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

Signing PDFs with Plain JavaScript and WebCrypto API

Digitally signing a PDF document using WebCrypto API and pure JavaScript presents certain challenges. While the WebCrypto API has evolved and gained support in browsers such as Chrome and Firefox, it lacks direct access to key stores or local crypto devices.

For secure PDF signing, it's not advisable to send the entire PDF file to the browser or API server. Creating a hash of the PDF for signing and sending only the hash to the browser is a recommended approach.

Browser-Based Signing Considerations

To address the limitations of WebCrypto API in accessing local keystores, browser extensions like Signer.Digital provide a solution. The extension bridges the gap between the browser and a local host application running on the user's system. This allows for accessing local key stores, crypto USBs, or smart cards.

Using the Signer.Digital extension, JavaScript code can call methods to sign the hash of a document. The host application then utilizes the user's certificate through Microsoft Certificate store (on Windows) or PKCS#11 library (on Linux) to obtain the signature. The extension returns a Base64 encoded pkcs7 signature, which can be injected into the PDF.

Browser Extension Host Functions

The JavaScript code in the browser can invoke the following methods from the Signer.Digital Browser Extension Host:

• signPdfHash(hash, certThumbPrint, hashAlgorithm)
• signHashCAdESBr(hash, certThumbPrint, hashAlgorithm) (for ICP-Brazil signatures)
• signCAdESEg(hash, certThumbPrint, hashAlgorithm) (for Egypt ITIDA CAdES-BES signatures)

These methods enable JavaScript to obtain signatures from local key stores and crypto devices without compromising the security of private keys.

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