ITextSharp Document Signing showing invalid - pdf

Question

So I've been working on signing of PDF documents lately, and today I came across a new and wonderful problem. So when I sign the document(The document is signed on a server actually) and open that document up in my machine, the signature shows valid and that it is LTV enabled, so pretty much works as expected. But when I open the same document on my boss's computer it shows that the identity of the signing could not be verified even after the certificate was trusted, but if I open the certificate properties it says that the certificate is valid and revocation was performed successfully. What could be the cause of this?
Figure 1: The certificate itself is trusted.
Figure 2: Intermediary certificate is trusted.
Figure 3: Root certificate trusted
Signed Document: https://drive.google.com/file/d/0B9RyqgJoa6W8WlBLemVETXJRU0U/view?usp=sharing
Another weird thing, for the timestamping signature, when I add the Root certificate as a trusted root certificate to adobe, it says that LTV is not enabled, but if I add the GlobalTrustFinder certificate itself as a trusted certificate it says that LTV is enabled. Any reason that it would do that?
Any help would really be appreciated
Code for adding LTV to the existing signature blocks as well as add the timestamping signature:
private void SignDocumentSigningBlockAddLTVVerification(PdfStamper stamper, Certificate certificate)
{
LtvVerification ltvVerification = stamper.LtvVerification;
List<string> signatureFieldNames = stamper.AcroFields.GetSignatureNames();
ITSAClient tsaClient = new TSAClientBouncyCastle(_settingManager["DocumentSigningTimestampingServiceAddress"], String.Empty, String.Empty, Int32.Parse(_settingManager["DocumentSigningEstimatedTimestampSize"]), _settingManager["DocumentSigningEncryptionHashAlgorithm"]);
IOcspClient ocspClient = new OcspClientBouncyCastle();
ICrlClient crlClient = new CrlClientOnline(SignDocumentSigningBlockBuildChain(new X509Certificate2(certificate.Bytes, certificate.Password, X509KeyStorageFlags.Exportable)).ToList());
PdfPKCS7 pkcs7 = stamper.AcroFields.VerifySignature(signatureFieldNames.Last());
if (pkcs7.IsTsp)
{
ltvVerification.AddVerification(signatureFieldNames.Last(), ocspClient, crlClient, LtvVerification.CertificateOption.SIGNING_CERTIFICATE, LtvVerification.Level.OCSP_CRL, LtvVerification.CertificateInclusion.NO);
}
else
{
foreach (string name in stamper.AcroFields.GetSignatureNames())
{
ltvVerification.AddVerification(name, ocspClient, crlClient, LtvVerification.CertificateOption.WHOLE_CHAIN, LtvVerification.Level.OCSP_CRL, LtvVerification.CertificateInclusion.NO);
}
}
ltvVerification.Merge();
PdfSignatureAppearance appearance = stamper.SignatureAppearance;
LtvTimestamp.Timestamp(appearance, tsaClient, null);
}
Kind regards

Answer 1

Revocation could not be checked
But when I open the same document on my boss's computer it shows that the identity of the signing could not be verified even after the certificate was trusted, but if I open the certificate properties it says that the certificate is valid and revocation was performed successfully. What could be the cause of this?
As I cannot check the computer of your boss, this is bound to be guesswork. So...
Giving it a shot
You say the certificate was trusted but there are many certificates involved. Thus, which one have you trusted? According to your screenshot
you have not trusted the user certificate (if you had, that certificate would not have been checked against a CRL but instead it would have been trusted without further tests).
Probably you have trusted the Root CA certificate on the computer of your boss. If you have, not only the end user certificate has to be checked for revocation but also the intermediary CA certificate (SAPO Class 4 CA)! Probably that is where your problem arises
Thus, when you are on the revocation tab, please select the intermediary certificate and see whether revocation checks for it could be performed. If not, that's the problem you ran into.
PS: It's not so
The OP tested the idea mentioned above but
So I went back and checked all the certificates, and it seems that the CRL check is performed as expected.
Thus, it is not as easy as I thought above
the difference in setup was that i trusted the certificate that I signed with as the root certificate hence was showing up as successfully signed on my machine, I've now set up my security settings so that I have the same results as on my boss's computer.
Ok, so the difference between the results on those two computers is explained.
To determine whether the problem is in determining the revocation status of the CA certificate or your user certificate, could you please re-configure your settings yet again and trust the CA certificate?
If after this change the problem remains, the issue most likely is linked to your user certificate and checking its revocation status. If it vanishes, the issue most likely is linked with checking the revocation status of the CA certificate.
I trusted the CA now, it still has the same problem, but if I trust the user cert marking the "Use this certificate as a trusted root" and Certified Documents" checkbox, the signature is shown as valid as well as LTV enabled, but only if I mark it as the root. In all other cases the signature has the warning stating that the revocation checks were not performed.
If you immediately trust your user certificate, the signature is verified positively without further ado. After all, you do trust that certificate...
Putting that aside, there already is a problem when checking the user certificate. I'll try and investigate some more.
PPS: So that is why...
If all else fails, start inspecting the signature or so I thought after I could not find anything really bad in the certificate. I opened the signature in an ASN.1 Dump utility and...
...
<30 42>
2174 66: SEQUENCE {
<06 09>
2176 9: OBJECT IDENTIFIER '1 2 840 113583 1 1 8'
<31 35>
2187 53: SET {
<30 33>
2189 51: SEQUENCE {
<A0 31>
2191 49: [0] {
<30 2F>
2193 47: SEQUENCE {
<2D 2D>
2195 45: Unknown (Reserved) {
<2D 2D>
2197 45: Unknown (Reserved) {
<2D 42>
2199 66: Unknown (Reserved) {
<45 47>
2201 71: [APPLICATION 5]
: 'IN X509 CRL-----.MIIfZzCCHU8CAQEwDQYJKo0...*.H..'
: '............0...Ix<..N+'
: Error: IA5String contains illegal character(s).
Error: Inconsistent object length, 7 bytes difference.
: }
Error: Inconsistent object length, 30 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
Error: Inconsistent object length, 32 bytes difference.
: }
...
So there are syntactical errors in the signature itself, more exactly inside the PDF signature certificate revocation information attribute (OID 1 2 840 113583 1 1 8).
(Looking at that region with a different viewer, one sees that there is a text(!) "-----BEGIN X509 CRL-----" - There seems to be a textual CRL representation which is invalid here.)
Adobe Reader, therefore, when trying to check for certificate revocation during signature verification, inspects the signature attribute which can be used to add revocation information already during signing, finds a broken attribute value and seems to stop the verification check with a failure...
The code used for filling the certificate details window seem to be separately coded.
Why that?
One might wonder how a textual CRL representation happens to be included in the PDF signature certificate revocation information attribute of your signature.
I looked into that embedded "CRL". It is in PEM format and cut off after 47 bytes. So I accessed the CRL at the URL given in the certificate (https://pki.trustcentre.co.za/crl/sapo_c4ca.crl) and I indeed retrieved a (complete) CRL in PEM format which is textual. The signature creation code seems to have tried to include it as is but it got cur off.
Is the PKI wrong in supplying that CRL in PEM format instead of in DER format? Or is the signature creation code wrong in assuming to find a DER encoded CRL at the position pointed to?
The current specification for the Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile (RFC 5280) specifies:
When the HTTP or FTP URI scheme is used, the URI MUST point to a single DER encoded CRL as specified in [RFC2585].
(Section 4.2.1.13. CRL Distribution Points)
But it does not say anything equivalent about the HTTPS URI scheme!
Because the PKI employs the HTTPS scheme, therefore, the PKI might be considered to be conform to the letter of the RFC.
Even though the HTTPS scheme essentially merely is the secured variant of the HTTP scheme and so the PKI definitively and unnecessarily violates the spirit of the RFC, it seemingly does not violate the letter of it.
Thus, CRL retrieving classes (like iText's CrlClientOnline) had better check the format of the retrieved CRL and transform it if necessary.
LTV-enabled or not LTV-enabled
An explanation
Another weird thing, for the timestamping signature, when I add the Root certificate as a trusted root certificate to adobe, it says that LTV is not enabled, but if I add the GlobalTrustFinder certificate itself as a trusted certificate it says that LTV is enabled. Any reason that it would do that?
This is pretty obvious:
If you trust the root certificate, revocation has to be checked for the intermediary CA and the end entity certificates. As there are no CRLs or OCSP responses for them embedded in your document, your time stamp is not LTV-enabled.
If on the other hand you explicitly trust the end entity certificate, the very certificate that created the timestamp, the Adobe validator immediately trusts the time stamp. No need to check for revocation of a certificate explicitly trusted... Thus, in this case the time stamp is LTV enabled.
(The term "LTV enabled" is an Adobe term with a very variable value. For backgrounds on this term and other LTV related terminology, cf. the section Background in this answer and the other answers referenced from there,)
Follow-up questions
For the timestamping I'm a bit confused, I followed the code examples in the post you recomended, and the code I've used was a variation of that but in principle the same, why would the CRL and the OCSP responses not be embedded?
The big difference is that the code in my referenced answer does not apply a document time stamp, it merely adds validation related information (certificates, CRLs, OCSP responses).
Adobe's proprietary term "LTV-enabled" refers to a document signature or document time stamp for which "all information required for verification of all relevant signatures (including those signing the CRLs, OCSP responses and time stamps used in the verification process) in the context of the security settings of the validating Adobe Reader are contained in the PDF". Your code, after adding the validation related information, adds a document time stamp, so the information required for validating that time stamp are in general not yet available in the PDF.
ETSI's standardized term "PDF document with LTV" refers to a signed PDF with validation related information (e.g. an "LTV-enabled" PDF) plus a final document time stamp.
The iText sample code you used attempts to create an ETSI "PDF document with LTV" and, therefore, has to be tweaked to instead attempt to create a PDF where all document signature and time stamps are Adobe "LTV-enabled".
As for adding the crls and ocsps, as far as I understood that for adding LTV, you have to add the validation to all signatures in the document and the final document time stamp, as I did in the code I provided, is this not the case?
There are different use cases in the context of LTV, and depending on one's very use case, different information may have to be added.