Secure Your Email, Part 1

Email is an indispensable part of your business. Most people think of email as a utility, akin to telephone service—so much so, that we have an expectation of privacy when we send email. Is that expectation justified? Are you doing everything you, as a Microsoft Exchange Server administrator, can do to ensure the privacy and security of your users' email? In Part 1 of this series, I discuss how Exchange Server 5.5's Advanced Security features work. In Part 2, I'll lead you through installing and configuring Advanced Security to work with a Windows 2000 certificate server and Microsoft Outlook 2002 clients.

Postcards and Pencils
When you send a postcard, anyone can read what you've written. Most of us write postcards in ink, but what if you used a pencil? Then, anyone could read or alter your message. Most email systems are like writing postcards in pencil—they don't do much to prevent eavesdropping or alteration. An email message that you send to an Internet recipient has the potential to go anywhere on the Internet before it reaches its target. The only way to protect such messages is to secure them before they leave the client workstation and keep them protected until the recipient opens them. Server-based encryption systems protect messages only as they travel between servers; an attacker can still access messages traveling between a server and a client. Advanced Security features target this vulnerability: The client protects the mail before it proceeds to the server, and only the intended recipients can read it. To understand Advanced Security features, you need to grasp the basics of public key cryptography.

Public Key Cryptography
Suppose Alice and Bob want to exchange email, and they don't want Ted to read it. The obvious solution is for Alice and Bob to encrypt (or scramble) their message. Alice and Bob might pick a secret phrase to use as an encryption key. This method—called a secret-key system because its security depends on keeping the key secret—might work if Alice and Bob already have a Ted-proof way to exchange messages. However, if they use unsecured email to exchange their secret key, Ted can read their email and gain access to all their encrypted traffic.

A better solution is to use public key cryptography, in which Alice and Bob each generate a keypair. A keypair contains a public key and a private key. The two keys are mathematically related, but deriving one from the other is impossible. Alice and Bob can use their keys to communicate as follows:

• Alice uses a copy of Bob's public key to encrypt a message to Bob. Only Bob's private key can decrypt the message. As long as Bob maintains control of the private key, his messages remain secure. This scenario is called public key encryption.
• Alice uses her private key to encrypt a message. Anyone who has a copy of her public key can decrypt the message and verify its authenticity (public keys can be widely distributed). This scenario is called a digital signature.
• Alice can combine public key encryption and her digital signature to encrypt and sign a message. For example, Alice can sign an encrypted message (so that anyone can verify the origin, but only Bob can read the message), or she can en-crypt a signed message (so that only Bob can verify the signature).

A digital certificate combines a user's or computer's public key with a set of attributes (e.g., an email address, ID number, expiration date). Because the certificate contains a public key, applications can use the certificate to perform public key operations. The certificate issuer uses its public key to digitally sign the certificate, so anyone who gets Alice's certificate can verify the issuer's signature, thereby checking the certificate—and by extension, the public key embedded in it—for validity.

Of course, this Alice-and-Bob scenario glosses over a few details. To successfully use public key systems, you don't necessarily need to know how they work—but I recommend that you take the time to learn. An excellent source for further information is Bruce Schneier's Applied Cryptography (John Wiley & Sons, 2000). Most important, you need to know how Exchange uses public key technology to secure your users' email messages, as well as how you administer and control it.

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