Represents the "DigSig" entry of the requirement dictionary object.

It is defined as part of the PDF 2.0 specification.

It contains a SignatureElement.

For definitive details see:.

The ISO PDF Specification, ISO 32000-1:2008 PDF 1.7; Table: 264, page 484.

The ISO PDF Specification, ISO 32000-2:2017 PDF 2.0; Table: 274, page 603.

When present this entry extends the requirement to include a specific digital signature capability, defined as a dictionary specifying the required signing algorithm and key usage.