PDF Signature with timestamping from TSA

Question

We are trying to embed Timestamp signature as a unsigned attribute in CMS format but after stamping signature in PDF, PDF viewer giving the signature includes an embeded timestamp but it is invalid message.

We have used internal TSA service (self sigined) TSA and bouncy castle API for crypto and signing operations. But don't know what is happening with timestamp. Could anyone know what is wrong I am doing.

hello_signed.pdf Also added Java code base and signed pdf sample for reference.

Any help would be appreciated.

Answer 1

There is an issue in the TSTInfo structure, its tsa member is

C = IN,S = MH,L = NSDL,O = NSDL,OU = NSDL,CN = NSDL,E = [email protected]

but your TSA certificate has the inverse subject

E = [email protected],CN = NSDL,OU = NSDL,O = NSDL,L = NSDL,S = MH,C = IN

According to RFC 3161, the purpose of the tsa field is to give a hint in identifying the name of the TSA. If present, it MUST correspond to one of the subject names included in the certificate that is to be used to verify the token.

Thus, an attentive validator cannot use the certificate you supplied for verifying the time stamp.

---

I don't know whether that's the only issue but it's definitively a show-stopper.

Answer 2

This means Acrobat cannot find the certificate used to create the timestamp. Looking at the ASN.1 decode for your PDF signature CMS, it looks like you haven't requested the Timestamp Authority's certificate in the request (search for timeStampToken to take you to the appropriate section of the CMS). In Bouncy Castle this is achieved by calling SetCertReq(true) on the TimeStampRequestGenerator instance as follows:

        TimeStampRequestGenerator reqGenerator = new TimeStampRequestGenerator();
        // Request the server to also include the signing certificate:
        reqGenerator.SetCertReq(true);

When this is set the Timestamping Authority will include the certificate, or more usually the timestamping certificate and its chain, as signed attribute(s) in the SignedCms of the actual timestamp.

To confirm that the certificate is included in the response you can write the timestamping authority's response to a text file as a hex string and decode it using this website https://lapo.it/asn1js/.