OWA-SMIME4Linux

This is a Linux implementation of the Outlook Web App SMIME control for Linux. Originally, SMIME functionality in OWA is only available for Windows.

OWA uses Native Messaging to communicate with a small piece of software installed on the user's computer to do SMIME mail de-/encryption. The browser hands the SMIME ciphertext to this software which decrypts it using the certificate from the local cert store, and returns it to the website so that the plaintext can be displayed (or vice versa when sending encrypted mails).

Native Messaging can only be initiated by a browser extension, that's why beside the locally installed SMIME handler, a browser extension is necessary. This Linux implementation works flawlessly with the original "Microsoft S/MIME" extension for Chrome, so there is no special or modified extension needed. Note that this browser extension is only compatible with Chrome/Chromium (and IE + Edge, but they do not apply to Linux).

OWA-SMIME4Linux internally calls the openssl command line utility for de-/encrypting and verifying the email payload.

Installation

1. Install dependencies: apt install openssl python3-cryptography
2. Copy owa-smime.py into /usr/bin/ and make it executable.
3. Register the OWA-SMIME4Linux Native Messaging app by copying com.microsoft.outlook.smime.chromenativeapp.json into:
   • /etc/opt/chrome/native-messaging-hosts/ for Chrome
   • /etc/chromium/native-messaging-hosts/ for Chromium
   • (/usr/lib/mozilla/native-messaging-hosts/ for Firefox, if supported in the future)
4. Install the "Microsoft S/MIME" extension in your Chrome browser.
   • The extension is currently not available in the normal Chrome web store. As suggested on the Microsoft documentation, you should use the Chrome policy "ExtensionInstallForcelist" to deploy it.
   • For testing purposes, you can also open the link https://outlook.office.com/owa/SmimeCrxUpdate.ashx from the MS docs, then follow the "codebase" URL to https://res-1.cdn.office.net/owasmime/<VERSION>/Microsoft.Outlook.Smime.crx, download and drag&drop the .crx archive into the Chrome extension page (dev mode must be enabled).
   • On-Prem Exchange only: open the "Microsoft S/MIME" extension settings page and enter your domain name to make the extension trust your domain.
5. Prepare certificate files. Remember to restrict access permissions to them.
   • Put your cert with private key in PEM format into ~/.config/owa-smime4linux/cert.pem.
     • To extract/convert your personal cert from .p12/.pfx into .pem, use:
       openssl pkcs12 -in cert.p12 -out cert.pem -nodes -legacy
   • Put the cert chain (optional) into ~/.config/owa-smime4linux/chain.pem.
     The chain will be included in your signatures. While it is not necessary, the chain helps your recipient verifying your signature if he doesn't have those intermediate certificates.
     • To extract/convert the cert chain from .p12/.pfx into .pem, use:
       openssl pkcs12 -in cert.p12 -out chain.pem -nodes -cacerts -nokeys -legacy

Usage

First, if you are still logged in, log out and log in again. OWA checks the SMIME control availability when logging in.

1. Open a SMIME encrypted and/or signed mail -> profit.
2. Sending a SMIME encrypted and/or signed mail unfortunately needs one more hack. Thanks to Microsoft, the encrypt/signing options only appear if the browser's user agent contains "Windows".
   • For a quick test, you can open the dev tools (F12), click on the 3 dots -> More Tools -> Network Conditions. There, disable "Use browser default" and select "Chrome - Windows". This only works as long as the dev tools are open.
   • For a durable solution, you can use the User Agent Switcher and Manager Chrome extension. Install it, open the extension settings, choose "White-List Mode" and enter your OWA URL there (this ensures that the custom user agent will only be used for this URL - we don't want to increase Windows page views in website statistics!). Don't forget to click "Save" on the bottom.
     Next, click the User Agent Switcher icon in the Chrome menu bar, choose "Chrome Windows 10" as user agent and click "Apply (active Window)".
   • Log out and log in again, now you shoud have the options to enable encryption/signing when writing an email.

Server Setup

S/MIME in OWA (regardless if used on Windows or Linux) requires a proper setup by your Exchange admin as described in this Microsoft article.

• Set the SMIMECertificateIssuingCA option in your Office 365 or on-prem Exchange by uploading an .SST file with the allowed CA certificates. This is basically the chain.pem as described in the installation instructions but in a proprietary SST format. Only an administrator can do this. If this is not set correctly, you will see an error like this:

  An error occurred while sending this S/MIME message. The certificate used to sign this message isn’t trusted by your organisation.

• Publish your personal certificate in your companies GAL (Global Address List) ie. Active Directory (only the public part, without private key of course). This can be done by yourself without admin rights. I have a tool for that too for Linux users who can't use Outlook, it's called CertUploader, but it focuses on on-prem AD. Linux-only solutions for Microsoft 365 are highly appreciated.

Debugging

In case of problems, you can create an empty file ~/.cache/owa-smime4linux/native.log to enable logging of the Native Messaging communication. Attention: this will contain decrypted messages in plaintext. Delete the file afterwards to disable logging!

Interesting OpenSSL Commands

• openssl smime -in mymail.eml -verify -noverify -signer scert.pem -out textdata
• cat message.eml | openssl smime -pk7out | openssl pkcs7 -print_certs
• openssl pkcs7 -in signature.p7 -inform DER -print_certs

Support

If you like this project please consider making a donation using the sponsor button on GitHub to support further development. If your financial resources do not allow this, you can at least leave a star for the Github repo.

Furthermore, you can hire me for commercial support or implementing new features. Please contact me if you are interested.