How Can I Securely Sign PDFs Using JavaScript and WebCrypto API?

Sign PDF with JavaScript and WebCrypto API

Signing a PDF document digitally using JavaScript and the WebCrypto API has been a sought-after capability, especially with the improved support for WebCrypto in Chrome and Firefox. However, there's limited information available on this topic.

The WebCrypto API currently has limitations in accessing key stores and local crypto devices. Moreover, sending the entire PDF file to the browser or signing API server raises security concerns.

A best practice is to create a hash of the PDF, send that hash to the browser, and use JavaScript to invoke an application on the local system through a browser extension. This local application can then access the key store and produce the signature. The signed hash can be returned to the server, where it can be injected back into the PDF.

One solution to facilitate this process is Signer.Digital, a browser extension that provides access to the local key store and allows JavaScript to sign the hash. The signed hash is then sent back to the server, where it can be added to the PDF.

Working Mechanism

1. Calculate the hash of the PDF file.
2. Pass the hash to the Signer.Digital browser extension via JavaScript.
3. The extension invokes the host application, which signs the hash using the local key store.
4. The signed hash (PKCS7 signature) is returned to JavaScript.
5. JavaScript sends the signed hash back to the server.
6. The server injects the signature into the PDF file.

Code Sample

// Sign the hash using the browser extension
SignerDigital.signPdfHash(hash, $("#CertThumbPrint").val(), "SHA-256")
  .then(
    function (signDataResp) {
      // Success: Send signed data to the server
    },
    function (errmsg) {
      // Failure: Handle error
    }
  );

By integrating Signer.Digital, developers can digitally sign PDF documents in a secure and efficient manner, even with the limitations of the WebCrypto API.

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