What is the difference between SAN and SNI SSL certificates?

SAN (Subject Alternative Name) is part of the X509 certificate spec, where the certificate has a field with a list of alternative names that are also valid for the subject (in addition to the single Common Name / CN). This field and wildcard names are essentially the two ways of using one certificate for multiple names.

SNI (Server Name Indication) is a TLS protocol extension that is sort of a TLS protocol equivalent of the HTTP Host-header.
When a client sends this, it allows the server to pick the proper certificate to present to the client without having the limitation of using separate IP addresses on the server side (much like how the HTTP Host header is heavily used for plain HTTP).

Do note that SNI is not something that is reflected in the certificate and it actually achieves kind of the opposite of what the question asks for; it simplifies having many certificates, not using one certificate for many things.

On the other hand, it depends heavily on the situation which path is actually preferable. As an example, what the question asks for is almost assuredly not what you actually want if you need certificates for different entities.

Leave a Comment