Network Monitoring

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 be observed by any other student using freely available PC software. You need to always be aware of this!

Encryption is a vast technical, scientific and political topic. We will look briefly at a few aspects.

Cryptography Basics

The security of the cyphertext depends on:

• The nature of the encryption method, or algorithm. It is nowadays generally agreed that open publication of details of the algorithm is a Good Thing.
• The secrecy of the key. Current opinion is that, given a suitably powerful encryption algorithm, the security of the system should depend entirely on:
  1. keeping the key secret and
  2. the length (usually measured in bits) of the key itself.

The Basic Algorithms

Substitution Cyphers

the simplest technique, whereby each character in the message is replaced by another using some rule. The order of the encrypted characters is the same as in the plaintext. 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.

Transposition Cyphers

here the order of the plaintext characters is changed, but the characters themselves are not.

Another approach (although rarely used) is the one-time pad, (or Vernam Cypher) where a simple algorithm is used in conjunction with a key of the same length as the message, and employing a brand new key for every message transmitted message. This is, in every respect, unbreakable, but rather impractical for real-world use in most cases (although see s/key).

DES - The Data Encryption Standard

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 cyphertexts. This is called the Electronic Code Book (ECB) mode of operation.

DES In Practice

In the Chain Block Cypher (CBC) mode, each block of plaintext is exclusive-ORed (XOR) with the cyphertext output from the previous encryption operation. Thus, the next block of cyphertext is a function of its corresponding plaintext, the 56-bit key and the previous block of cyphertext. Identical blocks of plaintext no longer generate identical cyphertext, 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.

A variation on CBC is used where the message may not be a multiple of 64 bits, or where interactive (character at a time) encryption and decryption is desired. This is called Cypher Feedback Mode (CBM), and uses shift registers to permit one byte at a time to be encrypted or decrypted.

Public Key Systems

• A public key is used to encrypt and a separate, different private key to decrypt the message.
• Each party involved generates a key pair.
• Each party publishes their public key.
• Each party secures their private key, which must remain secret.
• Assuming A desires to send a message to B, it first encrypts the message using B's public key
• B can decrypt the message using its private key. Since no one else knows B's private key, this is absolutely secure - no one else can decrypt it.
• There still remain difficult problems of authentication of public keys, compromised keys, bogus & out of date keys. Further, Public Key encryption is very, very slow compared to single key systems.

Other Encryption Applications

• This turns out to be one of the trickiest problems in cryptography.
• Can be done if both parties share a common secret key or keys, however...
• Diffie-Hellman key exchange protocol can be used to establish shared secret keys but is complex and unwieldy.
• More usefully, trusted Key Distribution Centres perform this function. See also Kerberos and Public Key-based systems.

• Single key and public key systems are known - both also encrypt the message, which may be unneccessary.
• To authenticate without encryption, message digest functions can be used. The most common is MD5 (previous version: MD4), available on the Bendigo Unix systems. Message digests are based on the idea that no two messages can ever generate the same MD.

Clipper, Key Escrow and the Whole Political Thing

• The original Clipper system was a hardware encryption device based on the (secret) skipjack encryption algorithm. It was originally intended to replace DES.
• Chip users (manufactuers?) register key with government: key escrow. The key is split into two halves, which are stored independently with different government agencies. Current version: key recovery.
• Court-approved "wiretap" (ie, decryption) operations require both halves of the key to access any data transmissions. In addition, a released key can be coded so that it only works for a limited time period.

• Privacy concerns
• Commercial organisations are nervous about trusting the government.
• some doubt about whether snooping decryption could be avoided by clever users.
• Opinion: it won't fly.