12-09-2009, 03:14 PM
Pretty Good Privacy (PGP) is a computer program that provides cryptographic privacy and authentication. PGP is often used for signing, encrypting and decrypting e-mails to increase the security of e-mail communications. It was originally created by Philip Zimmermann in 1991.
PGP and other similar products follow the OpenPGP standard (RFC 4880) for encrypting and decrypting data
a public key encryption program originally written by Phil Zimmermann in 1991. Over the past few years, PGP has got thousands of adherent supporters all over the globe and has become a de-facto standard for encryption of email on the Internet
How PGP encryption works
PGP encryption uses public-key cryptography and includes a system which binds the public keys to a user name and/or an e-mail address. The first version of this system was generally known as a web of trust to contrast with the X.509 system which uses a hierarchical approach based on certificate authority and which was added to PGP implementations later. Current versions of PGP encryption include both options through an automated key management server.
Digital signatures
PGP supports message authentication and integrity checking. The latter is used to detect whether a message has been altered since it was completed (the message integrity property), and the former to determine whether it was actually sent by the person/entity claimed to be the sender (a digital signature). In PGP, these are used by default in conjunction with encryption, but can be applied to plaintext as well. The sender uses PGP to create a digital signature for the message with either the RSA or DSA signature algorithms. To do so, PGP computes a hash (also called a message digest) from the plaintext, and then creates the digital signature from that hash using the sender's private keys.
Web of trust
Main article: Web of trust
Both when encrypting messages and when verifying signatures, it is critical that the public key one used to send messages to someone or some entity actually does 'belong' to the intended recipient. Simply downloading a public key from somewhere is not overwhelming assurance of that association; deliberate (or accidental) spoofing is possible. PGP has, from its first versions, always included provisions for distributing a user's public keys in an 'identity certificate' which is so constructed cryptographically that any tampering (or accidental garble) is readily detectable. But merely making a certificate which is impossible to modify without being detected effectively is also insufficient. It can prevent corruption only after the certificate has been created, not before. Users must also ensure by some means that the public key in a certificate actually does belong to the person/entity claiming it. From its first release, PGP products have included an internal certificate 'vetting scheme' to assist with this; a trust model which has been called a web of trust. A given public key (or more specifically, information binding a user name to a key) may be digitally signed by a third party user to attest to the association between someone (actually a user name) and the key. There are several levels of confidence which can be included in such signatures. Although many programs read and write this information, few (if any) include this level of certification when calculating whether to trust a key.
