Tuesday, April 8, 2008


Caesar Cipher is a substitution algorithm
Asymetric key are easy to distribute, this is a big advantage versus symetric ones.

online ppt

DES - Digital Encryption System

IMB developed DES on 1972, but never was approved by national security, but is a standard for unclassified government data.

Is a symetric private key algorithm that does 16 rounds of transpositions and substitutions.
blocksize 64bits, plain cyphertext.
keysize 56bit - 8bit of parity

Exists HW implementations of DES.

Double DES: key 112bits
Triple DES: very secure, encrypt first key, decrypt second key, encrypt first key.

El Gamal: Extends Diffie-Helman, enabling the encryption and the digital key management.

The only cipher system said to be unbreakable by brute force is one-time pad.

Electronic Code Book (ECB) is a cryptographic tool vulnerable to frequency analysis.
According to Bruce Schneier and Niels Ferguson, the best mode to select for a product would be CTR (counter) but failure to randomize the nonce, and preventing nonce reuse will decrease the security of CTR mode.

The running key cipher is based on modular arithmetic.

Cryptosystem attacks can be: timing, chosen plaintext an differential. But not Rubber hose which is used to attack the user and not the system.

Polyalphabetic ciphers are used for mitigating frequency analysis attack.

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