Work in progress: 06-Aug-03

Cryptography

As long as people have had interactive communication.  There has been a desire for the secrecy in those communications.

Definitions:

(All words are derived from Greek.)

Stegano: covered.
Crypto: hidden, secret.
Graphein: to write.

Steganography: Covered Writing.  Hiding a message within a larger one in such a way that others can not discern the presence or contents of the hidden message. For example, a message might be hidden within an image by changing the least significant bits to be the actual message.  Another variation of the image method is to hide the message within the image, possibly as a shadow on a branch of a tree.  This was one method used by the 9/11 terrorists to mask their communications.  Another example is microdots, where a message is scaled down to the size of a full-stop or period.  In either case the goal is to hide the message from the observer.  If the message is discovered, the complete text is revealed. 

Cryptography: Hidden Writing.  The practise and study of encryption and decryption - encoding data so that it can only be decoded by specific individuals. A system for encrypting and decrypting data is a cryptosystem. These usually involve an algorithm for combining the original data ("plaintext") with one or more "keys" - numbers or strings of characters known only to the sender and/or recipient. The resulting output is known as "ciphertext".

The security of a cryptosystem usually depends on the secrecy of (some of) the keys rather than with the supposed secrecy of the algorithm. A strong cryptosystem has a large range of possible keys so that it is not possible to just try all possible keys (a "brute force" approach). A strong cryptosystem will produce ciphertext which appears random to all standard statistical tests. A strong cryptosystem will resist all known previous methods for breaking codes ("cryptanalysis").

Recommended Reading / Reference List:

	Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography by Simon Singh
	Applied Cryptography: Protocols, Algorithms, and Source Code in C by Bruce Schneier