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Chapter: Security in Computing : Privacy in Computing

E-Mail Security

E-mail is exposed as it travels through the web. Furthermore, the privacy of an e-mail message can be compromised on the sender's or receiver's side, without warning.

E-Mail Security


E-mail is exposed as it travels through the web. Furthermore, the privacy of an e-mail message can be compromised on the sender's or receiver's side, without warning.


Consider the differences between e-mail and regular letters. Regular mail is handled by a postal system that by law is forbidden to look inside letters. A letter is sealed inside an opaque envelope, making it almost impossible for an outsider to see the contents. The physical envelope is tamper-evident, meaning it shows if someone opens it. A sender can drop a letter in any mailbox, making the sending of a letter anonymous. For these reasons, we have a high expectation of privacy with regular mail. (At certain times in history, for example during a war or under an autocratic ruler, mail was inspected regularly. In those cases, citizens knew their mail was not private.)


In this section we look at the reality of privacy for e-mail.


Where Does E-Mail Go, and Who Can Access It?


We cover e-mail and privacy -enhanced e-mail in Chapter 7. In this section we look only at the mechanics of transmitting e-mail with attention to privacy impacts.


E-mail is conceptually a point-to-point communication. If Janet sends e-mail to Scott, Janet's computer establishes a virtual connection with Scott, the computers synchronize, and the message is transferred by SMTP (simple mail transfer protocol). However, Scott may not be online at the moment Janet wants to send her message, so the message to Scott is stored for him on a server (called a POP or post office protocol server). The next time Scott is online, he downloads that message from the server. In the point-to-point communication, Janet's message is private; in the server version, it is potentially exposed while sitting on the server.


Janet may be part of a large organization (such as a company or university), so she may not have a direct outbound connection herself; instead, her mail is routed through a server, too, where the message's privacy is in jeopardy. A further complication is aliases and forwarding agents that add more midpoints to this description. Also, Internet routing can make many hops out of a conceptual point-to-point model.


What started as a simple case can easily have at least five parties: (a) Janet and her computer, (b) Janet's organization's SMTP server, (c) Janet's organization's ISP, (d) Scott's POP server, and (e) Scott and his computer. For now, we are most interested in the three middle parties: (b), (c), and (d). Any of them can log the fact that it was sent or can even keep a copy of the message.


Interception of E-mail


E-mail is subject to the same interception risks as other web traffic: While in transit on the Internet, e-mail is open for any interceptor to read.


In Chapter 7 we described techniques for encrypting e-mail. In particular, S/MIME and PGP are two widely used e-mail protection programs. S/MIME and PGP are available for popular mail handlers such as Outlook, Outlook Express, Eudora, Apple Mail, Netscape Communicator, and others. These products protect e-mail from the client's workstation through mail agents, across the Internet, and to the recipient's workstation. That protection is considered end-to-end, meaning from the sender to the recipient. Encrypted e-mail protection is subject to the strength of the encryption and the security of the encryption protocol.


A virtual private network, also described in Chapter 7, can protect data on the connection between a client's workstation and some edge point, usually a router or firewall, at the organization to which the client belongs. For a corporate or government employee or a university student, communication is protected just up to the edge of the corporate, government, or university network. Thus, with a virtual private network, e-mail is protected only from the sender to the sender's office, not even up to the sender's mail agent, and certainly not to the recipient.


Some organizations routinely copy all e-mail sent from their computers. Purposes for these copies include using the e-mail as evidence in legal affairs and monitoring the e-mail for inappropriate content.


Monitoring E-Mail


Companies and government agencies can legitimately monitor their employees' e-mail use. Schools and libraries can monitor the computer use of patrons. Network administrators and ISPs can monitor traffic for normal business purposes, such as to measure traffic patterns or to detect spam. Organizations must advise users of this monitoring, but the notice can be a small notice in a personnel handbook or in the fine print of a service contract. Organizations can use the monitoring data for any legal purpose, for example, to investigate leaks, to manage resources, or to track user behavior.


Network users should have no expectation of privacy in their e-mail or general computer use.


Anonymous E-mail and Remailers


We have described anonymity in other settings; there are reasons for anonymous e-mail, as well.


As with telephone calls, employees sending tips or complaining to management may want to do so anonymously. For example, consumers may want to contact commercial establishmentsto register a complaint, inquire about products, or request informationwithout getting on a mailing list or becoming a target for spam. Or people beginning a personal relationship may want to pass along some information without giving away their identities. These are some of the reasons people want to be able to send anonymous e-mail.


Free e-mail addresses are readily available from Yahoo, Microsoft Hotmail, and many other places. People can treat these addresses as disposable: Obtain one, use it for a while, and discard it (by ceasing to use it).


Simple Remailers


Another solution is a remailer. A remailer is a trusted third party to whom you send an e-mail message and indicate to whom you want it sent. The remailer strips off the sender's name and address, assigns an anonymous pseudonym as the sender, and forwards the message to the designated recipient. The third party keeps a record of the correspondence between pseudonyms and real names and addresses. If the recipient replies, the remailer removes the recipient's name and address, applies a different anonymous pseudonym, and forwards the message to the original sender. Such a remailer knows both sender and receiver, so it provides pseudonymity, not anonymity.


Mixmaster Remailers


A more complicated design is needed to overcome the problem that the remailer knows who are the real sender and receiver. This approach is similar to the concept of onion routing described in Chapter 7. The basic tool is a set of cooperating hosts that agree to forward mail. Each host publishes its own public encryption key.


The sender creates a message and selects several of the cooperating hosts. The sender designates the ultimate recipient (call it node n) and places a destination note with the content. The sender then chooses one of the cooperating hosts (call it node n-1), encrypts the package with the public key of node (n-1) and places a destination note showing node (n) with the encrypted package. The sender chooses another node (n-2), encrypts, and adds a destination note for (n-1). The sender thus builds a multilayered package, with the message inside; each layer adds another layer of encryption and another destination.


Each remailer node knows only from where it received the package and to whom to send it next. Only the first remailer knows the true recipient, and only the last remailer knows the final recipient. Therefore, no remailer can compromise the relationship between sender and receiver.


Although this strategy is sound, the overhead involved indicates that this approach should be used only when anonymity is very important.


Spoofing and Spamming


E-mail has very little authenticity protection. Nothing in the SMTP protocol checks to verify that the listed sender (the From: address) is accurate or even legitimate. Spoofing the source address of an e-mail message is not difficult. This limitation facilitates the sending of spam because it is impossible to trace the real sender of a spam message. Sometimes the apparent sender will be someone who knows the recipient or someone on a common mailing list with the recipient. Spoofing such an apparent sender is intended to lend credibility to the spam message.


Phishing is a form of spam in which the sender attempts to convince the sender to reveal personal data, such as banking details. The sender enhances the credibility of a phishing message by spoofing a convincing source address, or using a deceptive domain name


These kinds of e- mail messages entice gullible users to reveal sensitive personal data. Because of limited regulation of the Internet, very little can be done to control these threats. User awareness is the best defense.




E-mail is exposed from sender to receiver, and there are numerous points for interception. Unless the e-mail is encrypted, there is little to prevent its access along the way.


For businesses, governments, schools, and other organizations, network administrators and managers may read any e-mail messages sent.

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