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Classical Crypto

Classical Crypto. By: Luong-Sorin VA, IMIT Dith Nimol, IMIT. Agenda of Classical Crypto. Terms related to Crypto Simple Substitution Cipher Double Transposition Cipher One-Time Pad Codebook Cipher Conclusion. Terms related to Crypto.

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Classical Crypto

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  1. Classical Crypto By: Luong-Sorin VA, IMIT Dith Nimol, IMIT

  2. Agenda of Classical Crypto • Terms related to Crypto • Simple Substitution Cipher • Double Transposition Cipher • One-Time Pad • Codebook Cipher • Conclusion

  3. Terms related to Crypto • Cryptology: art and science of making and breaking “secret codes.” • Cryptography: making of “secret codes.” • Cryptanalysis: breaking of “secret codes.” • Crypto is a synonym for any or all of the above (and more).

  4. Simple Substitution Cipher • Ciphertext: substitute the letter of the alphabet (n) places ahead of the Plaintext. • Plaintext: substitute the letter of the alphabet (n) places backward of the Ciphertext. • n {0, 1, 2,…….,25} • (n) acts as Key • Ex: if (n=3) then

  5. Simple Substitution Cipher • Attack • Exhaustive key search? • A Long keys, Max = 26! ≈ 288 possible keys • It would take 8900 millennia to exhaust

  6. Another possible attack • Compare with English letters frequency counts • Ex: “E” is the most common • Same, try and guess, but more clever!

  7. Double Transposition Cipher • Transpose 2 times • Thwart learning from statistical information

  8. One Time Pad (OTP) • OTP strengths: • not easy to break if key is genuinely random • the cipher text is likewise completely random and so cannot be broken • not only function with 26 alphabets, but also 256 ASCII characters or other set. • also use XOR function • OTP weakness: • Key need to have same length as Plaintext so data to send is double making the traffic of network become slow • Key can be intercepted when sending • Difficulty of creating volumes of genuinely random numbers

  9. One Time Pad (OTP) • Spy named Alice want to encrypt the plaintext message • Spy Bob receives Alice’s message, he decrypts it using the same key.

  10. OTP with same key • In case messages are in depth • Attacker recognizes pattern of ciphertext • Try to guess Plaintext (Say P1) ! • Then figure out key (Say K) • Then use K to decrypt other chipertext

  11. OTP with same key

  12. OTP with same key

  13. Codebook Cipher • A kind of substitution cipher • By words or phrases • Used by Germany during WW1

  14. Codebook Cipher Codebook Cipher

  15. Conclusion • Weak : • Simple substitute cipher: attacker can guess in ciphertext message • Codebook cipher • Strong : • One time pad: is provably secure cryptosystem • Attacker can’t guess or break code • Random key never reused • Double Transposition Cipher • Employed by modern block cipher. • Which cipher to choose? • Secure • Large enough key space

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