Using Let's Encrypt Certificates for Exchange Server

Have you ever fantasized about using free SSL/TLS certificates for Exchange Server? If so, then this blog post is for you.

I’ve always hated the cost associated with SSL/TLS certificates. For what seemed like a pretty basic service some of the certificate authorities (CAs) were charging hundreds or thousands of dollars. You could always set up your own CA, but that didn’t work well with random clients on the Internet because they won’t trust certificates generated by your CA.

At the end of 2015, there was a game changing development. Let’s Encrypt started giving away SSL/TLS certificates for free. At the time, the certificates were only for a single name. So, without SAN support, not good for Exchange Server. However, now there is support for SAN/UCC certificates. And, in 2018 they are planning to support wildcard certificates.

What’s the Catch?

The certificates are free. There is no catch there. But, they do have a short lifetime of 90 days. The short lifetime is to ensure that compromised certificates are not available for an extended period of time. Because of the short lifetime, it is strongly recommended that you automate certificate renewal.

Note: This blog post only shows the manual steps for obtaining a certificate. I'll put up another one showing automation.

The process for generating and renewing a certificate is a bit complex. But, once the initial process is defined, it’s pretty easy to work with.

Unlike a typical CA, Let’s Encrypt does not provide a web site to manage your certificate requests. Instead you need client software that communicates with the Let’s Encrypt servers. Since I already work with Windows PowerShell on a regular basis, I like the ACMESharp module that provides PowerShell cmdlets for working with Let’s Encrypt.

Installing the ACMESharp Module

The ACMESharp module is available in the PowerShell Gallery. To download and install modules from the PowerShell Gallery, you use the Install-Module cmdlet that is part of the PowerShellGet module. The PowerShellGet modules is included as part of the Windows Management Framework 5 (part of Windows 10 and Windows Server 2016).

If you are not using Windows 10 or Windows Server 2016, you can download and install WMF 5 or a standalone MSI installer for PowerShellGet here:

• https://docs.microsoft.com/en-us/powershell/gallery/readme

After you have PowerShellGet, installed run the command:

Install-Module AcmeSharp

When you run this command, you might be prompted to install NuGet. If you are prompted, say yes to install it. NuGet is provides the functionality to obtain packages from the PowerShell Gallery. The PowerShellGet cmdlets use NuGet.

You might also be prompted that the repository PSGallery is untrusted. This is the PowerShell Gallery that you want to download files from. So, say yes to trust PSGallery.

Connecting to Let’s Encrypt

The first step after installing the ACMESharp module is creating a local data store for the ACMESharp client. The data store is used by the client for secure storage of requests and keys.
To create the local data store run the following command:

Initialize-ACMEVault

Then, to create an account with Let’s Encrypt, run the following command:

New-ACMERegistration -Contacts mailto:youremail@yourdomain.com -AcceptTos

Validating DNS Names

Let’s Encrypt requires you to verify ownership of each DNS name that you want to include in a certificate. Each DNS name is referred to as an identifier. For a SAN certificate, you will generate 2 or more identifiers then specify the identifiers when you create the certificate.

You can validate an identifier in three ways:

• Dns - You need to create a TXT record in DNS that is verified by Let’s Encrypt.
• HTTP - You need to place a file on your web server that is verified by Let’s Encrypt.
• TLS-SNI - You need to place an SSL/TLS certificate on your web server that is verified by Let’s Encrypt.

In the projects I work on, I typically do not have access to the main company web server/site, but do have access to create DNS records. So, I use DNS validation.

To create a new identifier:

New-ACMEIdentifier -dns server.domain.com -alias idAlias

You should include an alias each time you create an identifier. If you don’t create an alias, there is no easy way to refer to the identifier in later steps. If you forget to create an alias, just create a another new identifier with the same DNS name and include the alias.

In my example, the DNS name has four parts because I was testing by using a subdomain. In most cases, the DNS name will have only three parts.

Next you need to specify how the identifier will be verified. When you do this the cmdlet reports back the proof you need to provide. For HTTP verification, it identifies the file name. For DNS verification, it identifies the TXT record that needs to be created.

The command to start verifying the identifier is:

Complete-ACMEChallenge idAlias -ChallengeType dns-01 -Handler manual

Use the alias of the identifier to specify which identifier is being verified.

The manual handler identifies that you are specifying the challenge type. There are other automated handlers that automatically create the response for the HTTP-based challenges. The automated handlers are specific to different web servers.

The challenge type dns-01 identifies that you will create a TXT record in DNS. Note that dns-01 must be in lowercase. 

After you have created the DNS record that corresponds to the challenge, then you submit the challenge. When you submit the challenge, Let’s Encrypt verifies it.

To submit a challenge:

Submit-ACMEChallenge idAlias -ChallengeType dns-01

When you submit the challenge, you need to specify the alias of the identifier and the challenge type.
The validation may or may not complete immediately. You can verify the status of the validation with the following command:

(Update-ACMEIdentifier idAlias -ChallengeType dns-01).challenges

The Update-ACMEIdentifier cmdlet queries the status of the identifier from the Let’s Encrypt servers. The challenges property contains the challenges generated when the identifier was created. The status for the dns-01 challenge will change to valid when validation is complete.

Creating a Certificate

After you have validated all of the identifiers that will be included in your certificate, you can generate the certificate request. When you generate the certificate request, you need to specify the identifiers to include and a new alias for the certificate.

To generate the certificate request:

New-ACMECertificate idAlias -generate -AlternativeIdentifierRefs idAlias1,idAlias2 -Alias certAlias

The initial alias identifier that you provide is the subject for the certificate. This name is also added to the subject alternative names automatically. You don’t need to repeat it as an alternative identifier.
The -AlternativeIdentifierRefs parameter is used to identify additional identifiers that are included in the certificate. All identifiers used here need to be validated.

To submit the certificate request:

Submit-ACMECertificate certAlias

The certificate alias used here is the alias that was set when running the New-ACMECertificate cmdlet.

After submitting, notice that the IssuerSerialNumber is blank. This is how you can identify that the results from the request you submitted have not been retrieved. You need to update the data in the local vault before you can export it:

Update-ACMECertificate certAlias

To export the completed certificate as a pfx file that includes the certificate and private key:

Get-ACMECertificate certAlias -ExportPkcs12 filename.pfx -CertificatePassword “password”

The -ExportPkcs12 parameter can be given a filename or a full path. If there are spaces in either one, you need to put quotes around it.

When I first ran the Get-ACMECertificate cmdlet to export the pfx file, I got an error:

Issuer certificate hasn’t been resolved.

While I found some references to this being an issue with an intermediate certificate not being installed, my issue was simpler. I had forgotten to update the certificate information in the local vault with Update-ACMECertificate before trying to export to a PFX file.

However, if you have this error and you have already used Update-ACMECertificate then it may be caused by the missing intermediate certificate. You want the X3 intermediate certificate for the Let's Encrypt CA.

If necessary, you can get the X3 intermediate certificate here:

• https://letsencrypt.org/certificates/

After you have the pfx file, you can import it and assign to Exchange Server using the normal Exchange Management cmdlets.