title: External Signature with PDFBox 2.0.15
body: <p>I'm implementing an application to sign PDF files in the server, with the follow scenario (to make long history, short):</p>
  
  <ol>
  <li>Client start signature sending to server, date/time and watermark</li>
  <li>Server add signature dictionaries into file and send data to be signed</li>
  <li>Client sign content</li>
  <li>Server finish the signature</li>
  </ol>
  
  <p>I'm using PDFBox 2.0.15, and making use of new feature <code>saveIncrementalForExternalSigning</code> as shown in the code below:</p>
  
  <pre><code>try {
          String name = document.getID();
          File signedFile = new File(workingDir.getAbsolutePath() + sep + name + "_Signed.pdf");
          this.log("[SIGNATURE] Creating signed version of the document");
          if (signedFile.exists()) {
              signedFile.delete();
          }
          FileOutputStream tbsFos = new FileOutputStream(signedFile);
          ExternalSigningSupport externalSigning = pdfdoc.saveIncrementalForExternalSigning(tbsFos);
  
          byte[] content = readExternalSignatureContent(externalSigning);
          if (postparams.get("action").equalsIgnoreCase("calc_hash")) {
              this.log("[SIGNATURE] Calculating hash of the document");
              String strBase64 = ParametersHandle.compressParamBase64(content);
  
              // this saves the file with a 0 signature
              externalSigning.setSignature(new byte[0]);
  
              // remember the offset (add 1 because of "&lt;")
              int offset = signature.getByteRange()[1] + 1;
  
              this.log("[SIGNATURE] Sending calculated hash to APP");
              return new String[] { strBase64, processID, String.valueOf(offset) };
          } else {
              this.log("[SIGNATURE] Signature received from APP");
              String signature64 = postparams.get("sign_disgest");
              byte[] cmsSignature = ParametersHandle.decompressParamFromBase64(signature64);
  
              this.log("[SIGNATURE] Setting signature to document");
              externalSigning.setSignature(cmsSignature);
  
              pdfdoc.close();
  
              IOUtils.closeQuietly(signatureOptions);
  
              this.log("[DOXIS] Creating new version of document on Doxis");
              createNewVersionOfDocument(doxisServer, documentServer, doxisSession, document, signedFile);
  
              return new String[] { "SIGNOK" };
          }
      } catch (IOException ex) {
          this.log("[SAVE FOR SIGN] " + ex);
          return null;
      }
  </code></pre>
  
  <p>In the "IF" statement I'm generating data to be signed. In the "ELSE" statement adding the signature, that comes via post request (that is what <code>ParametersHandle.decompressParamFromBase64</code> does), into document. So I have two post requests for this method in this try.</p>
  
  <p>A second approach was doing each post request in one method, so I have this second code block:</p>
  
  <pre><code>// remember the offset (add 1 because of "&lt;") 
          int offset = Integer.valueOf(postparams.get("offset"));
          this.log("[PDF BOX] Retrieving offset of bytes range for this signature. The value is: "
                  + String.valueOf(offset));
  
          File signedPDF = new File(workingDir.getAbsolutePath() + sep + name + "_Signed.pdf");
          this.log("[SIGNATURE] Reloading document for apply signature: " + signedPDF.getAbsolutePath());
  
          // invoke external signature service
          String signature64 = postparams.get("sign_disgest");
          byte[] cmsSignature = ParametersHandle.decompressParamFromBase64(signature64);
  
          this.log("[SIGNATURE] Got signature byte array from APP.");
          // set signature bytes received from the service
  
          // now write the signature at the correct offset without any PDFBox methods
          this.log("[SIGNATURE] Writing signed document...");
          RandomAccessFile raf = new RandomAccessFile(signedPDF, "rw");
          raf.seek(offset);
          raf.write(Hex.getBytes(cmsSignature));
          raf.close();
          this.log("[SIGNATURE] New signed document has been saved!");
  </code></pre>
  
  <p>The problem is: I'm getting the error "The document has been altered or corrupted since the Signature was applied" when validating it on Adobe Reader.
  On my understanding it should not happen since the offset of the signature byte range is being remembered on the second post call.</p>
  
  <p>Any help or idea is appreciated,</p>
  
  <p>Thank you in advance.</p>
  
  <p>[EDIT]</p>
  
  <p>For a complete list of used files: <a href="https://drive.google.com/drive/folders/1S9a88lCGaQYujlEyCrhyzqvmWB-68LR3" rel="nofollow noreferrer">https://drive.google.com/drive/folders/1S9a88lCGaQYujlEyCrhyzqvmWB-68LR3</a></p>
  
  <p>[EDIT 2]</p>
  
  <p>Based on @mkl comment, here is the method where the signature is made:</p>
  
  <pre><code>public byte[] sign(byte[] hash)
          throws IOException, NoSuchAlgorithmException, InvalidKeyException, SignatureException {
      PrivateKey privKey = (PrivateKey) windowsCertRep.getPrivateKey(this.selected_alias, "");
      X509Certificate[] certificateChain = windowsCertRep.getCertificateChain(this.selected_alias);
  
      try
      {
          CMSSignedDataGenerator gen = new CMSSignedDataGenerator();
          X509Certificate cert = (X509Certificate) certificateChain[0];
          ContentSigner sha1Signer = new JcaContentSignerBuilder("SHA256WithRSA").build(privKey);
          gen.addSignerInfoGenerator(new JcaSignerInfoGeneratorBuilder(new JcaDigestCalculatorProviderBuilder().build()).build(sha1Signer, cert));
          gen.addCertificates(new JcaCertStore(Arrays.asList(certificateChain)));
          CMSProcessableInputStream msg = new CMSProcessableInputStream(new ByteArrayInputStream(hash));
          CMSSignedData signedData = gen.generate(msg, false);
          return signedData.getEncoded();
      }
      catch (GeneralSecurityException e)
      {
          throw new IOException(e);
      }
      catch (CMSException e)
      {
          throw new IOException(e);
      }
      catch (OperatorCreationException e)
      {
          throw new IOException(e);
      }
  
  }
  </code></pre>
  
  <p>I've tested the <code>CreateVisibleSignature2</code> examaple, replacing the <code>sign</code> method for one calling this service that returns me the signature, e it works.</p>