Key Exchange Protocols

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Basic Concepts in Cryptography


Encryption scheme:
functions to encrypt, decrypt data
key generation algorithm
Secret key vs. public key
Public key: publishing key does not reveal key-1
Secret key: more efficient, generally key = key-1
Hash function, MAC
Map input to short hash; ideally, no collisions
MAC (keyed hash) used for message integrity
Signature scheme
Functions to sign data, verify signature


What is a “secure” cryptosystem?


One idea
If enemy intercepts ciphertext, cannot recover plaintext

Issues in making this precise
What else might your enemy know?
The kind of encryption function you are using
Some plaintext-ciphertext pairs from last year
Some information about how you choose keys
What do we mean by “cannot recover plaintext” ?
Ciphertext contains no information about plaintext
No efficient computation could make a reasonable guess
Cannot use ciphertext for any nontrivial purpose