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Web Security, Privacy & Commerce, 2nd Edition by Gene Spafford, Simson Garfinkel

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What Cryptography Can’t Do

Cryptography is an incredibly powerful technology for protecting information, but it is only one of many technologies that play a role in web security and commerce. Unfortunately, cryptography plays such an important role that many people assume that any computer system is automatically secure, and that any system that does not use encryption can’t be made secure. As a matter of fact, the phrase secure web server is often used interchangeably with the phrase cryptographically enabled web server.

Encryption isn’t all-powerful. You can use the best cryptography that’s theoretically possible, but if other mistakes are made in either systems design or data handling, confidential information may still be revealed. For example, a document might be encrypted so that it could only be decoded by one person, but if that person prints out a document and then throws it out without first shredding the paper, the secrets that the document contains could still end up on the front page of the local newspaper.

Likewise, cryptography isn’t an appropriate solution for many problems, including the following:

Cryptography can’t protect your unencrypted documents

Even if you set up your web server so that it only sends files to people using 1024-bit SSL, remember that the unencrypted originals still reside on your web server. Unless you separately encrypt them, those files are vulnerable. Somebody breaking into the computer on which your server is located will have access to the data.

Cryptography can’t protect against stolen encryption keys

The whole point of using encryption is to make it possible for people who have your encryption keys to decrypt your files or messages. Thus, any attacker who can steal or purchase your keys can decrypt your files and messages. That’s important to remember when using SSL, because SSL keeps copies of the server’s secret key on the computer’s hard disk. (Normally it’s encrypted, but it doesn’t have to be.)

Cryptography can’t protect against most denial-of-service attacks

Cryptographic protocols such as SSL are great for protecting information from eavesdropping. Unfortunately, attackers can have goals other than eavesdropping. In banking and related fields, an attacker can cause great amounts of damage and lost funds by simply disrupting your communications or deleting your encrypted files. Although there has been some research on techniques by which cryptography can guard against some specific denial-of-service attacks (such as client puzzles and SYN hash cookies), the results of this research are not generally applicable.

Cryptography can’t protect you against records of message traffic or the fact that a message was sent

[62] Suppose you send an encrypted message to Blake Johnson, and Blake murders your lover’s spouse, and then Blake sends you an encrypted message back. A reasonable person might suspect that you have some involvement in the murder, even if that person can’t read the contents of your messages. Or suppose there is a record of your sending large, encrypted messages from work to your company’s competitor. If there is a mysterious deposit to your bank account two days after each transmission, an investigator is likely to draw some conclusions from this behavior.

Cryptography can’t protect against a booby-trapped encryption program

Someone can modify your encryption program to make it worse than worthless. For example, an attacker could modify your copy of Netscape Navigator so that it always uses the same encryption key. (This is one of the attacks that was developed at the University of California at Berkeley.)

  • Unless you write or inspect all of the programs that run on your computer, there is no way to completely eliminate these possibilities.[63] They exist whether you are using encryption or not. However, you can minimize the risks by getting your cryptographic programs through trusted channels and minimizing the opportunity for your program to be modified. You can also use digital signatures and techniques such as code signing to detect changes to your encryption programs.

Cryptography can’t protect you against a traitor or a mistak

Humans are the weakest link in your system. Your cryptography system can’t protect you if your correspondent is sending your messages to the newspapers after legitimately decrypting them. Your system also may not protect against one of your system administrators being tricked into revealing a password by a phone call purporting to be from the FBI.

Cryptography can’t protect you against a passerby

If you leave your computer unlocked when you go to the bathroom or to get a cup of coffee, somebody can use your computer and do things with your private keys.

Thus, while cryptography is an important element of web security, it is not the only part. Cryptography can’t guarantee the security of your computer if people can break into it through other means. But cryptography will shield your data, which should help to minimize the impact of a penetration if it does occur.



[62] In cryptanalysis, the study of such information is called traffic analysis.

[63] And unless you are a stellar programmer, writing the programs yourself may put you at even greater risk from bugs and design errors.

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