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5 Cryptography

# 5 Cryptography - CET 4663 Cryptography part 2 Last Time We...

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CET 4663 Cryptography part 2

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Last Time We covered a brief history of shared-key encryption At least up to modern times
Perfect Secrecy Is it achievable? Depends on how you define 'perfect' and 'secrecy' How about: An adversary, given an unlimited amount of time can learn nothing about the message from the ciphertext

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One-Time Pad Achieves this definition of perfect secrecy No eavesdropper can ever determine the original message Ok – that's it – lecture over, right? What's the catch?
One-Time Pad Extremely difficult to use correctly Requires long key length Each key can only be used a single time Insecure against known-plaintext attacks But let's look at how it works

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One-Time Pad Generate a string of random numbers as long as the message to be encrypted Combine the random numbers with the message XOR, modular or rotational arithmetic Now you have your ciphertext Why is this unbreakable?
One-Time Pad Assume we have Message : M Key : K Ciphertext : C, where C = M * K Let's just use numbers I tell you that C = 100 What is M?

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One-Time Pad We could have the ciphertext “JEUAHCUWHFZDION” Using one key it decrypts to “ATTACK AT DAWN!” Using another key it decrypts to “RETREAT AT ONCE”
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5 Cryptography - CET 4663 Cryptography part 2 Last Time We...

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