This is called eavesdropping or (in some circles) packet snarfing. Once the host has copies of all the frames it desires, it can then analyse them to discover the data they contain.
Most data transfers across the Internet are not encoded (or encrypted) in any way Ð the data is simply sent as plain text. Thus it is simple to observe messages transmitted by others. This is the origin of the (oft repeated, and generally true) assertion that "The Internet is insecure".
An area where this insecurity can present a serious problem is password authentication. At Bendigo, students can use the TELNET protocol to connect to the various Unix systems. The password which is typed by the student is transmitted across the LAN as plain text, and can potentially be observed by any other student using freely available PC software. You need to always be aware of this!The solution is encryption - encoding the message so that it is unintelligible to the intruder.
Encryption is a vast technical, scientific and political topic. We will look briefly at a few aspects.
The security of the ciphertext depends on:![]()
There are many examples of this technique. Most fall into the general category of monoalphabetic substitution, where the output alphabet is the same as the input. For example, in the classic Caesar Cipher, letters of the alphabet were shifted by 3 positions, hence a becomes D, b becomes E, etc. A more complex example:
hence bad is encrypted as WQR.plaintext: a b c d e f g h i j k l m n o p q r s t u v w x y z ciphertext: Q W E R T Y U I O P A S D F G H J K L Z X C V B N M
This type of cipher turns out to be relatively easy to break, despite the huge (26!) keyspace, by using known statistical characteristics of English (or other languages), eg:
Hence, the ciphertext is:![]()
Neither substitution nor transposition ciphers are regarded as secure for any serious use. The modern approach is to use basically the same ideas, but with much more complex algorithms (see DES, later).eatitnihmexnetmgmedt
For example, convert both the plaintext and the key to bit strings (which will be, necessarily, of the same length). Apply the XOR (Exclusive OR) function bitwise between the strings, giving the ciphertext. The recipient can then apply the same key to the ciphertext using XOR and thus recover the original plaintext.
This is, in every respect, unbreakable, but rather impractical for real-world use in most cases. Some reasons:
Neverthless, see s/key for a practical, working example of a one-time pad system.
Note that DES is designed so that decryption is performed by the exact same algorithm as encryption (using the same key - hence single key), except performed in reverse.![]()
The effectiveness of DES is based on the complexity of the 19 stages. In the above diagram, two identical 64-bit plaintexts will result in identical ciphertexts. This is called the Electronic Code Book (ECB) mode of operation.
In the Chain Block Cypher (CBC) mode, each block of plaintext is exclusive-ORed (XOR) with the ciphertext output from the previous encryption operation. Thus, the next block of ciphertext is a function of its corresponding plaintext, the 56-bit key and the previous block of ciphertext. Identical blocks of plaintext no longer generate identical ciphertext, which makes this system much more difficult to break.
The CBC mode of DES is the normal technique used for encryption in modern business data communications.![]()
DES has always been controversial: