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Cryptography is used to hide information. It is not only
used by spies but for phone, fax and e-mail communication,
bank transactions, bank account security, pin numbers and
passwords. It is also used for electronic signatures which
are used to prove who sent a message.
In our Cryptography course at Macquarie we cover vocabulary,
history, number theory, simple cryptosystems, simple cryptanalysis,
running time anaylsis and modern cryptosystems.
The Centre for Advanced Computing - Algorithms and Cryptography
(ACAC) acts as a focal point for joint research activities
between its members who come from diverse backgrounds in Physics,
Mathematics and Computing. Our vision is to be the leading
centre in Australasia for research into algorithms, complexity
and cryptography applied to advanced computing systems. We
have determined a mission to develop algorithms and cryptographic
methods and protocols for applications to advanced information
and computation systems.
The Centre's scope of interests include:
Algebraic and Combinatorial Algorithms and Their Complexity
Cryptography and Information Security
Quantum Computation
Parallel and Distributed Algorithms and Their Complexity
Graph Theory
Symbolic Computation
Computational Number Theory
Plaintext
or PT The message you want to send, like HELLO.
Ciphertext
or CT The disguised message, like XQABE.
Encrypt/Encipher Turn plaintext into cipher- text.
Decrypt/Decipher Turn ciphertext
back into plain-text.
Encode Turn plaintext into a number or num-
bers.
For example if we have A-0......Z-25, then we could encode
HELLO as 7 4 11 11 14.
Decode Turn number
or numbers back into plain- text.
Cryptosystem A pair of
enciphering and deciphering algorithms.
Private Key Cryptosystems: each user requires
the same secretely distributed key. Two users must agree on
a key ahead of time. They are also called symmetric cryptosystems
Public Key Cryptosystems: each user has
an encrypting key which is published and a decrypt- ing key
which is not. Ther ae also called Asymmetric cryptosystems
Cryptanalysis is the process by
which the enemy tries to turn CT into PT.
- Encryption
and decryption should be easy for the proper users. Decryption
should be hard for eavesdroppers/enemies.
- Security and
practicality of a cryptosystem are almost always tradeo s.
Practicality issues: time, storage, co-presence.
- Must
assume that the enemy will nd out about the nature of
a cryptosystem and will only be missing a key.
The 400 BC Greek transposition cipher. The Letters
were written on a long thin sheet of paper wrapped around
a cylinder. The diameter of the cylinder was the key.
__________________
/T/H/I/S/I/S/_/ / \
/ /H/O/W/I/T/ | |
/ /W/O/U/L/D/ \ /
-----------------------------
The Julius Caesar's substitution cipher. This Shifted
all letters three to the right. In our alphabet that would
send A-D;B-E,..............,Z-C.
The 1910's: British Playfair cipher. In World War
II it was shown that alternating substitution and transposition
ciphers is a very secure thing to do and complicated product
ciphers such as ENIGMA were used.
In the late 1960's, threats to computer security were considered
real problems. There was a need for strong encryption in the
private sector. One could now put very complex algorithms
on a single chip so one could have high-speed encryption.
There was also the possibility of high- speed cryptanalysis.
The problem
was
studied intensively between 1968 and 1975. In 1974, the
Lucifer cipher was introduced and in 1975, DES (the Data
Encryption
Standard) was introduced. Both are product ciphers. DES
uses a 64 bit key, 8 bits are for parity check. It alternates
16 substitu- tions with 15 transpositions.
The 1970's: Public key cryptography: RSA, Diffie-Hellman
And more recently 2001: Rijandel (Joan Daemen and
Vincent Rijmen) is to replace DES (which is not secure anymore).
We hope Crypto has been of interest to you. We recommend
visiting the following sites to learn more about research
in the areas of algorithms and cryptography.
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