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As we discussed, e-mails sent in plain text over SMTP can be read by anyone with access to an Internet gateway or router across which the packets happen to pass. The best solution to this problem is to encrypt each e-mail with a public key whose private key is possessed only by the person to whom you are sending the e-mail; there are freely available systems such as PGP and GPG for doing exactly this. But regardless of whether the messages themselves are protected, individual SMTP conversations between particular pairs of machines can be encrypted and authenticated using a method known as SSL/TLS. In this section, you will learn about how SSL/TLS fits in with SMTP conversations. Keep in mind that TLS protects only the SMTP hops that choose to use it if you carefully use TLS to send an e-mail to a server, you have no control over whether that server uses TLS again if it has to forward your e-mail across another hop toward its destination. For more details on TLS, please see 6; the code presented in this chapter cannot protect you from delivering a message to a fraudulent server without the certificate-handling described there. The general procedure for using TLS in SMTP is as follows: 1. 2. 3. 4. 5. 6. Create the SMTP object, as usual. Send the EHLO command. If the remote server does not support EHLO, then it will not support TLS. Check s.has_extn() to see if starttls is present. If not, then the remote server does not support TLS and the message can only be sent normally, in the clear. Call starttls() to initiate the encrypted channel. Call ehlo() a second time; this time, it s encrypted. Finally, send your message.

The Popfly Block Creator can be opened by selecting Create Stuff Block from the main menu. The Block Creator will open an unsaved block (see Figure 10-1). The Block Creator has two editing views: Block Description (which defines the operations and properties of the block) and Block Code (which executes the functionality of the block). Toggle between by clicking the tabs. When a block is first created, you see XML and JavaScript comments prompting you to place your description and code into the respective editors. To the left of the editors is the familiar Blocks window. If you recall, this window contains the categorized list of blocks that are available to use in Popfly. You can easily find blocks by typing keywords into the search box at the top of the window. In 7, we examined the code of the RSS block. To find that block here, type RSS into the search box. You should see the results in the Blocks window (see Figure 10-2).

3. readyReadStandardOutput() (several times) 4. stateChanged( NotRunning ) 5. finished( 0, NormalExit )

The first question you have to ask yourself when working with TLS is whether you should return an error if TLS is not available. Depending on your application, you might want to raise an error for any of the following: There is no support for TLS on the remote side. The remote side fails to establish a TLS session properly. The remote server presents a certificate that cannot be validated.

Figure 12-4. The uic process running and completing succesfully. The top image shows the top of the output text; the bottom image shows the end of the same text.

Let us step through each of these scenarios and see when they may deserve an error message. First, it is sometimes appropriate to treat a lack of support for TLS altogether as an error. This could be the case if you are writing an application that speaks to only a limited set of mail servers perhaps mail servers run by your company that you know should support TLS, or mail servers run by a bank that you know supports TLS. But since only a minority of mail servers on the Internet today support TLS, a mail program should not, in general, treat its absence as an error. Many TLS-aware SMTP clients will use TLS if available, but will fall back on standard, unsecured transmission otherwise. This is known as opportunistic encryption and is less secure than forcing all communications to be encrypted, but protects messages when the capability is present. Second, sometimes a remote server claims to be TLS-aware but then fails to properly establish a TLS connection. This is often due to a misconfiguration on the server s end. To be as robust as possible, you may wish to retry your transmission to such a server with a new connection that you do not even try to encrypt. Third, there is the situation where you cannot completely authenticate the remote server. Again, for a complete discussion of peer validation, see 6. If your security policy dictates that you must

Note You add the application s output to QTextEdit using append calls, which leads to each new chunk

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