Conventional Cryptography - A Classical Introduction to Cryptography: Applications for Communications Security - page 21

2
Conventional Cryptography

Content

DES: Feistel Scheme, S-boxes
Modes of operation: ECB, CBC, OFB, CFB, CTR, UNIX passwords
Classical designs: IDEA, SAFER K-64, AES

Case study: FOX, CS-CIPHER
Stream ciphers: RC4, A5/1, E0
Brute force attacks: exhaustive search, tradeoffs, meet-in-the-middle

In Chapter 1 we saw the foundations of cryptography. Shannon formalized secrecy
with the notion of entropy coming from information theory, and proved that secrecy was
not possible unless we used (at least) the Vernam cipher. Except for the red telephone
application, this is not practical. We can however do some cryptography by changing
the model and relying on a computational ability. Before carefully formalizing com-
putability in Chapter 8 we use an intuitive notion of complexity. Indeed, the need of
an industry for practical cryptographic solutions pushed toward adopting an empirical
notion of secrecy: a cryptographic system provides secrecy until someone finds an
attack against it.

We recall that symmetric encryption relies on three algorithms:

a key generator which generates a secret key in a cryptographically random or
pseudorandom way;
an encryption algorithm which transforms a plaintext into a ciphertext using a
secret key;
a decryption algorithm which transforms a ciphertext back into the plaintext
using the secret key.

Symmetric encryption is assumed to enable confidential communications over an inse-
cure channel assuming that the secret key is transmitted over an extra secure channel.
Fig. 2.1 represents one possible use of this scheme. Here the secret key is transmitted
from the receiver to the sender in a confidential way, and the adversary tries to get
information from the ciphertext only.