Understanding Certificate Requirements

Applies to: Exchange Server 2010 SP2

Digital certificates are an important part of securing the communication between the on-premises Exchange organization and the cloud-based service, other on-premises Exchange servers, and your clients. Certificates enable one entity to trust the identity of another. This helps to ensure that a client or server is communicating to the right source.

In a hybrid deployment, several services make use of certificates:

• Active Directory Federation Services (AD FS)   A certificate issued by a trusted third-party certificate authority (CA) is used to establish a trust between Web clients and federation server proxies, to sign security tokens, and to decrypt security tokens.

  Learn more at: Certificates

• Exchange federation   A self-signed certificate is used to create a secure connection between the on-premises Exchange 2010 hybrid server and the Microsoft Federation Gateway.

  Learn more at: Understanding Federated Delegation

• Exchange services   Self-signed certificates or certificates issued by a trusted third-party CA are used to help secure Secure Sockets Layer (SSL) communication between Exchange servers and clients. Services that use certificates include Outlook Web App, Exchange ActiveSync, Outlook Anywhere, and message transport.

• Existing Exchange servers   Your existing Exchange servers may make use of certificates to help secure Outlook Web App communication, message transport, and so on. Depending on how you use certificates on your Exchange servers, you might use self-signed certificates or certificates issued by a trusted third-party CA.

  Learn more at: Understanding Digital Certificates and SSL

Certificate Requirements for a Hybrid Deployment

When configure a hybrid deployment, you must configure certificates. You must purchase certificates from a trusted third-party CA. Multiple services, such as AD FS, Exchange 2010 federation, Exchange 2010 services, and Exchange, each require certificates. Depending on your organization, you may decide to do one of the following:

• Use a third-party certificate that's used by all services across multiple servers

• Use a third-party certificate for each server that provides services

Whether you choose to use the same certificate for all services, or dedicate a certificate for each service, depends on your organization and the service you're implementing. Here are some things to consider about each option:

• Third-party certificate across multiple servers   Third-party certificates that are used by services across multiple servers may be slightly cheaper to obtain, but they may complicate renewal and replacement. The complication occurs because, when a certificate needs replacement, you need to replace the certificate on every server where it's installed.

• Third-party certificate for each server   Using a dedicated certificate for each server that hosts services allows you to configure the certificate specifically for the services on that server. If you need to replace the certificate or renew it, you only need to replace it on the server where the services are installed. Other servers aren't impacted.

We recommend that you use a dedicated third-party certificate for the AD FS server, another certificate for the Exchange services on your hybrid server, and if needed, a certificate on your Exchange server. Federated delegation on the hybrid server uses a self-signed certificate by default. Unless you have specific requirements, there's no need to use a third-party certificate with federated delegation.

The services that are installed on a single server may require that you configure multiple fully qualified domain names (FQDNs) for the server. Purchase a certificate that allows for the required number of FQDNs. Certificates consistent of the subject, or principal, name, and one or more subject alternative names (SAN). The subject name is the FQDN that the certificate is issued to. SANs are additional FQDNs that can be added to a certificate in addition to the subject name. If you need a certificate to support five FQDNs, purchase a certificate that allows for five domains to be added to the certificate: one subject name and four SANs.

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