SSL

Secure Socket Layer (SSL) was developed by Netscape to enable e-commerce. It has since been formalised as a standard as TLS; sometimes the terms are used interchangeably.

SSL uses Public Key Cryptography to do two things:

Provide privacy by encrypting traffic on the network
Enhance trust by digitally signing transactions

Encryption is simple, conceptually - without the secret key, you can't read the traffic on the network - for instance, on wireless (WiFi).

Trust is more complex; here's how it works:

The user trusts the browser vendor (Mozilla Corporation, Microsoft etc.). If not to provide secure software, at least to not be part of a global criminal conspiracy to undermine the SSL infrastructure.
The browser vendor trusts a number of security companies (Verisign, RSA, Thawte etc.) to act as certificate authorities, to safeguard their root certificates, and to issue certificates to organizations only after a certain amount of "due diligence". The vendor includes the public key for these CAs in their browsers.
The certificate authority issues certificates to server operators after assuring themselves that the operator has the rights to the domain name. Thus, the CAs trust the Domain Name System and Internet Domain Registries to provide correct data. They may also require some proof that the applicant is a bona fide employee of the registrant company with authority to request a server certificate.

Thus, if you visit e.g. https://trmail.triumf.ca and see a closed padlock and no security alerts, then you can be reasonably sure you are connected to triumf.ca. But you have to know that TRIUMF uses triumf.ca, and not triumf.org or triumf.net etc. (which belong to other people).

The certificate authorities only check that an applicant matches the domain name. They don't check whether a business is honest, if it holds trademarks, or if it is the one "everyone expects". For instance, www.gap.ca does not belong to www.gap.com (GAP clothing).

Some have argued that this makes SSL largely useless for doing what people expect - verifying the identity of major banks and retailers. However, a site that does NOT have a valid certificate is almost certainly suspect.

Some common Security Alerts:

click to see screenshots

Domain Mismatch - the certificate belongs to a different server. In this case, the user did not enter the full domain name
Unknown Certificate Authority (Internet Explorer)
Unknown Certificate Authority (Netscape 4)
Unknown Certificate Authority (Mozilla)
In many cases, these are "self-signed" certificates. The server administrator has signed the certificate themselves. Data are still encrypted - the transaction is private - and the site (if academic) is usually legitimate. Users may check "accept permanently" if they are sure.
Expired Certificate - the server administrator has not renewed the certificate
Corrupt Certificate, Serial Number error - these errors are not usually seen

Andrew Daviel
TRIUMF, June 2006