How to calculate a digest of pdf document

I need to implement signing a pdf document in two steps. In the first step I need to calculate a pdf digest and in the second step, I need to sign this document.

I have a problem with step 1 (calculating pdf digest). If I run the digest calculation several times with identical data, I have different digests. When I checked the document that has been passed to the digest calculation method, I found the Modification date was different in these calls.But If I do it with the PdfBox library, I always get correct results, and the Modification Date doesn’t change

How can I calculate the pdf digest and don’t update the modified date?
My code looks like the following:

public class MyDigestCalculator extends ExternalBlankSignatureContainer { public byte[] pdfDigest; public MyDigestCalculator(PdfName filter, PdfName subFilter) { super(filter, subFilter); } @Override public byte[] sign(InputStream is) throws GeneralSecurityException { try { final byte[] bytes = is.readAllBytes(); final String pdf = new String(bytes); BouncyCastleDigest digestService = new BouncyCastleDigest(); pdfDigest = DigestAlgorithms.digest(new ByteArrayInputStream(bytes), “SHA-256”); return new byte[0]; } catch (IOException e) { throw new GeneralSecurityException("Can't get digest from file hash", e); } }
}
public class DocumentSigner { public String calculatePdfDigest(byte[] pdfBytes, SignatureProperties signature, boolean lockFields, Calendar signDate) throws IOException, GeneralSecurityException { final PdfDocument pdfDocument = createPdfDocument(pdfBytes); PdfAcroForm acroForm = PdfAcroForm.getAcroForm(pdfDocument, false); acroForm.setSignatureFlags(PdfAcroForm.SIGNATURE_EXIST); ByteArrayOutputStream baos = new ByteArrayOutputStream(); PdfReader reader = new PdfReader(new ByteArrayInputStream(pdfBytes)); PdfSigner signer = new PdfSigner(reader, baos, new StampingProperties().useAppendMode()); signer.setSignDate(signDate); MyDigestCalculator calcDigest = new MyDigestCalculator(PdfName.Adobe_PPKLite, PdfName.Adbe_pkcs7_detached); signer.signExternalContainer(calcDigest, 8120); pdfBytes = baos.toByteArray(); String digest = (digestHash != null ? new BigInteger(1, calcDigest.pdfDigest).toString(16) : null); return digest;
}
}

7 Related questions 4398 How do I avoid checking for nulls in Java? 4701 How do I read / convert an InputStream into a String in Java? 4089 How do I generate random integers within a specific range in Java? Related questions 4398 How do I avoid checking for nulls in Java? 4701 How do I read / convert an InputStream into a String in Java? 4089 How do I generate random integers within a specific range in Java? 3723 How can I create a memory leak in Java? 3966 How do I efficiently iterate over each entry in a Java Map? 3494 How do I convert a String to an int in Java? 2663 How do I call one constructor from another in Java? 3208 How do I test a class that has private methods, fields or inner classes? 2701 How do I determine whether an array contains a particular value in Java? 2718 How can I fix 'android.os.NetworkOnMainThreadException'? Load 7 more related questions Show fewer related questions Sorted by: Reset to default

Know someone who can answer? Share a link to this question via email, Twitter, or Facebook.

Your Answer

Sign up or log in

Sign up using Google Sign up using Facebook Sign up using Email and Password Submit

Post as a guest

Post Your Answer Discard

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.