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8.2. Digital Certificates

Real applications require a higher level of security than basic and digest authentication provide. They also need guaranteed confidentiality and integrity, as well as more reliable authentication. Digital certificate technology provides this.

The key concept is public key cryptography. In a public key cryptographic system, each participant has two keys that are used to encrypt or decrypt information. One is the public key, which is distributed freely. The other is a private key, which is kept secret. The keys are related, but one can not be derived from the other. To demonstrate, assume Jason wants to send a secret message to Will. He finds Will's public key and uses it to encrypt the message. When Will gets the message, he uses his private key to decrypt it. Anyone intercepting the message in transit is confronted with indecipherable gibberish.

Public key encryption schemes have been around for several years and are quite well developed. Most are based on the patented RSA algorithm developed by Ron Rivest, Adi Shamir, and Leonard Adelman. RSA uses very large prime numbers to generate a pair of asymmetric keys (i.e., each key can decode messages encoded with the other). Individual keys come in varying lengths, usually expressed in terms of the number of bits that make up the key. 1024- or 2048-bit keys are adequate for secure RSA communications.

Because keys are so large, it is not practical for a user to type one into her web brower for each request. Instead, keys are stored on disk in the form of digital certificates. Digital certificates can be generated by software like Phil Zimmerman's PGP package, or they can be issued by a third party. The certificate files themselves can be loaded by most security-aware applications, such as servers, browsers, and email software.

Public key cryptography solves the confidentiality problem because the communication is encrypted. It also solves the integrity problem: Will knows that the message he received was not tampered with since it decodes properly. So far, though, it does not provide any authentication. Will has no idea whether Jason actually sent the message. This is where digital signatures come into play. Because public and private keys are asymmetric, Jason can first use his private key to encode a message and then use Will's public key to encode it again. When Will gets the message, he decodes it first with his private key, and then with Jason's public key. Because only Jason can encode messages with his private key--messages that can be decoded only with his public key--Will knows that the message was truly sent by Jason.

This is different from simpler symmetric key systems, where a single key is used for encoding and decoding. While asymmetric keys have the significant advantage of allowing secure communication without ever requiring a secure channel, they have the disadvantage of requiring much more computational muscle. As a compromise, many encryption systems use asymmetric public and private keys to identify each other and then confidentially exchange a separate symmetric key for encrypting the actual exchange. The symmetric key is usually based on DES (Data Encryption Standard).

U.S. government restrictions currently limit symmetric key size to 56 bits (about 72 quadrillion possible keys). Messages encrypted with a 56-bit key are difficult to decode, but by no means impossible--large networks have been used to decode such messages within a matter of days. With the United States, however, many systems use 128-bit DES keys (about 3.40282 x 10^38 possible keys). Because there is no know way to decode a DES-encrypted message short of brute-force trial and error, messages sent using large keys are very, very secure.

This leaves one final problem--how does one user know that another user is who she says she is? Jason and Will know each other, so Will trusts that the public key Jason gave him in person is the real one.[2] On the other hand, if Lisa wants to give Jason her public key, but Jason and Lisa have never met, there is no reason for Jason to believe that Lisa is not actually Mark. But, if we assume that Will knows Lisa, we can have Will use his private key to sign Lisa's public key. Then, when Jason gets the key, he can detect that Will, whom he trusts, is willing to vouch for Lisa's identity. These introductions are sometimes called a "web of trust."

[2]To be truthful, people almost never meet in dark alleys to exchange their full public keys. Instead, they exchange keys digitally (via email, perhaps) and in person simply compare a small fingerprint hash of the key.

In the real world, this third-party vouching is usually handled by a specially established certificate authority, such as VeriSign Corporation. Because VeriSign is a well-known organization with a well-known public key, keys verified and signed by VeriSign can be assumed to be trusted, at least to the extent that VeriSign received proper proof of the receiver's identity. VeriSign offers a number of classes of digital IDs, each with an increasing level of trust. You can get a Class 1 ID by simply filling out a form on the VeriSign web site and receiving an email. Higher classes are individually verified by VeriSign employees, using background checks and investigative services to verify identities.

When selecting a certificate authority, it is important to choose a firm with strong market presence. VeriSign certificates, for instance, are included in Netscape Navigator and Microsoft Internet Explorer, so virtually every user on the Internet will trust and accept them. The following firms provide certificate authority services:

For more abstract information about digital certificates, we recommend Understanding Digital Signatures by Gail L. Grant (Mc-Graw Hill), which provides an excellent introduction to the subject suitable for programmers and nonprogrammers alike. For more on cryptography as it is related to Java, we recommend Java Cryptography by Jonathan Knudsen (O'Reilly).

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