Client Certificates vs. Server Certificates – What’s the Difference?

Mention PKI or ‘Client Certificates’ to many people and it may well conjure up images of businesses busily protecting and completing their customers’ online transactions, yet such certificates are to be found throughout our daily lives, in any number of flavours; when we sign into a VPN; use a bank card at an ATM, or a card to gain access to a building; within Oyster public transport smart cards, used in central London. These digital certificates are even to be found in petrol pumps, the robots on car assembly lines and even in our passports.

In Continental Europe and many so-called ‘emerging countries’, the use of client certificates  is particularly widespread, with governments issuing ID cards that have multiple uses, such as to pay local taxes, electricity bills and for drivers’ licenses. And the reason to see why is simple – client certificates play a vital role in ensuring people are safe on line. As the name indicates, they are used to identify a client or a user, authenticating the client to the server and establishing precisely who they are.

Server or SSL Certificates perform a very similar role to Client Certificates, except the latter is used to identify the client/individual and the former authenticates the owner of the site. Server certificates typically are issued to hostnames, which could be a machine name (such as ‘XYZ-SERVER-01’) or domain name (such as ‘’). A web browser reaching the server, and validates that an SSL server certificate is authentic. That tells the user that their interaction with the web site has no eavesdroppers and that the web site is exactly who it claims to be. This security is critical for electronic commerce, which is why certificates are now in such widespread use.

How do they do that? In practice, a web site operator obtains a certificate by applying to a certificate provider with a certificate signing request. This is an electronic document that contains all the essential information: web site name, contact email address and company information. The certificate provider signs the request, producing a public certificate, which is served to any web browser that connects to the web site and, crucially, proves to the web browser that the provider issued a certificate to the person he believes to be the owner of the web site. Before issuing a certificate, however, the certificate provider will request the contact email address for the web site from a public domain name registrar and check that published address against the email address supplied in the certificate request, ensuring the circle of trust has been closed.

Moreover, you can configure a web site so that any user wishing to connect is required to provide a valid client certificate, and valid user name and password. This is usually referred to as ‘two-factor authentication’ – in this instance, ’something you know’ (password) and ’something you have’ (certificate).

For those engaged in transactions on the web, certificates mean an end to anonymity and instead provide assurance that this is someone you can trust; that they are who they say they are. In an online world where our safety is being challenged constantly, such reassurance is invaluable.

For more information and resources visit our website security white paper resource website.

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