Specifications

While we are thinking about the security of our script, it is important to consider all flows of
information within our system. GPG will encrypt our email and allow our recipient to decrypt
it, but how does the information originally come from the sender? If we are providing a Web
interface to send GPG encrypted mail, the flow of information will look something like
Figure 15.5.
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3
Recipient’s
Mail
Server
2
Sender’s
Browser
Recipient’s
Mail
Client
Web
Server
1
FIGURE 15.5
In our encrypted email application, the message is sent via the Internet three times.
In this figure, each arrow represents our message being sent from one machine to another.
Each time the message is sent, it travels through the Internet and might pass through a number
of intermediary networks and machines.
The script we are looking at here exists on the machine labeled Web Server in the diagram. At
the Web server, the message will be encrypted using the recipient’s public key. It will then be
sent via SMTP to the recipient’s mail server. The recipient will connect to his mail server,
probably using POP or IMAP, and download the message using a mail reader. Here he will
decrypt the message using his private key.
The data transfers in Figure 15.5 are labeled 1, 2, and 3. For stages 2 and 3, the information
being transmitted is a GPG encrypted message and is of little value to anybody who does not
have the private key. For transfer 1, the message being transmitted is the text that the sender
entered in the form.
If our information is important enough that we need to encrypt it for the second and third leg
of its journey, it is a bit silly to send it unencrypted for the first leg. Therefore, this script
belongs on a server that uses SSL.
If we connect to our script using a port other than 443, it will provide a warning. This is the
default port for SSL. If your server uses a non-default port for SSL, you might need to modify
this code.
Rather than providing an error message, we could deal with this situation in other ways. We
could redirect the user to the same URL via an SSL connection. We could also choose to
ignore it because it is not usually important if the form was delivered using a secure connec-
tion. What is usually important is the details that the user has typed into the form are sent to us
securely. We could simply have given a complete URL as the action of our form.
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