It's simple. It's not particularly good. It parses PDFs.
Enjoy.
Useful links:
https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/PDF32000_2008.pdf https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSig/Acrobat_DigitalSignatures_in_PDF.pdf https://www.etsi.org/deliver/etsi_en/319100_319199/31914201/01.01.01_60/en_31914201v010101p.pdf http://www.libpng.org/pub/png/spec/1.2/PNG-Filters.html https://letsencrypt.org/docs/a-warm-welcome-to-asn1-and-der/