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https://issues.apache.org/jira/browse/PDFBOX-1613?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13671565#comment-13671565
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Thomas Chojecki commented on PDFBOX-1613:
-----------------------------------------
Maybe I miss a use case or it was just late and I make a mistake.
Here a part of the code from COSWriter
Long idTime = doc.getDocumentId() == null ? System.currentTimeMillis() :
doc.getDocumentId();
if documentId was not set (default case), we use the the current timestamp,
otherwise we use the documentId that the user set.
The document id will only be fix if someone set the documentId through the
setter, otherwise it will be always the timestamp.
The normal use case will be, creating or loading a pdf. After alter the
document and saving it, a new ID will be generated each time.
The only case where the ID stay fix is if the user set this Id through the
setter and save the document. Normally a user will save the document once.
After this maybe it will open the document once again and create a new
PDDocument object which will use the default behavior (current timestamp) and
not the fixed one.
If a user using the PDDocument object more than one time for saving (don't know
a reason for this) he can set the documentId to null ( #setDocumentId(null) )
so he will get the old behavior. I think this setter turn on a specific
feature, so the user will know what he is doing.
Please correct me if I'm wrong. I have no feeling if this is good or bad, but
it makes it possible using only the convenience classes to set an Id.
Your original code inject the Id in the COSWriter which is a low level api
call. Normally a user do not trigger the COSWriter.write(pdDocument) directly.
For this, he will need to understand what happens behind the scene (
#saveIncremental(....))
Best regards
Thomas
> The ability to inject the time/random component into the COSWriter process to
> write a PDF document allows some advanced signature creation scenarios where
> the signature is generated on a separate server that does not hold the full
> PDF document.
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: PDFBOX-1613
> URL: https://issues.apache.org/jira/browse/PDFBOX-1613
> Project: PDFBox
> Issue Type: Improvement
> Components: Writing
> Affects Versions: 1.8.1
> Environment: Any
> Reporter: Stefan Santesson
> Assignee: Thomas Chojecki
> Priority: Minor
> Labels: security
> Fix For: 2.0.0
>
>
> I have developed a prototype server based signing service for the Swedish
> National eID infrastructure.
> I'll skip the details, but I recently switched to PDFBox for the PDF signing
> process and it works great. However, I had to modify the COSWriter class to
> get this working.
> I'm writing to check whether you would consider adding the functionality I
> need to future version of PDFBox.
> The problem is the the signature service is just producing the signature, it
> is not trusted to handle the PDF document.
> The government service having the PDF document signed is using PDFBox in a 2
> step process.
> 1) To produce the SignedAttributes DER Object of the CMS signature to be
> created. This is the part that is hashed and signed by the signature service.
> 2) After receiving the signature and signature certs from the signature
> service, completing the PDF signature by delivering the complete PKCS#7
> object to PDFBox using the externally generated signature value and certs.
> There are probably a more pure way to handle this, but Since PDFBox allows me
> to create a signature interface that produces the SignedData. I found it to
> be the easiest way to run the signature process 2 times.
> 1st pass using dummy key and dummy certs. This only to obtain the
> SignedAttributes.
> 2nd pass by delivering a SignedData object that include the Signature value
> and certs produced by the signature service.
> Now in order to do this, I have to control the random seed added by the
> COSWriter, or else the signature created by the signature service will not
> match the hash in the SignedAttributes produced in the second pass.
> My modification is provided below.
> I simply provided an extra input parameter to the write function where I can
> provide the long seed
> I then added a backwards compatible write function where the long seed is
> current time.
> By providing the same seed to pass 1 and pass 2, I can get the externally
> created signature to match the SignedAttributes produced in the first pass.
> The write function below is identical to the original COSWriter function
> except that it takes the idTime value from the function input parameter
> instead of getting it from System.currentTimeMillis().
> Modified functions of COSWriter:
> /**
> * This will write the pdf document.
> *
> * @param doc The document to write.
> *
> * @throws COSVisitorException If an error occurs while generating the data.
> */
> public void write(PDDocument doc) throws COSVisitorException {
> write(doc, System.currentTimeMillis());
> }
> /**
> * This will write the pdf document.
> *
> * @param doc The document to write.
> * @param idTime The time seed used to generate the id
> *
> * @throws COSVisitorException If an error occurs while generating the data.
> */
> public void write(PDDocument doc, long idTime) throws COSVisitorException {
> document = doc;
> if (incrementalUpdate) {
> prepareIncrement(doc);
> }
> // if the document says we should remove encryption, then we shouldn't
> encrypt
> if (doc.isAllSecurityToBeRemoved()) {
> this.willEncrypt = false;
> // also need to get rid of the "Encrypt" in the trailer so readers
> // don't try to decrypt a document which is not encrypted
> COSDocument cosDoc = doc.getDocument();
> COSDictionary trailer = cosDoc.getTrailer();
> trailer.removeItem(COSName.ENCRYPT);
> } else {
> SecurityHandler securityHandler = document.getSecurityHandler();
> if (securityHandler != null) {
> try {
> securityHandler.prepareDocumentForEncryption(document);
> this.willEncrypt = true;
> } catch (IOException e) {
> throw new COSVisitorException(e);
> } catch (CryptographyException e) {
> throw new COSVisitorException(e);
> }
> } else {
> this.willEncrypt = false;
> }
> }
> COSDocument cosDoc = document.getDocument();
> COSDictionary trailer = cosDoc.getTrailer();
> COSArray idArray = (COSArray) trailer.getDictionaryObject(COSName.ID);
> if (idArray == null || incrementalUpdate) {
> try {
> //algorithm says to use time/path/size/values in doc to generate
> //the id. We don't have path or size, so do the best we can
> MessageDigest md = MessageDigest.getInstance("MD5");
> md.update(Long.toString(idTime).getBytes("ISO-8859-1"));
> COSDictionary info = (COSDictionary)
> trailer.getDictionaryObject(COSName.INFO);
> if (info != null) {
> Iterator<COSBase> values = info.getValues().iterator();
> while (values.hasNext()) {
>
> md.update(values.next().toString().getBytes("ISO-8859-1"));
> }
> }
> idArray = new COSArray();
> COSString id = new COSString(md.digest());
> idArray.add(id);
> idArray.add(id);
> trailer.setItem(COSName.ID, idArray);
> } catch (NoSuchAlgorithmException e) {
> throw new COSVisitorException(e);
> } catch (UnsupportedEncodingException e) {
> throw new COSVisitorException(e);
> }
> }
> cosDoc.accept(this);
> }
> Finally. The way I use this in my signature process is by using this altered
> static function saveIncremental from the PDFDocument class.
> Since this function is static, I just call this duplicated function instead
> of the one in the PDFDocument class.
> Here I use my altered COSWriter (CsCOSWriter).
> /**
> * Save the pdf as incremental. This method is a modification of the same
> * method of PDDcoument. This method use an altered COSWriter that allows
> * control over the time used to create the ID of the document. This way
> it
> * is possible to perform two consecutive signature generation passes that
> * produce the same document hash.
> *
> * @param doc The document being written with signature creation
> * @param input An input file stream of the document being written
> * @param output An output file stream for the result document
> * @param idTime The time in milliseconds from Jan 1st, 1970 GMT when the
> * signature is created. This time is also used to calculate the ID of the
> * document.
> * @throws IOException if something went wrong
> * @throws COSVisitorException if something went wrong
> */
> public static void saveIncremental(PDDocument doc, FileInputStream input,
> OutputStream output, long idTime) throws IOException, COSVisitorException {
> //update the count in case any pages have been added behind the
> scenes.
> doc.getDocumentCatalog().getPages().updateCount();
> CsCOSWriter writer = null;
> try {
> // Sometimes the original file will be missing a newline at the
> end
> // In order to avoid having %%EOF the first object on the same
> line
> // as the %%EOF, we put a newline here. If there's already one at
> // the end of the file, an extra one won't hurt. PDFBOX-1051
> output.write("\r\n".getBytes());
> writer = new CsCOSWriter(output, input);
> writer.write(doc, idTime);
> writer.close();
> } finally {
> if (writer != null) {
> writer.close();
> }
> }
> }
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