n change the bit. In this example, the modifier can be quite sure that he can change the message by changing the bit. In most cases this is not that trivial, but it is still possible to garble the message SO that the receiver does not notice. Another problem with plain encryption is that you cannot encrypt headers of the packet for communication reasons, however, the modifier can still often successfully attack by modifying headers. The two main purposes of cryptography are: to maintain the confidentiality of messages; and to guarantee the integrity of messages. Confidentiality is provided by encryption, whereas integrity can be provided by authen- tication codes or digital signatures. A fairly common standard for cryptography is called pretty good privacy (PGP). You are beautiful You are beautiful Key 1 Key 2 Public channel 17%g9+a# Message Encryption Decryption Message Ciphertext 17%g9+a# Eavesdropper Figure 6.19 The idea of confidentiality Understanding Changing Telecommunications - Building a Successful Telecom Business PGP was initially created by one person. It has become widespread because it is a freeware. PGP provides a data integrity for messages and data files by using existing technologies for security: Electronic signatures Both symmetric and public key (asymmetric) encryption Compression Key management When a user encrypts plain text with PGP, PGP first compresses the plain text. Data compression saves transmission time and disk space and, more importantly, strengthens cryptographic security. Files that are too short to compress or which don't compress well aren't compressed. 6.10.2 ENCRYPTION The encryption of packets protects IP traffic. Before sending a message the sender uses a key (k1) to encrypt it. The cipher text is then sent over a public channel that is open to eavesdroppers. To read the message, the recipient uses a key (k2) to decrypt the cipher text, thereby retrieving the message. Although active adversaries can insert packets and modify com- munication, for this discussion we assume o