Talking Talking - PowerPoint PPT Presentation

Jun 13, 2023 •318 likes •533 views

Online Cryptography Course Dan Boneh Online Cryptography Course Dan Boneh Introduction What is cryptography? Dan Boneh Crypto core Talking Talking Talking Talking to

  1. Online Cryptography Course Dan Boneh Online Cryptography Course Dan Boneh Introduction What is cryptography? Dan Boneh

  2. Crypto core Talking Talking Talking Talking to Alice to Alice to Bob to Bob Alice Secret key establishment: Bob attacker??? m 1 Secure communication: k k m 2 confidentiality and integrity Dan Boneh

  3. But crypto can do much more • Digital signatures • Anonymous communication Alice Alice Who did I Who did I signature signature just talk to? just talk to? Alice Bob Dan Boneh

  4. But crypto can do much more • Digital signatures • Anonymous communication • Anonymous digital cash – Can I spend a “digital coin” without anyone knowing who I am? – How to prevent double spending? Who was Who was 1$ Alice that? that? Internet (anon. comm.) Dan Boneh

  5. Protocols • Elections • Private auctions Dan Boneh

  6. Protocols • Elections • Private auctions trusted trusted Goal: compute f(x 1 , x 2 , x 3 , x 4 ) authority authority “ Thm :” anything the can done with trusted auth. can also be done without • Secure multi-party computation Dan Boneh

  7. Crypto magic • Privately outsourcing computation What did she What did she search for? search for? search E[ query ] query Alice E[ results ] results • Zero knowledge (proof of knowledge) ??? ??? Alice I know the factors of N !! N= p∙q N Bob p roof π Dan Boneh

  8. A rigorous science The three steps in cryptography: • Precisely specify threat model • Propose a construction • Prove that breaking construction under threat mode will solve an underlying hard problem Dan Boneh

  9. Online Cryptography Course Dan Boneh Online Cryptography Course Dan Boneh Introduction History Dan Boneh

  10. History David Kahn, “The code breakers” (1996) Dan Boneh

  11. Symmetric Ciphers Dan Boneh

  12. Few Historic Examples (all badly broken) 1. Substitution cipher k := Dan Boneh

  13. Caesar Cipher (no key) Dan Boneh

  14. What is the size of key space in the substitution cipher assuming 26 letters? | ࣥ | = 26 𝒧 = 26! ( 26 factorial) | ࣥ | = 2 26 | ࣥ | = 26 2 Dan Boneh

  15. How to break a substitution cipher? What is the most common letter in English text? “X” “L” “E” “H” Dan Boneh

  16. How to break a substitution cipher? (1) Use frequency of English letters (2) Use frequency of pairs of letters (digrams) Dan Boneh

  17. An Example UKBYBIPOUZBCUFEEBORUKBYBHOBBRFESPVKBWFOFERVNBCVBZPRUBOFERVNBCVBPCYYFVUFO FEIKNWFRFIKJNUPWRFIPOUNVNIPUBRNCUKBEFWWFDNCHXCYBOHOPYXPUBNCUBOYNRVNIWN CPOJIOFHOPZRVFZIXUBORJRUBZRBCHNCBBONCHRJZSFWNVRJRUBZRPCYZPUKBZPUNVPWPCYVF ZIXUPUNFCPWRVNBCVBRPYYNUNFCPWWJUKBYBIPOUZBCUIPOUNVNIPUBRNCHOPYXPUBNCUB OYNRVNIWNCPOJIOFHOPZRNCRVNBCUNENVVFZIXUNCHPCYVFZIXUPUNFCPWZPUKBZPUNVR  E  IN  THE B 36 NC 11 UKB 6  AT N 34 PU 10 RVN 6  T U 33 UB 10 FZI 4  A P 32 UN 9 trigrams C 26 digrams Dan Boneh

  18. 2. Vigener cipher (16’th century, Rome) k = C R Y P T O C R Y P T O C R Y P T (+ mod 26) m = W H A T A N I C E D A Y T O D A Y c = Z Z Z J U C L U D T U N W G C Q S s uppose most common = “H” first letter of key = “H” – “E” = “C” Dan Boneh

  19. 3. Rotor Machines (1870-1943) Early example: the Hebern machine (single rotor) A A K K E E N N B B S S K K E E C C T T S S K K . . . . T T S S . . . . . . T T X X R R . . . . Y Y N N R R . . key Z Z E E N N R R Dan Boneh

  20. Rotor Machines (cont.) Most famous: the Enigma (3-5 rotors) # keys = 26 4 = 2 18 (actually 2 36 due to plugboard) Dan Boneh

  21. 4. Data Encryption Standard (1974) DES: # keys = 2 56 , block size = 64 bits Today: AES (2001) , Salsa20 (2008) (and many others) Dan Boneh

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