An electronic signature or digital signature is any electronic tool that indicates either that a person adopts the contents of an electronic message, or more generally that the person who claims to have written a message is the one who wrote it (and that the message received is exactly the one that was sent). In commerce and the law, a signature on a document is an indication that the person adopts the intentions recorded in the document.

For electronic invoices the European law provides three kinds of electronic signatures, each with different juridical value:

1. electronic signature
2. advanced electronic signature
3. advanced electronic signature which is based on a qualified certificate

For further information see also:

• Directive 1999/93/EC of the European Parliament and of the Council http://europa.eu/legislation_summaries/information_society/other_policies/l24118_en.htm
• German Signature Law of 2001, changed in 2005http://www.gesetze-im-internet.de/sigg_2001/index.html