node-signpdf0.3.0 • Public • Published
Simple signing of PDFs in node.
The purpose of this package is not as much to be used as a dependendency, although it could. The main purpose is to demonstrate the way signing can be achieved in a piece of readable code as it can take a lot of hours to figure out.
Simply said this could be used in two steps.
npm i -S node-signpdf node-forge.
;const signedPdf = signer;
In practice we expect that most people will just read through the code we've written in the testing part of this package and figure it out themselves. If that's your case, you should read the [Signing PDF in simple steps] section.
To verify a signed pdf call
;...const signedPdfBuffer = signer;const verified = signer;
- The process of signing a document is described in the Digital Signatures in PDF document.
- This lib:
- requires the signature placeholder to already be in the document;
- requires the
Contentsdescriptor in the
Sigbe placed after the
Buffers of the PDF and a P12 certificate to use when signing;
- does not cover multiple signatures, incremental updates, etc. Only the basic scenario of signing a freshly created PDF. We actually only worked with documents created with PDFKit;
- Feel free to copy and paste any part of this code. See its defined Purpose.
Signing PDF in simple steps
Generate a PDF
See the unit-testing code. PDFKit is used there for generating the document. This also allows easy addition of the signature placeholder.
Append a signature placeholder
What's needed is a
Sig element and a
Widget that is also linked in a
Form. The form needs to be referenced in the root descriptor of the PDF as well. A (hopefully) readable sample is available in the helpers. Note the
Contents descriptor of the
Sig where zeros are placed that will later be replaced with the actual signature.
Note: Signing in detached mode makes the signature length independent of the PDF's content length, but it may still vary between different signing certificates. So every time you sign using the same P12 you will get the same length of the output signature, no matter the length of the signed content. It is safe to find out the actual signature length your certificate produces and use it to properly configure the placeholder length.
Generate and apply signature
node-signpdf kicks in. Given a PDF and a P12 certificate a signature is generated in detached mode and is replaced in the placeholder. This is best demonstrated in the tests.
Verifying PDF signature
The signed PDF file has the public certificate embeded in it, so all we need to verify a PDF file is the file itself.
node-forge is used for working with signatures.
PDFKit is used in the tests for generating a PDF with a signature placeholder.
- The whole signing flow is a rework of what's already in pdfsign.js so thanks go to @tbocek
- Thanks to the guys of PDFKit as well. They've made PDF generation incredibly easy.
- This incredible Stack Overflow answer for describing the whole process of verifying PKCS7 signatures.
- All PRs are welcome in the
- This is a git-flow repo. We use the default git flow with a
- Note that gitmoji is used in the commit messages. That's not a must but we think it's nice.