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Cryptography



Secrecy
Ciphers
Secret Key Cryptography
Key Exchange
Public Key Cryptography
Digital Signatures
Internet applications

Secrecy


Scenario: Alice wants to send a message (plaintext p) to Bob. The communication channel is insecure and can be eavesdropped by Trudy. If Alice and Bob have previously agreed on an encryption scheme (cipher), the message can be sent encrypted (ciphertext c)

Issues:
What is a good cipher?
What is the complexity of encrypting/decrypting?
What is the size of the ciphertext, relative to the plaintext?
If Alice and Bob have never interacted before, how can they agree on a cipher?

Traditional Cryptography

Ciphers were already studied in ancient times
Caesar’s cipher:
replace a with d
replace b with e
...
replace z with c
A more general monoalphabetic substitution cipher maps each letter to some other letter.