Pretty Good Privacy (PGP)

How PGP works PGP uses both public-key cryptography and symmetric key cryptography, and includes a system which binds the public keys to user identities.public-key cryptography symmetric keyuser identities The first version of this system is generally known as a web of trust as opposed to developed later X.509 system with top-down approach based on certificate authority. Later versions of PGP have included something more akin to a public key infrastructure (PKI) that includes certificate authority.web of trustX.509certificate authoritypublic key infrastructurecertificate authority

PGP – Contd. PGP uses asymmetric key encryption algorithms. In these, the recipient must have previously generated a linked key pair, a public key, and a private key. The sender uses the recipient's public key to encrypt a shared key (aka a secret key or conventional key) for a symmetric cipher algorithm. That key is used, finally, to encrypt the plaintext of a message. Many PGP users' public keys are available to all from the many PGP key servers around the world which act as mirror sites for each other.asymmetric key encryptionpublic keyprivate keysecret key symmetric cipherencryptplaintextkey servers

PGP – Cont. The recipient of a PGP-protected message decrypts it using the session key for a symmetric algorithm. That session key was, of course, included in the message in encrypted form and was itself decrypted using the recipient's private key. Use of two ciphers in this way is sensible because of the very considerable difference in operating speed between asymmetric key and symmetric key ciphers (the differences are often times). decryptssession key

PGP – Cont. A similar strategy is (by default) used to detect whether a message has been altered since it was completed, or (also by default) whether it was actually sent by the person/entity claimed to be the sender.

PGP – Cont. To do both at once, the sender uses PGP to 'sign' the message with either the RSA or DSA signature algorithms.RSADSA 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 key.message digestdigital signature The message recipient computes a message digest over the recovered plaintext, and then uses the sender's public key and the signed message digest value with the signature algorithm. If the signature matches the received plaintext's message digest, it must be presumed (to a very high degree of confidence) that the message received has not been tampered with, either deliberately or accidentally, since it was properly signed.

PGP – Cont. It is critical that the public key one uses to send messages to some person or entity actually does 'belong' to the intended recipient. Users must also verify by some means that the public key in a certificate actually does belong to the person/entity claiming it. From its first release, PGP has used a web of trust. A given public key (or more specifically, information binding a person to a key) may be digitally signed by a third party to attest the association between the person and the key. There are several levels of confidence that can be expressed in this signature.web of trust

PGP – Cont. In the (more recent) OpenPGP specification, trust signatures can be used to support creation of certificate authorities. A trust signature indicates both that the key belongs to its claimed owner and that the owner of the key is trustworthy to sign other keys at one level below their own.certificate authorities –A level 0 signature is comparable to a web of trust signature, since only the validity of the key is certified. –A level 1 signature is similar to the trust one has in a certificate authority because a key signed to level 1 is able to issue an unlimited number of level 0 signatures. –A level 2 signature is highly analogous to the trust assumption users must rely on whenever they use the default certificate authority list in Internet Explorer; it allows the owner of the key to make other keys certificate authorities.Internet Explorer

PGP – Cont. PGP has also included a way to 'revoke' identity certificates which may have become invalid. More recent PGP versions have also supported certificate expiration dates. The problem of correctly identifying a public key as belonging to some other user is not unique to PGP. All public key and private key cryptosystems have the same problem.

Downloading PGP etc. PGP – Tutorial privacy and PGP – Tutorial