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From Stefan Santesson <ste...@aaa-sec.com>
Subject Request to control random seed for signing PDF
Date Thu, 16 May 2013 23:20:37 GMT
Hi,

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.

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.
Modifications/additions are highlighted in bold.

/Stefan Sanesson

    /**
     * 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);
    }



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