# CST(30)-Security better than C2 on LAN

**Encryption:**

This allows two principals to hold a secure communication. Each principal must obtain a copy of a session key from a trusted third party. This session key can be used for encoding and decoding messges. Another approach is to use a private / public key encryption techniques. The advantage of this is it can be used for electronic signatures and non repudiation.

**Cryptographic checksums:**

A less extreme solution, ensure that data is not altered as ist passes through the network. The sender calculates the checksum on the data, using a session key to encrypt it and appends the result to the message. The receiver recalculates the checksum, decrypts the one received in the message using the session key and then compares the two. If they don’t match the message is suspect. Without the session key the intruders will not be able to alter the data and update the checksum.

**The shared private key approach:**

Uses a single key to encrypt and decrypt information. Each pair of users who need to exchange messages must agree on a private key and use it as a cipher to encode and decode their messages. This method works well as long as both the sides maintain the secrecy of the private key.

The Data Encryption Standard is based on the public key approach and has been the official US National Cryptographic standard. IBM originally proposed DES as 128 bi ciphers. DES has been the algorithm of choice for interbank electronic funds transfer. A machine performing one DS decryption per microsecond would take 2000 years to crack a given key.

**The Public Key Approach:**

This approach uses 2 keys, a public key and a private key. The public key may be listed in directories and available for all to see. Message can be encrypted with the private key and the recipient uses your public key to decode it.

RSA is a public key algorithm invented at MIT. An RSA stand for the initials of its three inventors. It is considered the public key algorithm of choice and is used mostly for authentication. The problem is that RSA is too slow for encrypting longer messages and requires DES to do that.

CST(30)-Security better than C2 on LAN
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