How Can I Digitally Sign a PDF Using JavaScript Without Direct Key Access?

Sign PDF with Plain JavaScript

Challenge:

Digitally signing a PDF document using the WebCrypto API poses a challenge, as the current API doesn't directly allow access to key storage or local crypto devices.

Solution:

To circumvent this limitation, it's recommended to:

1. Create a Hash of the PDF: Generate a hash of the PDF file instead of sending the entire document to the browser.
2. Utilize a Browser Extension: Install a browser extension such as Signer.Digital to access local keystores (e.g., Windows Certificate store or PKCS#11 on Linux).
3. Sign the Hash: Use the extension's API to sign the hashed PDF using the private key obtained from the local keystore.
4. Receive the Signature: The browser extension will return the signature in a suitable format (e.g., PKCS7).
5. Inject Signature into PDF: On the server side, use an appropriate library (or the one provided by Signer.Digital) to inject the signature back into the original PDF file.

Signer.Digital Extension:

The Signer.Digital browser extension provides the necessary functionality to perform the signing process as follows:

• Function: SignerDigital.signPdfHash(hash, certThumbPrint, hashingAlgo)

• Parameters:

  • hash: Hash of the PDF in Base64-encoded string
  • certThumbPrint: Thumbprint of the certificate to use for signing
  • hashingAlgo: Algorithm used to hash the PDF (e.g., "SHA256")

Example:

// Assuming hash is a Base64-encoded PDF hash
SignerDigital.signPdfHash(hash, $("#CertThumbPrint").val(), "SHA-256")
.then(
   function (signDataResp) {
      // Send signDataResp (contains Base64 PKCS7 signature) to server
   },
   function (errmsg) {
      // Handle error
   }
);

Note:

• The private key is stored locally in the certificate settings of the browser.
• In the future, with the advancement of WebCrypto API, direct access to key storage might become possible, eliminating the need for browser extensions.

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