PGP is often used to secure email communication. Describe the steps that a pair of users using PGP
must take before they can exchange email messages with privacy and authnticity guarantees. What
scope is there to make the preliminary negotiations invisible to the users? (The PGP negotiation
is an instance of the hybrid scheme.)

What will be an ideal response?

PGP is based on a hybrid protocol like those described on pages 264 and 281. Its primary use is for secure
email communication. It provides digital signatures for the authentication of messages string encryption for
their secrecy and integrity. The signatures are made using the SHA-1 algorithm to make a digest of the message
and RSA or DSS for signing with the sender’s private key.
The message is (optionally) encrypted with 3DES or IDEA, using a one-time session key generated by
the sender, which is encrypted using RSA with the recipient’s public key and sent with the message.
PGP is required to generate public/private key pairs for each user and the one-time session keys used to
encrypt messages. Users’ public/private keys should be changed from time-to-time. (No keys should be used
indefinitely in a secure system because of the danger thst they may be compromised through inadvertent
disclosure or as a result of an attack.) To achieve the rotation of public/private key pairs, PGP must generate
and store multiple key pairs and give each pair a label or identifer.
Key management is based on a key ring held by each user and a collection of PGP key servers accessible
on the Internet that hold only the public keys of registered PGP users. The key ring is simply a small database
holding keys in data structures that are secure. They are secured using secret key encryption with a pass phrase
that the use must type in order to allow applications to access the keys in the keyring.
If PGP is thoroughly integrated into an email or other application the necessary actions to generate keys,
access the key ring and perform signing and encryption on email messages can all be triggered automatically.
The only user action required is the input of the pass phrase to decrypt the keyring entries. If users are equipped
with smart cards or other physical access keys, the pass phrase could be supplied from the card.