I don't think we need to check if the byte range covers the entire
document. Our job, when verifying the signature, is to use the byte
ranges provided in the signature dictionary. It is up to the pdf
producer to ensure the byte range covers the entire document (excluding
the signature value).

All we need to do is ensure we check all bytes ranges in the ByteRange
array. We should also check that each byte range is within the file. eg
check that each offset is >= 0 and offset + length <= file size.

While it would be nice to check if the byte range covers the entire
document, poppler does not provide any easy way to determine the file
offsets of a dictionary value. This makes it difficult to check if the
excluded range only covers the signature value.

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https://bugs.launchpad.net/bugs/740506

Title:
  verify digital signatures

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