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DynaPDF Manual - Page 57

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Digital Signatures
Page 57 of 766
CreateSigField() on the page where the field should appear. If multiple signature fields exist,
DynaPDF uses the last signature field to sign the PDF file.
How to modify the appearance of a signature field?
The appearance of a signature field can be fully user defined. The function CreateSigFieldAP()
can be used to create a user defined signature appearance template. You can draw anything you
want into this template such as images, vector graphics, text, and it is also possible to import a
PDF page into or to draw an EMF file into the template.
What is stored in a signature field?
When signing a PDF file a signature handler, whether internal or external, creates a PKCS#7
signature object that contains the file’s signature, optionally a time stamp, and a PKCS#1
certificate that was extracted from the PKCS#12 certificate. The difference between PKCS#1 and
PKCS#12 is that a PKCS#1 certificate contains no private key.
A viewer application validates the signature by using the public key that is stored in the
PKCS#1 certificate object. Because the private key is not stored in the PDF file it is impossible to
sign other PDF files with the certificate that can be extracted from the PDF file.
Adobe's Acrobat supports defined length encoded ASN1 objects only for encryption key lengths
stronger than 1024 bits.
How to validate a signature?
Acrobat validates signatures from unknown certificates not automatically. The certificate must
first be added to the list of trusted identities. Once a certificate was added to the list of trusted
identities, signatures of other documents which use the same certificate will be automatically
validated.
 

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