Ubuntu – How to digitally sign a PDF with a S/MIME certificate

Part One: With Firefox, back up the certificate as a .p12 file.

1. In Firefox, click Edit > Preferences to display the Preferences dialog.

2. On the Preferences dialog, click the Advanced button.

3. On the Advanced form, click the the View Certificates button to display the Certificate Manager dialog.

4. On the Certificate Manager dialog, click the Your Certificates tab.

5. Expand the relevant certificate authority name and click the relevant certificate to select it.

6. Click the Backup button to display a Save As dialog titled File Name to Backup.

7. On the File Name to Backup dialog, in the Name text box, type the name you want to give to the backup copy of the file.

8. Select the folder in which you want to save the file.

9. Click the Save button to display the Choose a Certificate Backup Password dialog.

10. Type your chosen password twice. Click the OK button.

11. An Alert dialog will appear, informing you that your certificates and private keys were successfully backed up. Click the OK button.

12. Close the other two open dialogs.

Part 2: With Thunderbird, import the .p12 file to Thunderbirds certificate store.

1. In Thunderbird, click Edit > Prefereces, to display the Thunderbird Preferences dialog.

2. Click the Advanced button on the top menu.

3. On the Advanced form, click the Certificates tab.

4. Click the View Certificates button to display the Certificate Manager dialog.

5. Click the Your Certificates tab.

6. Click the Import button to display the Certificates to Import dialog.

7. Find the .p12 file which you backed from Firefox, click it and click the Open button to display the Password Entry dialog.

8. Type the password you used to backup the file from Firefox and click the OK button.

9. An Alert dialog will appear, indicating that the operation was successfull.

10. Close the other two open dialogs.

Part 3: In Thunderbird, edit the root certificate of your certificate authority to make it trusted for identifying websites, mail users and software makers.

1. Display the Certificate Manager dialog as described in Part 2.

2. On the Certificate Manager dialog, click the Authorities tab.

3. In the list of root certificates, click on your certificate authority's certificate to select it.

4. Click the Edit Trust... button to display the Edit Trust Settings dialog.

5. Check the three checkboxes to make your certificate authority trusted for websites, email and software.

6. Click the OK button.

7. Close all open dialogs.

Part 4: In LibreOffice Writer, verify that the certificate is available for digital signatures.

1. Close all Writer windows which may have been running.

2. Open an existing Writer document (because you can only view the certificates from a file which is saved to disk).

3. Click File > Digital Signatures... to display the Digital Signatures dialog.

4. Click the Sign Document button to display the Select Certificate dialog.

Your certificate will appear in the list of certificates.

I use DocuSign, which is free web app (for single signatures). It also serves as a (hopefully) trusted third party.

From DocuSign - Wikipedia:

DocuSign, Inc. is an American company headquartered in San Francisco, California that allows organizations to manage electronic agreements. As part of the DocuSign Agreement Cloud, DocuSign offers eSignature, a way to sign electronically on different devices. DocuSign claims it has over 475,000 customers and hundreds of millions of users in more than 180 countries. Signatures processed by DocuSign are compliant with the US ESIGN Act. and the European Union's eIDAS regulation, including EU Advanced and EU Qualified Signatures.