Operation Manual

Figure 300: Sending a document as an email message
Digital signing of documents
To sign a document digitally, you need a personal key, also known as a certificate. A personal key
is stored on your computer as a combination of a private key, which must be kept secret, and a
public key, which you add to your documents when you sign them. You can get a certificate from a
certification authority, which may be a private company or a governmental institution.
When you apply a digital signature to a document, a kind of checksum is computed from the
document’s content plus your personal key. The checksum and your public key are stored together
with the document.
When someone later opens the document on any computer with a recent version of LibreOffice,
the program will compute the checksum again and compare it with the stored checksum. If both
are the same, the program will signal that you see the original, unchanged document. In addition,
the program can show you the public key information from the certificate. You can compare the
public key with the public key that is published on the web site of the certificate authority.
Whenever someone changes something in the document, this change breaks the digital signature.
On Windows operating systems, the Windows features of validating a signature are used. On
Solaris and Linux systems, files that are supplied by Thunderbird, Mozilla or Firefox are used. For
a more detailed description of how to get and manage a certificate, and signature validation, see
“About Digital Signatures” in the LibreOffice Help.
To sign a document:
1) Choose File > Digital Signatures. If you have set LibreOffice to warn you when the
document contains comments (see “Removing personal data” below), you may see a
message box asking whether you want to want to continue signing the document.
Chapter 10 Printing, Exporting, E-mailing | 309