How many public certificates do I *really* need?

– How many public certificates do I need, in order to configure external access to my OCS 2007 pool?
– 3. One for the HTTP Reverse Proxy, one for the OCS Access Edge and one for the OCS Web Conferencing Edge.
– Even if I'm using a consolidated Edge topology?
– Yes!
– But can't I just use a super-mega-jumbo SAN certificate with all the required alternative names?
– No!
– Why not?
– Because!

Well, to tell you the truth it's not "because", it is the official support policy written in the OCS 2007 Supportability Guide, the OCS 2007 Planning Guide and the OCS 2007 Edge Server Deployment Guide.

Here's a summary of the external certificate requirements:

  • For each unique IP address on the external interface that you use for the Access Edge Server and Web Conferencing Edge Server, you will need a separate certificate. We recommend that you use a separate external IP addresses for each server role, even if all servers are collocated. An external certificate is not required on the A/V Edge Server.
  • Office Communications Server 2007 will support certificates with a length of up to 1024 bits.
  • Office Communications Server 2007 server certificates must be configured with an enhanced key usage (EKU) extension for server authentication.
  • For a list of public certificate authorities who have partnered with Microsoft to ensure that their certificates comply with specific requirements for Office Communications Server, see
  • For both internal and remote clients, the CA certificate chain for the Office Communications Server 2007 deployment must be downloaded and installed to the certificate store of the client computer in the Trusted Root Certification Authorities folder.
  • All server certificates must support server authorization (Server EKU
  • All server certificates must contain a CRL Distribution Point (CDP).
  • Auto-enrollment is supported for internal Office Communications Server servers, including an array of Standard Edition Servers configured as Director.
  • Auto-enrollment is not supported for Office Communications Server edge servers.
  • For the A/V Edge Server, an additional certificate is required for audio/video authentication. The private key of the A/V authentication certificate is used to generate authentication credentials. As a security precaution, you should not use the same certificate for A/V authentication that you use for the internal interface of the A/V Edge Server. We recommend that you issue this certificate from an internal CA, but you can also use a certificate from a public CA

– But why can't I just use my internal CA?
– Well, to tell you the truth, it's technically possible, as long as you remember these guidelines:

  • Public certificates are required if you enable Web conferencing and enable your users to invite anonymous participants (individuals from outside your organization that do not have Active Directory credentials).
  • Public certificates are required for public IM connectivity, and they are highly recommended for enhanced federation. The public certificate must be from a public CA that is on the default list of trusted root CAs installed on the server.
  • It is possible to use your Enterprise subordinate CA for direct federation, as well as for testing or trial purposes if all partners agree to trust the CA or cross-sign the certificate.

It is very unlikely that these requirements will change with the release of Office Communications Server 2007 R2.

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