Symmetric Ciphers
The Data Encryption Standard (DES, 1976)
is known as a symmetric cipher, often
a private key algorithm -
in which the sender and receiver use
the same key that must be kept private.
In general, longer keys provide stronger encryption, but it is a mistake
to imply the strength of an encryption algorithm in terms of bits alone.
Brute force attacks are the most successful.
Some popular examples of symmetric ciphers:
- DES:
a block based cipher of 64bit blocks in,
64bit blocks out, 56 bit key filled to 64bits (8 odd-parity bits).
- Triple-DES: encrypts the same plaintext with DES three times.
Three or two keys are provided, the plaintext is encrypted with the 1st key,
decrypted with the second, and finally encrypted with the third (or 1st
again).
A double-DES scheme (with only 2 keys) does not require 22n
brute-force tests but 2n+1
tests with a meet-in-the-middle attack.
- Ron Rivest's RC2 block cipher employs keys up to 1024 bits,
and executes at a speed independent of key length.
- Ron Rivest's RC4 stream cipher
(as used in WiFi's WEP encryption)
employs keys of 40 to 256 bits,
but has the property that if two messages are encrypted with the
same RC4 key,
their encryptions are related in a known way.
CITS3002 Computer Networks, Lecture 12, Cryptography's role in networking, p6, 22nd May 2024.
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CITS3002