Share via

Understanding Digital Certificates and SSL

  • Article

Applies to: Exchange Server 2010

Secure Sockets Layer (SSL) is a method for securing communications between a client and a server. For a Microsoft Exchange Server 2010 Client Access server, SSL is used to help secure communications between the server and clients. Clients include mobile devices, computers inside an organization's network, and computers outside an organization's network. These include clients with and without virtual private network (VPN) connections.

By default, when you install Exchange 2010, client communications are encrypted using SSL when you use Microsoft Office Outlook Web App, Exchange ActiveSync, and Outlook Anywhere. By default, Post Office Protocol version 3 (POP3) and Internet Message Access Protocol Version 4 rev1 (IMAP4) aren't configured to communicate over SSL.

SSL requires you to use digital certificates. This topic summarizes the different types of digital certificates and information about how to configure the Client Access server to use these types of digital certificates.

Contents

Overview of Digital Certificates

Digital Certificates and Proxying

Digital Certificates Best Practices

Client Limitations

Overview of Digital Certificates

Digital certificates are electronic files that work like an online password to verify the identity of a user or a computer. They're used to create the SSL encrypted channel that's used for client communications. A certificate is a digital statement that's issued by a certification authority (CA) that vouches for the identity of the certificate holder and enables the parties to communicate in a secure manner using encryption.

Digital certificates do the following:

  • They authenticate that their holders—people, Web sites, and even network resources such as routers—are truly who or what they claim to be.
  • They protect data that's exchanged online from theft or tampering.

Digital certificates can be issued by a trusted third-party CA or a Microsoft Windows public key infrastructure (PKI) using Certificate Services, or they can be self-signed. Each type of certificate has advantages and disadvantages. Each type of digital certificate is tamper-proof and can't be forged.

Certificates can be issued for several uses. These uses include Web user authentication, Web server authentication, Secure/Multipurpose Internet Mail Extensions (S/MIME), Internet Protocol security (IPsec), Transport Layer Security (TLS), and code signing.

A certificate contains a public key and attaches that public key to the identity of a person, computer, or service that holds the corresponding private key. The public and private keys are used by the client and the server to encrypt the data before it's transmitted. For Windows-based users, computers, and services, trust in a CA is established when there's a copy of the root certificate in the trusted root certificate store and the certificate contains a valid certification path. For the certificate to be valid, the certificate must not have been revoked and the validity period must not have expired.

Types of Certificates

There are three primary types of digital certificates: self-signed certificates, Windows PKI-generated certificates, and third-party certificates.

Self-Signed Certificates

When you install Exchange 2010, a self-signed certificate is automatically configured. A self-signed certificate is signed by the application that created it. The subject and the name of the certificate match. The issuer and the subject are defined on the certificate. A self-signed certificate will allow some client protocols to use SSL for their communications. Exchange ActiveSync and Outlook Web App can establish an SSL connection by using a self-signed certificate. Outlook Anywhere won't work with a self-signed certificate. Self-signed certificates must be manually copied to the trusted root certificate store on the client computer or mobile device. When a client connects to a server over SSL and the server presents a self-signed certificate, the client will be prompted to verify that the certificate was issued by a trusted authority. The client must explicitly trust the issuing authority. If the client confirms the trust, then, SSL communications can continue.

Frequently, small organizations decide not to use a third-party certificate or not to install their own PKI to issue their own certificates. They might make this decision because those solutions are too expensive, because their administrators lack the experience and knowledge to create their own certificate hierarchy, or for both reasons. The cost is minimal and the setup is simple when you use self-signed certificates. However, it's much more difficult to establish an infrastructure for certificate life-cycle management, renewal, trust management, and revocation when you use self-signed certificates.

Windows Public Key Infrastructure Certificates

The second type of certificate is a Windows PKI-generated certificate. A PKI is a system of digital certificates, certification authorities, and registration authorities (RAs) that verify and authenticate the validity of each party that's involved in an electronic transaction by using public key cryptography. When you implement a PKI in an organization that uses Active Directory, you provide an infrastructure for certificate life-cycle management, renewal, trust management, and revocation. However, there is some additional cost involved in deploying servers and infrastructure to create and manage Windows PKI-generated certificates.

Certificate Services are required to deploy a Windows PKI and can be installed through Add Or Remove Programs in Control Panel. You can install Certificate Services on any server in the domain.

If you obtain certificates from a domain-joined Windows CA, you can use the CA to request or sign certificates to issue to your own servers or computers on your network. This enables you to use a PKI that resembles a third-party certificate vendor, but is less expensive. These PKI certificates can't be deployed publicly, as other types of certificates can be. However, when a PKI CA signs the requestor's certificate by using the private key, the requestor is verified. The public key of this CA is part of the certificate. A server that has this certificate in the trusted root certificate store can use that public key to decrypt the requestor's certificate and authenticate the requestor.

The steps for deploying a PKI-generated certificate resemble those required for deploying a self-signed certificate. You must still install a copy of the trusted root certificate from the PKI to the trusted root certificate store of the computers or mobile devices that you want to be able to establish an SSL connection to Microsoft Exchange.

A Windows PKI enables organizations to publish their own certificates. Clients can request and receive certificates from a Windows PKI on the internal network. The Windows PKI can renew or revoke certificates.

Trusted Third-Party Certificates

Third-party or commercial certificates are certificates that are generated by a third-party or commercial CA and then purchased for you to use on your network servers. One problem with self-signed and PKI-based certificates is that, because the certificate is not automatically trusted by the client computer or mobile device, you must make sure that you import the certificate into the trusted root certificate store on client computers and devices. Third-party or commercial certificates do not have this problem. Most commercial CA certificates are already trusted because the certificate already resides in the trusted root certificate store. Because the issuer is trusted, the certificate is also trusted. Using third-party certificates greatly simplifies deployment.

For larger organizations or organizations that must publicly deploy certificates, the best solution is to use a third-party or commercial certificate, even though there is a cost associated with the certificate. Commercial certificates may not be the best solution for small and medium-size organizations, and you might decide to use one of the other certificate options that are available.

Choosing a Certificate Type

When you choose the type of certificate to install, there are several things to consider. A certificate must be signed to be valid. It can be self-signed or signed by a CA. A self-signed certificate has limitations. For example, not all mobile devices let a user install a digital certificate in the trusted root certificate store. The ability to install certificates on a mobile device depends on the mobile device manufacturer and the mobile service provider. Some manufacturers and mobile service providers disable access to the trusted root certificate store. In this case, neither a self-signed certificate nor a certificate from a Windows PKI CA can be installed on the mobile device.

Most mobile devices have several trusted third-party commercial certificates preinstalled. For the optimal user experience, implement certificate-based authentication for Exchange ActiveSync by using devices that are running Windows Mobile 6.0 or a later version and use a digital certificate from a trusted third-party CA.

Default Exchange Certificates

By default, Exchange installs a default self-signed certificate so that all network communication is encrypted. Encrypting all network communication requires that every Exchange server have an X.509 certificate that it can use. You should replace this self-signed certificate with one that is automatically trusted by your clients.

“Self-signed” means that a certificate was created and signed only by the Exchange server itself. Because it wasn't created and signed by a generally trusted CA, the default self-signed certificate won't be trusted by any software except other Exchange servers in the same organization. The default certificate is enabled for all Exchange services. It has a Subject Alternative Name (SAN) that corresponds to the server name of the Exchange server that it's installed on. It also has a list of SANs that include both the server name and the fully qualified domain name (FQDN) of the server.

Although other Exchange servers in your Exchange organization trust this certificate automatically, clients like Web browsers, Outlook clients, mobile phones, and other e-mail clients in addition to external e-mail servers won't automatically trust it. Therefore, consider replacing this certificate with a trusted third-party certificate on your Exchange servers that have the Client Access server role installed and also on any external-facing Hub Transport servers. If you have your own internal PKI, and all your clients trust that entity, you can also use certificates that you issue yourself.

Certificate Requirements by Service

Certificates are used for several things in Exchange. Most customers also use certificates on more than one Exchange server. In general, the fewer certificates you have, the easier certificate management becomes.

IIS

All the following Exchange services use the same certificate on a given Exchange server:

  • Outlook Web App
  • Exchange Control Panel
  • Exchange Web Services
  • Exchange ActiveSync
  • Outlook Anywhere
  • Autodiscover
  • Outlook Address Book distribution

Because only a single certificate can be associated with a Web site, and because all these services are offered under a single Web site by default, all the names that clients of these services use must be in the certificate (or fall under a wildcard name in the certificate).

POP/IMAP

Certificates that are used for POP or IMAP can be specified separately from the certificate that's used for IIS. However, to simplify administration, we recommend that you include the POP or IMAP service name in your IIS certificate and use a single certificate for all these services.

SMTP

A separate certificate can be used for each receive connector that you configure on your Hub Transport servers or Edge Transport servers. The certificate must include the name that SMTP clients (or other SMTP servers) use to reach that connector. To simplify certificate management, consider including all names for which you have to support TLS traffic in a single certificate.

Live Federation

The certificate that's used for federating with Windows Live for Exchange sharing scenarios can include any name. During the federation process, you identify the certificate that you want your Exchange server to use. This certificate must be issued by a third-party certification authority that's trusted by Windows Live. If you get your Exchange certificate for other services from a third-party certification authority that's trusted by Windows Live, you can use a single certificate for those services and also for federation with Windows Live.

Unified Messaging

When you connect Exchange Unified Messaging servers to Microsoft Office Communications Server 2007 servers or to third-party SIP gateways or Private Branch eXchange (PBX) telephony equipment, you must use a certificate that matches the FQDN of the Unified Messaging servers. This can be achieved by using a wildcard certificate that matches all your Unified Messaging servers, by including the FQDNs of your Unified Messaging servers in the certificate SAN list, or by having separate certificates for each Unified Messaging server with the FQDN in the certificate's SAN. The recommended approach is to include the Unified Messaging server FQDNs in the certificate SAN list.

Outlook Web App Instant Messaging with Office Communications Server 2007 R2

Outlook Web App in Exchange 2010 includes a programming interface that allows Instant Messaging providers to write add-ins to control presence and instant messaging functionality. An add-in exists for Communications Server 2007 R2. When you use this add-in, you must use a certificate to secure the connection between the Communications Server 2007 R2 server and the Exchange 2010 Client Access server. The certificate must be installed on the Exchange Client Access servers.

Legacy Exchange Servers

If you follow the best practices for transitioning from Microsoft Exchange Server 2003 or Exchange Server 2007 to Exchange 2010, you'll be introducing a new host name - legacy.contoso.com for use during the coexistence period when you have mailboxes both on legacy versions of Exchange and on Exchange 2010. This legacy host name should also be included in the certificate that you use. For more information about how to upgrade from Exchange Server 2003 and Exchange 2007 to Exchange 2010, see Upgrade from Exchange 2003 Client Access and Upgrade from Exchange 2007 Client Access.

Digital Certificates and Proxying

Proxying is the method by which one Client Access server sends client connections to another Client Access server. There are two scenarios where one Client Access server will proxy traffic to another Client Access server.

  1. Client Access servers in an Internet-facing Active Directory site will proxy traffic to Client Access servers in a non-Internet-facing site. This ensures that all processing of client requests is handled as close to the client's Mailbox server as possible.
  2. Client Access servers for Exchange 2010 will proxy connections from clients that have mailboxes on Exchange 2003 or Exchange 2007. These client connections will be proxied to Exchange 2007 Client Access servers. This happens because, for many Exchange services, the Client Access server can't process requests for Mailbox servers that are running an older version of Exchange.

When Client Access servers proxy requests, SSL is used for encryption but not for authentication. In most cases, a self-signed certificate can be used for Client Access server proxying. If your organization requires extraordinary security rules, there's a configuration key you can set to require trusted certificates for Client Access server proxying. You can configure the following key by setting it to false for this scenario:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeOWA\AllowInternalUntrustedCerts

Incorrectly editing the registry can cause serious problems that may require you to reinstall your operating system. Problems resulting from editing the registry incorrectly may not be able to be resolved. Before editing the registry, back up any valuable data.

For more information about proxying, see Understanding Proxying and Redirection.

Reverse Proxies and Certificates

Most Exchange deployments use reverse proxies to publish Exchange services on the Internet. Examples of popular Reverse Proxy products are Microsoft Internet Security and Acceleration (ISA) Server and Checkpoint. These reverse proxies can be configured to terminate SSL encryption, examine the traffic in the clear on the server, and then open a new SSL encryption channel from the reverse proxy server to the Exchange servers behind them. This is known as SSL bridging. Another way to configure the reverse proxy servers is to let the SSL connections pass straight through to the Exchange servers behind the reverse proxy servers. With either deployment model, the clients on the Internet connect to the reverse proxy server using a host name for Exchange access, such as mail.contoso.com. Then the reverse proxy server connects to Exchange using a different host name, such as the machine name of the Exchange Client Access server. You don't have to include the machine name of the Exchange Client Access server on your certificate because most common reverse proxy servers are able to match the original host name that's used by the client to the internal host name of the Exchange Client Access server.

Load Balancing and Certificates

If you have more than one Client Access server, consider configuring a Client Access server array. This array will allow all clients to connect to your Exchange Client Access servers through a single host name. You can add as many Client Access server computers as you want to one Client Access server array provided that all the Client Access servers are located within the same Active Directory site. For more information about load balancing and Client Access server arrays, see Understanding Proxying and Redirection and Managing Client Access Servers.

SSL and Split DNS

Split DNS is a technology that allows you to configure different IP addresses for the same host name, depending on where the originating DNS request came from. This is also known as split-horizon DNS, split-view DNS, or split-brain DNS. Split DNS can help you reduce the number of host names that you must manage for Exchange by allowing your clients to connect to Exchange through the same host name whether they're connecting from the Internet or from the Intranet. Split DNS allows requests that originate from the Intranet to receive a different IP address than requests that originate from the Internet.

Split DNS is usually unnecessary in a small Exchange deployment because users can access the same DNS endpoint whether they're coming from the Intranet or the Internet. However, with larger deployments, this configuration will result in too high of a load on your outgoing Internet proxy server and your reverse proxy server. For larger deployments, configure split DNS so that external users access mail.contoso.com and internal users access internal.contoso.com. Using split DNS for this configuration ensures that your users won't have to remember to use different host names depending on where they're located.

Remote PowerShell

Kerberos authentication and Kerberos encryption are used for Remote PowerShell access, both from the Exchange Management Console and the Exchange Management Shell. Therefore, you won't have to configure your SSL certificates for use with Remote PowerShell.

Digital Certificates Best Practices

Although the configuration of your organization's digital certificates will vary based on its specific needs, information about best practices has been included to help you choose the digital certificate configuration that's right for you.

Best Practice: Use a Trusted Third-Party Certificate

To prevent clients from receiving errors regarding untrusted certificates, the certificate that's used by your Exchange server must be issued by someone that the client trusts. Although most clients can be configured to trust any certificate or certificate issuer, it's simpler to use a trusted third-party certificate on your Exchange server. This is because most clients already trust their root certificates. There are several third-party certificate issuers that offer certificates configured specifically for Exchange. You can use the Exchange Management Console to generate certificate requests that work with most certificate issuers.

How to Select a Certification Authority

A certification authority is a company that issues and ensures the validity of certificates. Client software (for example, browsers such as Microsoft Internet Explorer, or operating systems such as Windows or Mac OS) have a built-in list of CAs they trust. This list can usually be configured to add and remove CAs, but that configuration is often cumbersome. Use the following criteria when you select a CA to buy your certificates from:

  • Ensure the CA is trusted by the client software (operating systems, browsers, and mobile phones) that will connect to your Exchange servers.
  • Choose a CA that says it supports “Unified Communications certificates” for use with Exchange server.
  • Make sure that the CA supports the kinds of certificates that you’ll use. Consider using Subject Alternative Name (SAN) certificates. Not all CAs support SAN certificates, and other CAs don't support as many host names as you might need.
  • Make sure that the license you buy for the certificates allows you to put the certificate on the number of servers that you intend to use. Some CAs only allow you to put a certificate on one server.
  • Compare certificate prices between CAs.

Best Practice: Use SAN Certificates

Depending on how you configure the service names in your Exchange deployment, your Exchange server may require a certificate that can represent multiple domain names. Although a wildcard certificate, such as one for *.contoso.com, can resolve this problem, many customers are uncomfortable with the security implications of maintaining a certificate that can be used for any sub-domain. A more secure alternative is to list each of the required domains as SANs in the certificate. By default, this approach is used when certificate requests are generated by Exchange.

Best Practice: Use the Exchange Certificate Wizard to Request Certificates

There are many services in Exchange that use certificates. A common error when requesting certificates is to make the request without including the correct set of service names. The certificate request wizard in the Exchange Management Console will help you include the correct list of names in the certificate request. The wizard lets you specify which services the certificate has to work with and, based on the services selected, includes the names that you must have in the certificate so that it can be used with those services. Run the certificate wizard when you've deployed your initial set of Exchange 2010 servers and determined which host names to use for the different services for your deployment. Ideally you'll only have to run the certificate wizard one time for each Active Directory site where you deploy Exchange.

Instead of worrying about forgetting a host name in the SAN list of the certificate that you purchase, you can use a certification authority that offers, at no charge, a grace period during which you can return a certificate and request the same new certificate with a few additional host names.

Best Practice: Use As Few Host Names as Possible

In addition to using as few certificates as possible, you should also use as few host names as possible. This practice can save money. Many certificate providers charge a fee based on the number of host names you add to your certificate.

The most important step you can take to reduce the number of host names that you must have and, therefore, the complexity of your certificate management, is not to include individual server host names in your certificate's subject alternative names.

The host names you must include in your Exchange certificates are the host names used by client applications to connect to Exchange. The following is a list of typical host names that would be required for a company named Contoso:

  • Mail.contoso.com   This host name covers most connections to Exchange, including Microsoft Office Outlook, Outlook Web App, Outlook Anywhere, the Offline Address Book, Exchange Web Services, POP3, IMAP4, SMTP, Exchange Control Panel, and ActiveSync.
  • Autodiscover.contoso.com   This host name is used by clients that support Autodiscover, including Microsoft Office Outlook 2007 and later versions, Exchange ActiveSync, and Exchange Web Services clients.
  • Legacy.contoso.com   This host name is required in a coexistence scenario with Exchange Server 2003 or Exchange 2007. If you'll have clients with mailboxes on both a legacy version of Exchange and Exchange 2010, configuring a legacy host name prevents your users from having to learn a second URL during the upgrade process. For more information about upgrade and coexistence, see Upgrade from Exchange 2003 Client Access and Upgrade from Exchange 2007 Client Access.

Client Limitations

Several Exchange clients limit the certificates they support. These clients and their limitations are summarized as follows:

  • Outlook on Windows XP or earlier operating systems   The Windows RPC over HTTP component used for Outlook Anywhere requires that the SAN or common name of the certificate must match the Certificate Principal Name configured for Outlook Anywhere. Outlook 2007 and later versions use Autodiscover to obtain this Certificate Principal Name. To configure this value on your Exchange 2010 Client Access server, use the Set-OutlookProvider command with the -CertPrincipalName parameter. Set this parameter to the external host name that Outlook clients use to connect to Outlook Anywhere.
  • Versions of Outlook earlier than Outlook 2010 don't support SAN certificates for POP3 and IMAP4 access   A hotfix is available for SAN support in Outlook 2007 Service Pack 2. That hotfix can be found here.
  • Mobile devices   Some mobile devices, including those running Windows Mobile 5.0 and some Palm devices, don't support wildcard certificates.