PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

426 Chapter 18 Implementing Secure Transactions with PHP and MySQL
We used the function tempnam() to create a unique temporary filename.You can
specify both the directory and a filename prefix.You are going to create and delete these
files in around 1 second, so it’s not very important what you call them as long as they are
unique.We specified a prefix of ‘pgp’ but let PHP use the system temporary directory.
The statement
$command = "/usr/local/bin/gpg -a \\
--recipient 'Luke Welling ' \\
--encrypt -o $outfile $infile";
sets up the command and parameters that will be used to call GPG.You need to modify
this statement to suit your situation. As when you used it on the command line, you
need to tell GPG which key to use to encrypt the message.
The statement
system($command, $result);
executes the instructions stored in $command and stores the return value in $result.You
could ignore the return value, but it lets you have an if statement and tells the user that
something went wrong.
After you finish with the temporary files you use, you delete them using the
unlink() function.This means that your user’s unencrypted email is stored on the server
for a short time. If the server fails during execution, it is even possible that the file could
be left on the server.
While you are thinking about the security of your script, it is important to consider
all flows of information within your system. GPG encrypts your email and allows your
recipient to decrypt it, but how does the information originally come from the sender?
If you are providing a web interface to send GPG-encrypted mail, the flow of information
will look something like Figure 18.5.
Sender’s
Browser
1
Web
Server
2
Recipient’s
Mail
Server
3
Recipient’s
Mail
Client
Figure 18.5
In the encrypted email application, the message is sent via the
Internet three times.
In this figure, each arrow represents the 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.