Microsoft PowerPoint - Crypto.ppt

Short Description
Traditional Cryptography. Ciphers were already studied in ancient times … Cryptography. 5. Statistical Attacks. Armed with statistical knowledge about the …

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Content
6/8/2002 2:08 PM Cryptography 1
Cryptography
plaintext encrypt ciphertext
6/8/2002 2:08 PM Cryptography 2
Outline
Traditional cryptography
Statistical attacks
Secret-key encryption
Public-key encryption
6/8/2002 2:08 PM Cryptography 3
Encryption
Scenario:
.. Alice wants to send a message (plaintext p) to Bob.
.. The communication channel is insecure and can be eavesdropped
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 encryption scheme?
.. 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 an encryption scheme?
plaintext encrypt ciphertext decrypt plaintext
6/8/2002 2:08 PM Cryptography 4
Traditional Cryptography
Ciphers were already studied in ancient times
Caesar’s cipher:
.. replace a with d
.. replace b with e
.. …
.. replace z with c
Caesar’s cipher is an example of a monoalphabetic substitution
cipher, which permutes the characters
Armed with simple statistical knowledge, one can easily break a
monoalphabetic substitution cipher
…..