Securing Email

One of the most fundamental changes wrought by the global interconnectivity of networked computers has been the rise of what has come to be known as electronic mail (email) . Originally used to send messages between systems operators on the early Bitnet and other pre-Internet networks, email is a widespread method of communication between individuals and business partners . It is also used to facilitate financial transactions and electronic commerce. Email has been used successfully as evidence in several court trials, and it now forms the fundamental method of communication within many organizations.

The global nature of email distribution and the speed of delivery (often only seconds separate transmission and receipt, even between users on separate continents) make email a valuable tool. However, the speed and accessibility of this technology also carry several security considerations. Public transfer of sensitive information could potentially expose this information to undesired recipients. In addition, undesired and often unsolicited email can require a significant amount of time to review and discard. Email messages may also contain any number of hazardous programmatic file attachments directed at unsuspecting users. In the following sections, we cover S/MIME, PGP, and other email vulnerabilities such as Spam and email hoaxes .

We do not focus on potentially hazardous payloads here, beyond mentioning that many viruses, Trojan horses, worms, and other forms of viral programming agents transmit themselves using electronic mail as their carrier. A detailed discussion of viral programming agents is provided in Chapter 3, "Nonessential Services and Attacks."

Secure Multipurpose Internet Mail Extension (S/MIME)

The Multipurpose Internet Mail Extension (MIME) protocol extends the original Simple Mail Transfer Protocol (SMTP) to allow the inclusion of nontextual data within an email message. Embedding data within an email message allows for a simple method of transferring and receiving images, audio and video files, application programs, and many other types of non-ASCII text.

To provide a secure method of transmission, the Secure Multipurpose Internet Mail Extension (S/MIME) standard was developed. S/MIME uses the RSA asymmetric encryption scheme to encrypt email transmissions over public networks. Modern versions of Netscape and Internet Explorer include S/MIME support in their role as email clients .

Pretty Good Privacy (PGP)

An alternative to the use of S/MIME is the proposed PGP/MIME standard, derived from the Pretty Good Privacy application program developed by Phillip R. Zimmerman in 1991. This program encrypts and decrypts email messages using either the RSA or the Diffie-Hellman asymmetric encryption schemes. The PGP application must be purchased and is available for individual and corporate use.

One useful feature of the PGP program is that it can include a digital signature, which validates an email to its recipient. This calculated hash value can be used by the recipient to verify that the received email has not been tampered with.

Undesirable Email

One of the strengths of email is its capability to be rapidly transmitted to one or many recipients, who rapidly receive the directed message, generally without per-item charges (in contrast to surface mail, which requires a stamp for each item). This allows small organizations to rapidly reach a tremendously large potential base of consumerswhether with a possible item for sale, a request for donation, a notice of service, or any other manner of information. Unfortunately, this very strength of email can result in undesirable email such as spam and hoax messages, as described in the following sections.

Spam

Obviously, with the entire world only a single click of the Send button away, the volume of messages a user may receive can become too great to manage quickly and easily. Undesired or unsolicited email has gained the nickname spam (derived from an amalgamated meat product of the same name ). These electronic "junk mail" messages can rapidly overtax the capacity of email servers and consume a large amount of user time in order for the user to review, respond to, or discard each item.

Many solutions are available to attempt to halt the rising tide of spam messages flowing into users' inboxes, such as subscriptions to email blacklists , which register known spam senders so that email matching a sender's address can be discarded before any messages are received by an organization's clients. Most email clients allow you to configure automatic rules that can handle many types of spam, discarding items from particular senders or items that contain a particular set of words or phrases.

Obviously, the subjective nature of any type of email filtering can be problematic to implement, especially when it is critical that messages are received from clients or vendors who might inadvertently put the wrong words or phrases within the body of an important message.

Hoaxes

Another form of problematic email includes messages that contain incorrect or misleading information. These hoax messages may warn of pending legislation that does not exist or instruct users to delete certain files to ensure their security against a new virus while actually only rendering the system more susceptible to later viral agents.

Hoaxes might offer users great sums of money if they will simply provide all their personal and financial information to the source, or they might even tell of an expensive cookie recipe that the sender will be glad to make available for only a fraction of the price. These and many more hoax items circulate in a growing thread of tales and ideas that encapsulates everything from urban myths to detailed instructions that can result in a loss of functionality, damage to the user's computer, or some other later security vulnerability.