Using Self-Signed S/MIME Certificates in Outlook

If you followed my guide to create self-signed S/MIME certificates, then you will have the necessary files to begin digitally signing and receiving encrypted e-mail. As long as the e-mail client supports S/MIME, which Outlook does support, then you can create and use your own certificates for any e-mail address including custom domains, Gmail, iCloud, or even AOL. This guide describes the process for using self-signed S/MIME certificates with Microsoft Outlook 2019 found in the Microsoft Office Professional Plus 2019 suite running on Windows 10.

Part 1 – Trusting the Self-Signed Certificate Authority

Since we’re using self-signed certificates, Windows and Outlook will not automatically recognize your personal certificate authority. If a certificate authority is not trusted, then any certificates issued by that certificate authority are not trusted and they are considered invalid by Outlook. If you try to send a digitally signed e-mail using your personal certificate before the certificate authority is trusted, then Outlook displays the message “Microsoft Outlook cannot sign or encrypt this message because your certificate is not valid.”

Invalid Certificate
Invalid Certificate

Step 1 – Install Certificate

Find the certificate authority certificate. If you followed my guide, the file is named ca.crt. Double-click on the file in File Explorer to open it. The Certificate Information screen is displayed. Confirm that the Issue to, Issued by, and Valid from/to dates match the expected values.

Click the Install Certificate… button.

Install Self-Signed Certificate Authority Certificate
Install Self-Signed Certificate Authority Certificate

Step 2 – Select Store Location

The Certificate Import Wizard screen is displayed. Choose Current User and click the Next button.

Certificate Import Wizard – Store Location
Certificate Import Wizard – Store Location

Step 3 – Select Certificate Store

The Certificate Store screen is displayed. Select Place all certificates in the following store and click the Browse… button.

Certificate Import Wizard – Store Selection
Certificate Import Wizard – Store Selection

The Select Certificate Store screen is displayed. Select Trusted Root Certification Authorities and click the OK button.

Certificate Import Wizard – Trusted Root Certification Authorities
Certificate Import Wizard – Trusted Root Certification Authorities

Confirm the details on the Select Certificate Store screen and click the Next button.

Step 4 – Complete the Certificate Import Wizard

The Completing the Certificate Import Wizard screen is displayed. Confirm the displayed details and click the Finish button.

Certificate Import Wizard – Completion
Certificate Import Wizard – Completion

Step 5 – Security Warning

A Security Warning screen is displayed requesting confirmation that the certificate should be installed. Confirm the displayed details and click the Yes button.

Security Warning
Security Warning

The Import Successful dialog box is displayed.

Import Successful
Import Successful

Close the Certificate Information screen by clicking the OK button.

Step 6 – Confirm Trusted Certification Authorities in CertMgr

From the Windows start menu, run certmgr (Manager user certificates). Under Current User, expand Trusted Root Certification Authorities and click Certificates. Review the list of certificates to confirm your certificate authority is in the store. Close the application.

CertMgr – Trusted Root Certification Authorities
CertMgr – Trusted Root Certification Authorities

Part 2 – Installing the Self-Signed S/MIME Certificate in Outlook

With the certificate authority certificate in the Windows trust store, we can now add our self-signed S/MIME certificate to Outlook.

Step 1 – Open Trust Center

Open Outlook and select File and then Options. The Outlook Options screen is displayed. Select Trust Center.

Outlook Options – Trust Center
Outlook Options – Trust Center

Step 2 – Open Email Security

Click the Trust Center Settings… button. The Trust Center screen is displayed. Select Email Security.

Trust Center – Email Security
Trust Center – Email Security

Step 3 – Import Self-Signed S/MIME Certificate

Click the Import/Export… button. The Import/Export Digital ID screen is displayed. In the Import existing Digital ID from a file section, click the Browse… button. Find the relevant PKCS12 file. If you followed my guide, the file is named smime_test_user.p12. Enter the password for the package.

Trust Center – Import/Export Digital ID
Trust Center – Import/Export Digital ID

Click the OK button.

Step 4 – Import Certificate

The Importing a new private exchange key dialog box is displayed. Click the OK button.

Importing a New Private Exchange Key
Importing a New Private Exchange Key

Step 5 – Change Security Settings

Returning to the Trust Center screen, click the Settings… button. The Change Security Settings screen is displayed. Confirm the displayed information is correct. Change the Security Settings Name value to a unique name for the certificate.

Trust Center – Change Security Settings
Trust Center – Change Security Settings

If the Signing Certificate or Encryption Certificate are blank, then click either Choose… button. The Windows Security Confirm Certificate dialog box is displayed. Click the OK button.

Windows Security – Confirm Certificate
Windows Security – Confirm Certificate

Confirm the information on the Change Security Settings dialog box are correct and click the OK button.

Step 6 – Confirm Default Settings

On the Trust Center Email Security settings screen, confirm the Default Setting references the security settings name created in the prior step. Click the OK button to close the Trust Center screen. On the Outlook Options screen, click the OK button to close the screen.

Trust Center – Email Security Settings Completed
Trust Center – Email Security Settings Completed

Part 3 – Sending a Digitally Signed E-mail

Finally, we can send a digitally signed e-mail in Outlook using a self-signed S/MIME certificate issued by a personal certificate authority. As a reminder, this does not allow you to send encrypted e-mails since public-key cryptography requires the sender to have the public key for the recipient (we only have the sender keys). However, this does allow you to send a digitally signed e-mail to a recipient. Since the digital signature contains your public key, the recipient can than respond with an encrypted e-mail after establishing trust in their e-mail client.

Step 1 – Draft and Sign E-mail

Create a new e-mail using the e-mail address associated with the S/MIME certificate. From the Options ribbon, click Sign.

Outlook – Digitally Signed E-mail
Outlook – Digitally Signed E-mail

Step 2 – Send E-mail

Complete the e-mail and click the Send button. A Windows Security dialog box is displayed requesting access to the private key. Click the Allow button.

Outlook – Credential Required
Outlook – Credential Required

Result

When the e-mail is received, the recipient’s e-mail client displays an indicator that the e-mail is digitally signed. Outlook’s indicator for digitally signed e-mail is a small ribbon. The recipient may then need to trust the certificate (public key) contained in your digital signature in order to respond with an encrypted message.

Outlook – Digital Signature Indicator
Outlook – Digital Signature Indicator

If you send a digitally signed e-mail from the e-mail address back to itself, you can respond to that e-mail with an encrypted message. Open the e-mail and click Reply. From the Options ribbon, click Encrypt. Add a response to the message body and click the Send button. When the response is received, Outlook displays a lock icon to indicate that the e-mail is encrypted. Outlook automatically handles the decryption when the e-mail is opened.

Outlook – Encryption Indicator
Outlook – Encryption Indicator

Part 4 – Establishing Recipient Trust

When a recipient receives a digitally signed e-mail where the sender used a self-signed S/MIME certificate and a personal certificate authority, the message is flagged by Outlook with the message “There are problems with the signature. Click the signature button for details.” The signature button is the yellow triangle with an exclamation point. As the sender, we added trust in Part 1 of this guide to the sending machine, however, the recipient machine does not recognize the certificate authority so the digital signature certificate is not trusted and flagged as invalid.

Outlook – Digital Signature is Invalid Warning
Outlook – Digital Signature is Invalid Warning

Step 1 – Invalid Digital Signature

Click the yellow triangle with an exclamation point icon. The Digital Signature: Invalid dialog box is displayed.

Outlook – Digital Signature Invalid
Outlook – Digital Signature Invalid

Step 2 – Message Security Properties (Invalid)

Click the Details… button. The Message Security Properties screen is displayed. There are many intimidating red circles with exclamation points.

Outlook – Message Security Properties (Invalid)
Outlook – Message Security Properties (Invalid)

Step 3 – Trust Certificate Authority

If available, click the Trust Certificate Authority… button. The Trust Certificate Authority screen is displayed. Click the Trust button. The Trust Certificate Authority screen closes. If the button is not active, proceed to the next step.

Outlook – Trust Certificate Authority
Outlook – Trust Certificate Authority

Step 4 – View Certificate and Edit Trust

Returning to the Message Security Properties screen, click the Edit Trust… button. The View Certificate screen is displayed. In the Edit Trust section, select Explicitly Trust this Certificate. Click the OK button.

Outlook – View Certificate
Outlook – View Certificate

Step 5 – Message Security Properties (Valid)

Returning to the Message Security Properties screen again, we find all the red circles have been replaced with green check marks. Click the Close button.

Outlook – Message Security Properties (Valid)
Outlook – Message Security Properties (Valid)

Step 6 – Valid and Trusted Digital Signature

The Digital Signature: Invalid dialog box is now the Digital Signature: Valid dialog box. Click the Close button. The digital signature is now trusted and flagged as valid.

Outlook – Digital Signature Valid
Outlook – Digital Signature Valid

Further Reading

Leave a Comment