There are various types of digital signatures that are used in practice, and there is the well-known manual signature. However, if this is to be applied to a digital document, things can get a bit more complicated.
First of all, the digital document must be printed out so that it can be signed by hand. Once the analog signature has been applied, the document must be scanned so that it can be sent in digital form. This is referred to as a media discontinuity (digital - analog - digital).
However, if you sign digitally, you can avoid a media discontinuity and save time. If, for example, an image of the signature is added to the document (with the help of an image editing tool), this is referred to as a simple electronic signature (SES).
To be able to sign advanced (AES) or qualified (QES), you need a tool like sproof sign. After placing the signature card on the document, a digital signature is added to the PDF.
Digital signatures are created purely digitally. The information needed to verify a digital signature is NOT visually attached to the document (in theory, digital signatures can even be "invisible") and is lost when the document is printed and rescanned. A digital signature is also only created for a specific version of a document. As soon as the document is edited, the digital signature is invalid. However, it is possible to apply multiple digital signatures to a PDF document. This is also supported by sproof sign. sproof sign only uses digital signatures and offers the choice between an advanced and a qualified signature.
The following table shows a few examples of possible combinations of the different signatures. The signatures are applied sequentially to the document.
Digital signatures that are inserted one after the other in a document are valid because they are always linked to the document and the sequence is also saved. As soon as a digitally signed document is edited afterwards, e.g. an image is inserted with a signature, a previously created digital signature, e.g. cell phone signature, loses its validity. Even if a document already signed with a cell phone signature is printed out and signed by hand, the digital signature automatically loses its validity.
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