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Signing

Signing PDF files enables the receipient to verfiy, that the document is from the proper person and has not been modified after signing. This is done with a private/public key mechanism. The private key is used to encrypt a fingerprint of the document (the hash). This can be decrypted with the public key and then be compared with a hash created from the present document. These only match, when private and public key match and the document has not been modified.

To digitally sign a PDF, there are several requirements, that need to be met:

  1. A digital Certificate in a PFX\P12 file that is enable to create digital signatures.
  2. .Net 1.1 must be installed. That means, that digital signatures in PDFCreator have no support for Windows 95 and Windows NT 4 Workstation, as .Net 1.1 is not available for these operating systems.
  3. The file pdfforge.dll has to be located in the application directory of PDFCreator under PlugIns\pdfforge. If .Net 1.1 is installed, this DLL will be automatically installed with PDFCreator.

Pfx/P12 file: The file with the signature. It stores the private key which is used to encrypt the hash value.

Signature reason: The reason for the cerfitifacte. This is a piece of text, describing, why the document is signed.

Signature contact: The name of the contact person, that signed the file.

Signature location: Place (Country, City etc.), where the file has been signed.

Signature position: Allows to let Acrobat Reader show an icon showing the validity of the signature. The coordinates for this icon can be set with the LeftX/Y and RightX/Y values. You can also define the page on which the signature will be displayed. If set to a value below zero, it will be displayed on the last page.

Multi signature allowed: Allows the PDF to be signed by other people afterwards.

You can get free certificates at http://www.cacert.org. Please note, that this is not a register-and-go service. CACert is based on a web of trust, where you are verified by people and can verify other people. It takes a bit of work to get enough points for a trusted status, but it really is free and you should give it a try.