Cryptography

• Private key: Same key for encryption and decryption.
• It requires that a secure channel be used to distribute the key.

• Public key: Encryption key is public E whereas the decryption key D is private
• Requirements:
• D(E(M))=M ,
• E and D efficient
• impractical to derive D from  E
• Algorithms: most uses mathematical properties (eg: prime numbers)

Authentification

• When : signature, client/server, e-mail
• Why: to protect from replay of messages or interferences
• How: digital signature

Digitial signature

• Goal: make sure that a message comes from a known reader
• How: by using cryptography between a (Ea/Da) and b (Eb/Db)

Sender(a): C=Eb(Da(M) ) then send C

Receiver(b): receive C and M=Ea(Db(C))