Special problems of secure messaging
An important application of public-key cryptography is secure email. When you send an encrypted message to Sue, you can encrypt it with Sue's public key and expect that only Sue can decrypt it (with her private key) because she is the only one who has her secret key.
Further, by using your own private key, you can also digitally sign the message, either in addition to encrypting it, or in lieu of encryption (in cases where the content does not require privacy protection, but authentication and integrity are needed).
As previously described, the digital signature is created by encrypting a message digest with your private signing key. This digital signature is then sent along with your message. However, signing and encrypting messages is easier said than done. The program you use to send signed and/or encrypted messages must have access to all of your recipients' public keys, to your encryption key, and to your signing key.
As mentioned, secure email is one of the few applications that requires use of a public-key cryptography infrastructure (PKI). Managing keys for a PKI is difficult to do on the scale necessary for an institution such as the UW. Some of the challenges relating to managing a PKI, for digital signatures in particular, include:
- Interoperability, given that there are incompatible methods in use
- Policies for accepting a digital signature and determining which applications should be required to use them
- Procedures for accessing and distributing public keys
- Policies relating to certificate authorities and certificate expiration
Key management is even more difficult when you would like to use your private keys from more than one computer. Almost all applications using PKI assume that your private key exists on the local hard disk of a single computer. In the future, use of "smart cards" to store these keys may mitigate this problem, but there are other challenges to deploying a public-key infrastructure that are either unique to or exacerbated by secure messaging:
- Different key pairs should be used for signing and for encrypting
- The keys used to sign or to encrypt a message must be available for as long as anyone needs to verify or decrypt the message
- The email PKI management problem is also more complex when messages cross international boundaries, since the U.S. government does not permit export of certain forms of encryption technology (even when it is readily available outside the United States)
There are a number of incompatible methods of doing secure messaging. The two most common are S/MIME and PGP. S/MIME has the advantage that it is supported "out of the box" by Netscape's and Microsoft's email programs, though many have found it challenging to follow all the steps necessary to obtain the requisite keys and certificates.
S/MIME's leading competitor is PGP (Pretty Good Privacy). PGP is not directly supported by either Netscape or Microsoft, but add-on software is available to use it with those and many other mail programs.
More information about PGP and cryptography is available at www.pgpi.com/overview/ and, about S/MIME, at en.wikipedia.org/wiki/S/MIME
Although widespread deployment of a public-key infrastructure to support secure email is still a ways off at the UW, a few individuals here are successfully using secure email on a small-scale basis. This involves acquiring suitable software, because all correspondents must have a compatible encryption package, and then exchanging public keys or publishing them on public-key servers.
