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To add a certification signature field to a PDF document and sign it:
Digitally sign PDF files
Full code sample which demonstrates using the digital signature API to digitally sign and/or certify PDF documents.
Unlike, approval signatures, there can be only one certification per PDF document. Only the first signature in the PDF document can be used as the certification signature. Certifying a document is like notarizing a document. The process of certifying a document is almost exactly the same as adding approval signatures with the exception of certification signatures requires an entry in the "Perms" dictionary.
If you want to certify a PDF/A document, it is best to convert to PDF/A first, then certify. This is because PDF/A changes the contents of the document, while digital signatures, including certifications, rely on the document's bytes remaining the same so that they can be digested and compared with the embedded cryptographic digital signature.
A DigitalSignatureField
can be added before or after PDF/A conversion, since there aren't any requirements in PDF/A upon it.
The PDF/A-2
specification allows adbe.pkcs7.detached
and adbe.pkcs7.sha1 certification-type
or UR3-type
cryptographic digital signatures, with or without secure timestamps, with or without embedded revocation information, which must be signed if present. A single SignerInfo
must be present. Attribute certificates must not be used. The PDFNet SDK's signing support is sufficient to meet the requirements of PDF/A-2
compliance if used properly.
There shouldn't be any problem with retaining PDF/A compliance after digitally signing a document, so long as there is no annotation appearance for the digital signature field, or there is an appearance and that appearance conforms to PDF/A, i.e. e.g. sections 6.3.2 and 6.3.3 of the PDF/A-2 specification (ISO-19005-2).
An additional limitation of PDF/A for digital signing is the implementation limit that says that a conforming file shall not contain any string longer than 32767 bytes. Sometimes, signatures with a large amount of data will cause the Contents byte string in the digital signature dictionary to exceed this limit.
The PDF/A-2 specification also mentions the following:
Here is what 6.3.2 and 6.3.3 say about the annotation:
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