Digital Signatures

Hi,

I saw that ERPNext can include with Signature, a kind of touch signature.

But, also saw that exists a demand to digitally sign some documents, including quality documents, as SOPs, meetings reports, etc.

Has somebody already deal with it? With some of these services (http://wiki.cacert.org/PdfSigning) or something similar?

It would be awesome to have, as it can end with a lot of paper!!

Thanks!