Verifying digital signatures in PDF documents

Question

I'm working on verifying PDFs digital signatures.
I know that when a PDF is signed, a byterange is defined, the certificates get embedded, and from what i've read, the signed message digest and the timestamp are also stored in the PDF.
I already can extract the certificates and validate them. Now I'm trying to validate the pdf's integrity and my problem is I don't know where the signed message digest is located.
In this sample signed pdf (http://blogs.adobe.com/security/SampleSignedPDFDocument.pdf), I can clearly identify the digest since it is down below the embedded certificates: /DigestMethod/MD5/DigestValue/ (line 1520).
But that PDF sample seems to be from 2009, and I suspect the message digest is stored in a different way now, because I signed a PDF with Adobe Reader and I can't find any message digest field like the previous one. Can someone tell if the digests are now stored in a different way? Where are they located?
Anyway, for now I'm using that sample document, and trying to verify its integrity. I'm getting the document's bytes to be signed acording to the specified byterange, and digesting them with MD5 algorithm, but the digest value I get doesn't match with the one from the message digest field... Am I doing something wrong? Is the digest also signed with the signer's private key?
I appreciate any help.

Answer 1

You cannot rely on the digest to be in a certain place in PDF. If you want to manually verify the digest in a PDF signature here's what you need to do.
1. Open PDF in a Text Editor.
2. Find Signature Dictionary for your signature.
3. Get the Hex String which is the value of the /Contents entry in the Signature Dictionary.
4. Convert Hex String to binary string and discard trailing zeros. Remember that in a Hex string each byte is represented with two characters and the last one might be a zero. So, when you discard zeros make sure that what you get left has even number of bytes.
5. Use one of the commercially available BER Viewers (you can find free BER Viewers on the Web) to convert the binary string to ANSI.1 representation.
6. Analyze the BER-decoded PKCS#7 signature object (RFC 2315 describes it) and find the digest that you are looking for in it. It is an OCTET STRING.
If you want to programmatically validate a signature, you need to write code that does all that. Signature validation includes much more than checking the digest. You need to build chain, validate each certificate in the chain, check revocation for each certificate in the chain, etc. RFC 5280 is the guide what to do.
Good luck!