You can get a certificate from a certification authority. No matter if you choose a governmental institution or a private company it is common to be charged for this service, for example when they certify your identity. Few other authorities issue certificates free of costs, like the Open Source Project CAcert which is based on the well-known and reliable Web of Trust model and is of growing popularity.
Choose File - Digital Signatures - Digital Signatures.
A message box advises you to save the document. Click Yes to save the file.
After saving, you see the Digital Signatures dialog. Click Add to add a public key to the document.
In the Select Certificate dialog, select your certificate and click OK.
You see again the Digital Signatures dialog, where you can add more certificates if you want. Click OK to add the public key to the saved file.
A signed document shows an icon in the status bar. You can double-click the icon in the status bar to view the certificate.
The result of the signature validation is displayed in the status bar and within the Digital Signature dialog. Several documents and macro signatures can exist inside an ODF document. If there is a problem with one signature, then the validation result of that one signature is assumed for all signatures. That is, if there are ten valid signatures and one invalid signature, then the status bar and the status field in the dialog will flag the signature as invalid.
Normally, macros are part of a document. If you sign a document, the macros inside the document are signed automatically. If you want to sign only the macros, but not the document, proceed as follows:
Choose Tools - Macros - Digital Signature.
Apply the signature as described above for documents.
When you open the Basic IDE that contains signed macros, you see an icon in the status bar.
You can double-click the icon in the status bar to view the certificate.