reeware. 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 only passive listening. If k1 and k2 are equal, the system is symmetric. Otherwise, it is said to be asymmetric. To guarantee security, k2 must always be kept secret, whereas k1 can be made public, provided it is infeasible to derive k2 from k1. If indeed this is the case, then the system is called a public key system. Public key systems offer many interesting possibilities. For instance, anyone can send an encrypted credit card number to an online shop using the shop's public k1. Since only the shop possesses k2, no one but the shop can determine the number. If a symmetric system were used, the shop would have to exchange unique keys - privately and advance - with every potential customer. The security of public key systems is always based on the difficulty of solving certain mathematical problems, whereas symmetric schemes are more ad hoc in nature. The main drawback of public key systems is that their mathematical nature always makes them less efficient than symmetric systems; in particular, because the size of keys in public key systems is measured in kilobits - the keys of symmetrical systems are only 