Long story short... You're mixing up your apples with your oranges. The
subject of a certificate has nothing to do with the
key usages of certificate.
Wildcard certificates are certificates where the domain-name has a wild-card (an asterisk) instead of a specific domain name. For example: A certificate that has "*.example.com" as it's
subject, would be valid for www.example.com, fish.example.com and even lvl1.lvl2.lvl3.lvl4.example.com. These are much cheaper than buying multiple domain-specific certificates when you have additional sub-domains that you use on the same top-level domain. Unfortunately, if your private key becomes compromised... a hacker could setup anything under your domain and it would appear as valid.
Now, for the
key-usage portion of the certificates. Certificates can have very specific usages defined. Different "Certificate Authorities" have some pre-defined certificate templates that assign a set of 'key usages'. For example, a "Secure Web certificate" typically defines that the certificate has the
Server Authentication usage and the
Client Authentication usage. This means that the web-server should be trusted to handle server/client authentication for the domain defined in the subject of the certificate. (could be a wild-card domain or a specific domain) Other secure services can also use that same certificate as long as it provides the
key usages that are required by that service. For example, if you were to setup an XMPP server, you can use that same certificate to securely authenticate your clients for that domain, as XMPP does not need additional
key usages to establish a connection.
There are many other potential uses for certificates, such as:
- Code Signing
- Secure Email
- Time Stamping
- OCSP Signing
- Encrypting File System
- IP security tunnel termination
- IP security user
- IP security IKE intermediate
Different certificate authorities have different requirements and costs associated with the various certificate usages.