Pretty Good Privacy
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Purpose:

Encryption is used to transmit messages in various formats for hundreds of years; it is not a new concept created just for the Internet. As technology has evolved, so have the methods of encryption—from manually coding text to using complex computer programs. Encryption uses a mathematical formula and an encryption key to scramble information so that an unauthorized person cannot understand the information.
The scrambled information is decoded—or converted back—into the original format using the same mathematical formula and a decryption key so an authorized person can understand it. While the information is encrypted, it cannot be viewed. With 128-bit Secure Sockets Layer Version 3.0 (SSLV3) encryption, the privacy of information passing between your Web browser and encryption enabled Web servers is ensured.
Encrypting the information allows it to be safely transmitted and authenticated. Data cannot be compromised when SSL is in use. Through SSL the identity of the server computer can be verified. Although it is also possible to identify the user as well, most servers do not use this method of identification.
When you send data using SSL encryption, the data is broken down into small, separate packages of information called blocks. SSL then encrypts each block. These encrypted blocks are sent over the Internet as individual network packets, and are individually addressed. Once all the packets have reached the safety of our secure Web server, they are reassembled and decrypted.
PGP combines some of the best features of both conventional and public key cryptography. When you encrypt plaintext with PGP, it will first compress the plaintext. Data compression saves modem transmission time and disk space and, more importantly, strengthens cryptographic security.

Compression reduces these patterns in the plaintext, thereby greatly enhancing resistance to cryptanalysis. (Files that are too short to compress or which don't compress well aren't compressed.)

PGP then creates a session key, which is a one-time-only secret key. This key is a random number generated from the random movements of your mouse and the keystrokes you type. This session key works with a very secure, fast conventional encryption algorithm to encrypt the plaintext; the result is ciphertext.

Once the data is encrypted, the session key is then encrypted to the recipient's public key. This public key-encrypted session key is transmitted along with the ciphertext to the recipient.

The same way PGP encryptions works to protect , decryption is the same in the reverse. The recipient's copy of PGP uses his or her private key to recover the temporary session key. Then, PGP uses their private key to decrypt the conventionally-encrypted ciphertext.

The combination of the two encryption methods combines the convenience of public key encryption with the speed of conventional encryption. Conventional encryption is about 1, 000 times faster than public key encryption. Public key encryption in turn provides a solution to key distribution and data transmission issues. Used together, performance and key distribution are improved without any sacrifice in security.