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Public key Encryption

Encryption and certificates

The defense-in-depth strategy toward security requires system administrators to take every possible action to improve security. One significant improvement to security can be obtained by widespread use of encryption. With respect to the UNIX workstation, the following are security advantages to be gained:
  1. If a workstation gets compromised and taken over by an attacker, previously encrypted files are likely to be protected. This assumes that passphrases used to encrypt the data are kept in the users memory and not on the workstation.
  2. By encrypting traffic on the local area network (LAN), the risk of being attacked from a local source is greatly reduced. Many organizations consider their biggest security feature to be the firewall between the LAN and the Internet. Hower, other workstations on the LAN also pose a significant threat. For example, if the LAN is hubbed, any workstation can listen in on all instant messaging to and from another worksation. Even if the network is switched there are readily available tools, such as ettercap, that can monitor all traffic in and out of a workstation.
  3. Much of the traffic that travels over the Internet, such as e-mail or FTP, is in the clear or unencrypted. The only protection afforded to this traffic is security through obscurity. In other words, the telnet, e-mail, and FTP traffic can be read in many places as the traffic is routed, but who would want to? Most users would not find this level of security very comforting.

Guide to Network Security
To understand how public-key cryptosystems work, consider two users, Alice and Bob. Both users have a public/private key pair.

When Alice wants to send a signed, encrypted message to Bob, she indicates that she wants the message signed and encrypted, and sends the message.

Alice's email program selects a random symmetric key to encrypt the message and creates a message digest of the message.

The program encrypts the digest with Alice's private key to create a signature. The random symmetric key is encrypted to Bob's public key.


The entire package containing the encrypted symmetric key, the encrypted message digest or signature, and the encrypted message is sent to Bob.

Bob's email program uses his secret key to decrypt the encrypted random key, which is then used to decrypt the message.

To ensure the message was from Alice, the program decrypts the message digest using Alice's public key. If Alice's public key decrypts the message digest, then Bob knows that only Alice could have encrypted it.

Bob's program cannot reverse the message digest because of its one-way nature. The program can ensure that the message was not tampered with by calculating a message digest from the original message and matching it to the message digest sent from Alice.