Using Self-Signed S/MIME Certificates in Thunderbird

This guide describes the process for using self-signed S/MIME certificates with Mozilla Thunderbird on Windows 10. If you need an S/MIME certificate, please follow the guide to Create Self-Signed S/MIME Certificates. Certificates allow you to both send digitally signed messages and receive encrypted messages as part of public-key cryptography (also known as asymmetric cryptography). By creating and self-signing your own certificates, you avoid paying recurring fees but you lose the inherent trust that comes with a certificate generated by a well-known certificate company. After having written similar guides for iOS / iPadOS and Outlook, in my opinion, Thunderbird is probably the easiest to establish trust and set up your own certificates.

Part 1 – Trusting Self-Signed S/MIME Personal Certificates

Step 1 – Account Settings

From the Thunderbird menu (the hamburger icon button or collapsed menu icon button), select Account Settings.

Thunderbird – Account Settings Menu
Thunderbird – Account Settings Menu

Step 2 – End-To-End Encryption

The Account Settings screen is displayed. Select End-To-End Encryption from the account settings menu. Click the Manage S/MIME Certificates button.

Account Settings – End-To-End Encryption
Account Settings – End-To-End Encryption

Step 3 – Establish Personal Certificate Trust

The Certificate Manager window is displayed. From the available tabs, select Your Certificates followed by the Import… button. The Certificate File to Import window is displayed. Find the .p12 file containing the appropriate self-signed certificate for this e-mail account and click the Open button.

Certificate Manager
Certificate Manager

The Password Required prompt is displayed. Enter the password set when the .p12 package was created and then click the Sign in button.

PKCS12 Package Password
PKCS12 Package Password

The personal certificate is now displayed in the Your Certificates tab of the Certificate Manager.

Certificate Manager – Personal Certificates
Certificate Manager – Personal Certificates

Step 4 – Establish Certificate Authority Trust

Now the certificate authority certificate needs to be added to the Certificate Manager so that the personal certificate can be trusted. Select the Authorities tab and click the Import… button.

Certificate Authorities – Authorities
Certificate Authorities – Authorities

The Select File containing CA certificate(s) to import window is displayed. Find the .crt file containing the appropriate certificate authority certificate used to create the personal certificate and click the Open button. If the guide Create Self-Signed S/MIME Certificates was followed, then the file is named ca.crt.

Added Certificate Authority Certificate
Added Certificate Authority Certificate

The Downloading Certificate prompt is displayed. Confirm the displayed information and CA certificate are correct. The Trust this CA to identify email users checkbox should be checked. Click the OK button when complete.

Downloading Certificate – Authority Certificate
Downloading Certificate – Authority Certificate

The certificate authority certificate is now displayed in the Certificate Manager window on the Authorities tab. In this example, the certificate authority certificate was established using the Organization Name as TEST COMPANY and the Common Name as TEST COMPANY CERTIFICATE AUTHORITY. Click the OK button to close the Certificate Manager window.

Added Certificate Authority Certificate
Added Certificate Authority Certificate

Step 5 – S/MIME Settings for Digital Signing and Encryption

Now that trust has been established for the certificate authority certificate as well as the personal certificate, the S/MIME section of the End-To-End Encryption settings screen can be completed.

Account Settings – End-To-End Encryption (Completed)
Account Settings – End-To-End Encryption (Completed)

In the Personal certificate for digital signing field, click the Select… button next to the field. The Select Certificate prompt is displayed. Select the appropriate certificate from the drop-down list and confirm the displayed details. Click the OK button.

Digital Signature Certificate
Digital Signature Certificate

A prompt may be displayed asking “You should also specify a certificate for other people to use when they send you encrypted messages. Do you want to configure an encryption certificate now?” Click the Yes button. The Personal certificate for encryption field is populated with the same certificate specified for the digital signature.

Configure Encryption Certificate
Configure Encryption Certificate

If the question prompt was not displayed, Thunderbird returns to the End-To-End Encryption settings screen. If the Personal certificate for encryption is not populated in the S/MIME section, then click the Select… button next to the Personal certificate for encryption field. The Select Certificate prompt is displayed. Select the appropriate certificate from the drop-down list and confirm the displayed details. Click the OK button. I can’t think of a reason why the digital signature and encryption fields would use different certificates, but I’m sure someone has an edge case where the distinction is needed. Generally, I would expect both fields to reference the same personal certificate.

Encryption Certificate
Encryption Certificate

Close the Account Settings screen.

Part 2 – Sending a Digitally Signed and Encrypted Message

At this point, trust has been established for both the certificate authority certificate and the personal certificate. The personal certificate has been established for use in digital signatures as well as encryption and Thunderbird is ready to send a digitally signed message.

Step 1 – Draft New Message

Click the Write button to draft a new message. A new message window opens.

Step 2 – Set Security Options

In the Options menu for the message, select Digitally Sign This Message.

Message Options
Message Options

I was also able to select Require Encryption because the sender and recipient are the same e-mail address in this example (I sent an e-mail to myself). Since the sender and recipient are the same, Thunderbird already has a trusted recipient certificate. If Require Encryption is selected and a recipient certificate isn’t available, then Thunderbird displays the error “Sending of the message failed. You specified encryption for this message, but the application failed to find an encryption certificate for <e-mail address>.”

Encryption Error
Encryption Error

The Digitally Sign This Message and Require Encryption options may also be set by clicking the Security button. View Security Info will display the recipient certificate, if available.

Message Security Options
Message Security Options

Step 3 – Send Message

Complete drafting the message and click the Send button. A Message Security prompt is displayed. Confirm the displayed information is correct and click the OK button to send the message.

Send Message – Security Warning
Send Message – Security Warning

Result

The recipient receives a digitally signed message using the sender’s self-signed certificate. In this example, since the sender and recipient are the same, the recipient also received an encrypted message.

Digitally Signed and Encrypted Message
Digitally Signed and Encrypted Message

Further Reading

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

Using Self-Signed S/MIME Certificates in iOS Mail App

If you read an earlier post detailing the steps to create self-signed S/MIME certificates using OpenSSL, I left off at the point where the certificate is created and packaged in the PKCS12 format. In order for the certificate to be of any use, you’ll need to install it in an e-mail client. This post details the steps for installing digital certificates on an iOS device and enabling S/MIME in the iOS Mail app.

Step 1 – Load the .p12 File on the iOS Device

Upload a copy of the .p12 file to your iCloud Drive or e-mail a copy of the file to an e-mail address accessible on your iOS device. E-mail isn’t the most secure way of loading it on the device, but it is suggested only for simplicity in this guide. Once the .p12 file is accessible on the device, tap the file to begin the installation process.

iCloud Drive – p12 File
iCloud Drive – p12 File

If prompted to “Choose a Device”, select the appropriate device to install the profile and confirm.

Choose Device
Choose Device

Step 2 – Install the Profile

Open the Settings app and tap Profile Downloaded.

Settings – Profile Downloaded
Settings – Profile Downloaded

The device will prompt to install the profile. Tap Install in the upper right corner.

Install Profile
Install Profile

If the device is password protected, a prompt to enter the device passcode is displayed.

Enter Passcode
Enter Passcode

Since the digital certificate is self-signed and not signed by a well-known trusted certificate authority, a warning message that the profile is not signed is displayed. Continue by tapping Install in the upper right corner.

Warning – Profile Not Signed
Warning – Profile Not Signed

Another prompt to install the profile is displayed. Tap the Install button at the bottom of the screen.

Warning – Profile Not Signed (Install)
Warning – Profile Not Signed (Install)

A prompt is displayed to enter the PKCS12 export/import password created when the .p12 file was assembled. Enter the password and tap Next in the upper right corner of the screen.

Certificate Password
Certificate Password

The profile and certificate are now installed. The configuration profile appears in Settings, General, VPN & Device Management. Tap Done and exit the Settings app.

Profile Installed
Profile Installed

Step 3 – Enabling S/MIME in iOS Mail

Now that you have the digital certificate loaded and a profile created, you may begin using it in the iOS or iPadOS Mail app. For these steps, I am using an e-mail address associated with iCloud. These steps may vary slightly (in steps 4 and 5) if you are using a different account type such as Gmail.

  1. Open the Settings app
  2. Tap Mail
  3. Tap Accounts
  4. Tap on an existing account name in the Accounts section (for this example I am using an iCloud e-mail address)
  5. Tap iCloud
  6. Tap iCloud Mail
  7. Tap Advanced
  8. In the S/MIME section, tap Sign
  9. Enable Sign
  10. Ensure the appropriate certificate is selected
  11. Return to the Advanced screen
  12. In the S/MIME section, tap Encrypt by Default
  13. Enable Encrypt by Default
  14. Ensure the appropriate certificate is selected.
  15. Return to the Advanced screen
  16. Confirm changes and exit the Settings app

S/MIME is now enabled and ready to use your personal digital certificate the next time e-mail is sent from this account.

Step 4 – Sending Encrypted E-Mail

Recall that S/MIME uses public-key encryption, so you won’t be able to send an encrypted e-mail to another address until you have the recipient’s public key. If you receive a digitally signed e-mail from a sender who used a self-signed certificate, the Mail app will flag the certificate as not trusted or invalid. You can test this by using two different e-mail addresses where each address has its own self-signed certificate.

  1. Send a digitally signed (not encrypted) e-mail to another e-mail address where the certificate profile has not been installed on the device.
  2. When the e-mail is received, the sender’s e-mail address will be red with a red circle containing a question mark inside next to the address.
iOS Mail – Untrusted Signature Indicator
iOS Mail – Untrusted Signature Indicator
  1. Tap the question mark or the address (twice) to view sender’s details. iOS indicates that the sender’s digital signature is an untrusted signature and that it is unable to verify the authenticity of the S/MIME certificate provided by the sender.
iOS Mail – Untrusted Signature Message
iOS Mail – Untrusted Signature Message
  1. Tap View Certificate. Details of the certificate are displayed.
  1. Tap Install.
iOS Mail – Trusted Certificate
iOS Mail – Trusted Certificate
  1. Tap Done to exit the certificate details screen.
  2. Tap Done to exit the contact details screen.
  3. iOS now recognizes the public key and allows encrypted e-mail communications. Reply to the signed e-mail and the contact is now in blue font with a blue lock icon next to the name/address.
iOS Mail – Encrypted Message Indicator
iOS Mail – Encrypted Message Indicator

Common Issues

If, after installing the profile, you receive a message stating no valid certificates found when attempting to enable S/MIME, then the extension “extendedKeyUsage = e-mailProtection” was most likely missing when the certificate was signed.

Further Reading