Protect Your E-Mail

By Bruce Schneier
Macworld
November 1995

Safeguard your messages today, and prepare for electronic commerce tomorrow

You may have just started using the Internet for your business, but scientists, academics, and computer programmers have been using it for years. It was designed specifically as a public network for sharing information. Because the availability of information was the priority, provisions for data security were not considered essential. But now that you're sending proprietary business information over the Internet that openness can become a drawback. You need to take steps to protect your communications.

After all, you may prefer transmitting business correspondence over the Internet to sending it through the U.S. Postal Service, and you may even find E-mail more convenient than trying to reach someone by telephone. But remember--it's not hard for someone to intercept your electronic mail. The news media are filled with stories of hackers tapping and tiptoeing into places they're not supposed to be. And when you send E-mail, it's almost like you're talking into a cordless telephone, except that every eavesdropper can record your conversation. Of course, as with phone lines, it's illegal to tap electronic communications, and law-enforcement agencies need a warrant to do so.

Before you start imagining your most confidential documents suddenly being scrutinized by your competitors, and you swear off ever using the Send command again, be aware that you can protect your communications. I'll show you some online-security methods using shareware, software, and some plain old common sense about how to incorporate them easily into your routine. You may also be wondering how the heck to transact business over the Internet if you can't even protect your E-mail--so we'll look at how major financial institutions are already addressing these concerns (see the sidebar "Money on the Line").

Secure Electronic Mail

You can buy lots of networking hard-ware and communications software that incorporate a variety of security features into file transfers, but they're mostly for corporate use. If you're a small-business owner interested in protecting your Internet mail, there's a simple, effective two-step process that you can use: encrypt your E-mail and sign it. If you ever sent encoded messages to friends as a kid, this is the same process, only digital. Encrypting means scrambling your messages so that only the authorized recipient can unscramble it. Think of it as putting your electronic mail into envelopes that only the recipient can open. Use encryption when you want to keep a secret.

Think of signing as the digital analog to a handwritten signature on the bottom of a letter. Signing proves to the recipient that the message came from you, and it was not forged or modified in transit. Send signed messages when you want to prove authenticity. For instance, if you receive a purchase order over the Internet, you will want a valid signature on it. Sending messages that are encrypted and signed provides both confidentiality and authentication.

Why go to the trouble of all this secret agent stuff? Because when electronic mail is broken down into digital bits, it becomes vulnerable to interception and eavesdropping. It's like a postcard that anyone can read along the way. Spoofing an E-mail message--sending a message purporting to come from someone else--is easy. Even Macworld columnist David Pogue found himself the victim of a spoofer. It is only a little harder to alter a legitimate message in transit. Savvy Net users don't send credit card numbers--or any other sensitive information--over the Internet, and they think twice before accepting a suspicious E-mail message as genuine.

There are several security programs for electronic mail available on the Internet, but only one of them comes in a Macintosh version. It's called PGP, which stands for "pretty good privacy." (Why only pretty good? Because the original author of the program, Phil Zimmermann, was a fan of Garrison Keillor's Prairie Home Companion radio show, which occasionally mentioned Lake Wobegon's Pretty Good Grocery.)

All these programs work similarly, but unfortunately they're not interoperable across platforms. If you are using the Macintosh version of PGP, you can't send a secure message to someone who is using other security programs like RSA Data Security's MailSafe or AT&T's SecretAgent. One Mac version of PGP-- ViaCrypt PGP (ViaCrypt, 602/944-0773, $124.98 per user)--is interoperable, though, with the company's versions of PGP for DOS, Windows, and Unix, as well as with MIT's PGP for DOS, Unix, and Macintosh.

The More the Merrier

PGP is like a telephone. Just as you can call only those people who have telephones, you can send PGP-encrypted mail only to someone who has PGP. Obviously, the more people there are who have the program, the more advantage you derive from it. You can get a free copy of PGP for personal use by downloading it from the Internet (see the screen shot "How to Get PGP"). If you're a business user, contact ViaCrypt.

To understand how PGP works, start with the concept of keys. The first thing you have to do is generate your own public key and private key, which are simply two long bit strings. The public key is just what its name implies: you can mail it to your friends, post it on Usenet newsgroups, and send it to a PGP public-key server on the Internet. Think of it as part of your identity information--others will need it to send you secret E-mail and to verify that messages you write are authentic. You want this key to be as widely available as possible; it's how other people can communicate securely with you.

The private key is yours alone. PGP stores it on your hard drive, encrypted with a pass phrase, as opposed to a single word, of your choosing. Think of your private key as the way to keeping your E-mail safe. You use it to decrypt encoded messages that others send to you and to digitally sign messages you send.

When you want to send an encrypted message to someone, you need that person's public key. You can either get it from that person or find it on the Internet. Just as you can save E-mail addresses, with PGP you can also store frequently used public keys. On the Macintosh, PGP works with the Clipboard. When you copy your message to the Clipboard, PGP encrypts it with the recipient's public key; you can then use your mail program to transmit the encrypted message. On the other end, the recipient decrypts the message using his or her private key.

When you want to make sure a recipient can confirm that it was indeed you who sent the message, you encrypt it with your private key. The recipient decrypts the message with your public key--which, as you recall, is publicly available--and reads the message. Since it could only have been encrypted with your private key, it must have come from you.

The actual encryption and signing operations are more complicated and involve a lot of mathematical calculation, but that's the general idea. (PGPhandles all of that math.) If you want to learn more about the encryption process, the PGP documentation goes into further detail. For additional reading, check out PGP: Pretty Good Privacy, by Simson Garfinkel (O'Reilly & Associates, 1995; $24.95), and Protect Your Privacy, by William Stallings (Prentice-Hall, 1995; $19.95).

Other Options

You can use any Macintosh application that can encrypt files to create secure files to send via electronic mail--for example, Kent Marsh's FolderBolt (713/522-5625, $129); usrEZ Software's ultraSecure (714/756-5140, $239); or the "encrypt" feature in Symantec's Norton Utilities for Macintosh (503/334-6054, $149.95). The only difference is that both you and the recipient have to share a secret key; this is different from PGP's concept of public and private keys. You encrypt the file using the shared key and attach the file to your E-mail message. At the other end, the recipient uses the shared key to decrypt your file.

If you don't want to worry about whether the recipient has a copy of your encryption program, consider Kent Marsh's alternative FolderBolt Pro ($159). Though it was designed primarily as a security application for a single Macintosh, you can use FolderBolt Pro to send secure electronic files (or to secure floppy disks). Like any security program, it allows you to encrypt a file before sending it, and the recipient has to decrypt it before reading it. But in this case, the recipient doesn't need a copy of FolderBolt Pro. You can create self-decrypting files (similar to the self-expanding files of Aladdin Systems' StuffIt products). The recipient simply double-clicks on the encrypted file, and it decrypts itself--assuming that the recipient knows the secret key, of course. FolderBolt Pro comes with several different encryption algorithms--some strong but slow and others weak but fast. Choose the one that best fits your needs.

The Last Word

Don't send another E-mail message before you at least start thinking about security. Encryption is kind of like insurance--it's something you must have, but hope you never need. If you make encryption a habit by incorporating it into your E-mail routine, you won't have to worry that one day your credit card number--or even your rantings about someone you dislike--will end up in an embarrassingly public forum.

Money on the Line

You may be wondering, "If even my E-mail isn't secure, how are companies going to conduct financial transactions over the Internet?" Nevertheless, several financial institutions are putting mechanisms in place to allow for digital payments, some of which are more secure--and in some cases more private--than simply sending your credit card number over E-mail.

Credit card leaders Visa and MasterCard, along with credit card processing leader First Data, are all working with Netscape Communications, which has created security software to go along with its World Wide Web browser. First Virtual Holdings, a company so virtual that it has no main office (its president works in Rancho Santa Fe, California), has its own credit card-based system, while England's National Westminster Bank and Midland Bank are collaborating on a multicurrency digital-cash card system called Mondex to handle international transactions.

At the same time, CyberCash, a Virginia-based firm headed by Verifone founder Bill Melton and Internet guru Dan Lynch, is bringing conventional credit card transactions to the Internet. The CyberCash electronic-payment system uses cryptography to prevent eavesdroppers from stealing and unscrupulous merchants from overcharging. By contrast, NetCheque, developed at the University of Southern California, brings the model of a paper check to the Internet. An account holder can send an electronic document that a recipient can deposit as a check. The document contains the name of the payer and the financial institution, the payer's account number, the payee's name, and the amount of the check. And like a paper check, it contains a signature--in this case a digital one. The check will be endorsed by the payee, again with a digital signature, and deposited electronically into a bank account.

None of these systems, however, can protect your privacy on the Internet. They can all compile statistics about customers who use these electronic forms of money: how much they spend, what kinds of things they buy, and when they buy them. Before electronic consumers can get true personal privacy, we need the electronic equivalent of cash. That's where DigiCash, a Netherlands-based firm founded by cryptography expert David Chaum, comes in. Its Internet payment system, called eCash, provides digital money without an audit trail, protecting the privacy of its users. The eCash system is currently being tested in a worldwide pilot program. Pretty soon, you'll see money flying all over the Internet.

Ten Tips for Internet Privacy

1. Choose a pass phrase (not just a password) that is hard to guess. Don't use English words. The best pass phrases contain nonalphanumeric characters and both upper- and lowercase letters (like CHAnel #5).

2. Change your pass phrase regularly. Don't use the same one on multiple accounts. Don't store your pass phrase in your Internet account. And don't share it or write it down. If you accidentally reveal your pass phrase, change it immediately.

3. If you use Telnet's remote-access capability to get to your Internet account from a public place (for example, at a conference), change your pass phrase as soon as is practical afterward.

4. Don't give out personal information to strangers on the Internet.

5. Don't save sensitive files in your Internet account.

6. Remember that electronic-mail messages are like postcards. Do not use unencrypted E-mail to send, receive, or store any messages you want to keep private.

7. Think of E-mail as a shared file system. Assume that messages sent, received, or stored are available for review by authorized personnel.

8. Don't walk away from your computer with your E-mail window open.

9. Review your electronic-messages carefully before you send them. E-mail may feel like voice conversation, but messages can be saved to keep records of your words. Don't say anything you may regret later.

10. Don't send your credit card number unencrypted over E-mail.

earlier essay: Electronic Speech - For Domestic Use Only
later essay: Why Cryptography is Harder Than It Looks
categories: Computer and Information Security
back to Essays and Op Eds

Photo of Bruce Schneier by Per Ervland.

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