Add a funtion for guessing the signature count of a PDF file

We require the signature count in the future to force users to
do a manual positioning when there are already signatures in the file
since we can't add a new page for them.

This uses a simple regex, matching parts of the signatures we use (PAdES).

In case the result is wrong, worst case the user has to do extra work by positioning
or the auto positioning will put it at the top of the first page, which might not
be ideal, but the document is still signed either way.

src/dbp-pdf-preview.js

@@ -10,6 +10,7 @@ import * as commonStyles from '@dbp-toolkit/common/styles';
 import pdfjs from 'pdfjs-dist/es5/build/pdf.js';
 import buildinfo from 'consts:buildinfo';
 import {name as pkgName} from './../package.json';
+import {getPDFSignatureCount} from './utils.js';
 
 const i18n = createI18nInstance();
 
@@ -238,6 +239,8 @@ export class PdfPreview extends ScopedElementsMixin(DBPLitElement) {
         };
 
         reader.readAsBinaryString(file);
+
+        console.log(`Signature count: ${await getPDFSignatureCount(file)}`);
     }
 
     getSignatureRect() {

src/utils.js

@@ -80,3 +80,36 @@ export const fabricjs2pdfasPosition = (data) => {
         p: data.currentPage
     };
 };
+
+/**
+ * Given a PDF file returns the amount of signatures found in it.
+ *
+ * Note that this uses an heuristic, so the result can be wrong
+ * (improvements welcome).
+ *
+ * @param {File} file The PDF file object
+ * @returns {number} The amount of signatures found
+ */
+export const getPDFSignatureCount = async (file) => {
+    const sigRegex = new RegExp(
+        "/Type\\s*/Sig\\s*/Filter\\s*/Adobe.PPKLite\\s*/SubFilter\\s*(/ETSI\\.CAdES\\.detached|/adbe\\.pkcs7\\.detached)",
+        "g");
+
+    const promise = new Promise((resolve, reject) => {
+        let reader = new FileReader();
+        reader.onload = async () => {
+            let result = reader.result;
+            let matches = 0;
+            while (sigRegex.exec(result) !== null) {
+                matches++;
+            }
+            resolve(matches);
+        };
+        reader.onerror = async () => {
+            reject(reader.error);
+        };
+        reader.readAsBinaryString(file);
+    });
+
+    return promise;
+};
\ No newline at end of file

test/unit.js

@@ -2,6 +2,7 @@ import {assert} from 'chai';
 
 import '../src/dbp-official-signature-pdf-upload';
 import '../src/dbp-signature.js';
+import {getPDFSignatureCount} from '../src/utils.js';
 
 suite('dbp-official-signature-pdf-upload basics', () => {
   let node;
@@ -39,3 +40,24 @@ suite('dbp-signature-app basics', () => {
   });
 });
 
+suite('pdf signature detection', () => {
+
+    function getPDFFile(data) {
+        return new File([new Blob([data])], 'test.pdf', {type: 'application/pdf'});
+    }
+
+    test('getPDFSignatureCount', async () => {
+        let sig1 = "/Type\n/Sig\n/Filter\n/Adobe.PPKLite\n/SubFilter\n/ETSI.CAdES.detached";
+        let sig2 = "/Type\n/Sig\n/Filter\n/Adobe.PPKLite\n/SubFilter\n/adbe.pkcs7.detached";
+
+        assert(await getPDFSignatureCount(getPDFFile(sig1)) === 1);
+        assert(await getPDFSignatureCount(getPDFFile(sig2)) === 1);
+        assert(await getPDFSignatureCount(getPDFFile(sig1 + sig2)) === 2);
+        assert(await getPDFSignatureCount(getPDFFile("foo" + sig1 + "bar" + sig2 + "quux")) === 2);
+        assert(await getPDFSignatureCount(getPDFFile("\nfoo" + sig1 + "bar\n")) === 1);
+        assert(await getPDFSignatureCount(getPDFFile("\nfoo" + sig2 + "bar\n")) === 1);
+
+        assert(await getPDFSignatureCount(getPDFFile("foobar")) === 0);
+        assert(await getPDFSignatureCount(getPDFFile("")) === 0);
+    });
+});