How Can I Digitally Sign PDFs Using Pure JavaScript and Browser Extensions?

Signing PDFs with Pure JavaScript and WebCrypto API

WebCrypto API, with its increasing support in popular browsers like Chrome and Firefox, offers a compelling solution for digitally signing PDF documents. While the API supports signing operations, it faces limitations in accessing local key stores or utilizing external crypto devices (such as USB/Smartcards).

To address these challenges, a pragmatic approach is to create a hash of the PDF file for signing, rather than sending the entire file to the browser. This hash is then sent to the browser, where JavaScript can invoke a browser extension leveraging the WebCrypto API. The browser extension, in turn, interacts with a local application or crypto device (running behind the scenes) to perform the actual signing operation. Once signed, the signature is returned to the browser and subsequently sent to a server for embedding into the original PDF.

For convenient browser-based signing scenarios, Signer.Digital offers a free browser extension and .NET library for integration with server-side processes. The extension facilitates secure hash signing using the user's local keystore, USB tokens, or smartcards. The extension's JavaScript API allows for effortless interaction with the local signing host:

//Calling the extension method to sign the hash
SignerDigital.signPdfHash(hash, $("#CertThumbPrint").val(), "SHA-256")
  .then(
    function (signDataResp) {
      //Send signed data to server
    },
    function (errmsg) {
      //Handle error message from extension
    }
  );

Upon successful signing, the browser extension returns a base64-encoded PKCS7 signature, which can be used to inject the signature into the PDF using appropriate libraries. In case of errors, the extension provides a meaningful error message.

Signer.Digital offers a comprehensive digital signing solution from within the browser. The approach ensures robust and secure signing without compromising the user's private key or exposing the sensitive PDF content to the public internet.

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