Generating signed messages using CMS and PKI.js

One of the most common signature formats on the web is known as CMS SignedData, this is the signature format used in PDF files, CAdES, S/MIME and several other digital signature solutions.

As a signature it has a few notable features:

  1. Having multiple signers.
  2. Including meta-data that will be signed along with the data that is being signed.
  3. Including meta-data that is outside the scope of the signature.
  4. Signing data contained within the signature or data referenced by it.

These traits mean you can do some interesting things like implementing counter-signing in-turn enabling notarization scenarios.

Utilizing PKI.js you can now create and verify this signature format, bellow is an example of how creating one of these messages looks using this library:

// #region Put a static values 
var sample_data = new Uint8Array(sample_data);
sample_data[0] = 0x00;
sample_data[1] = 0x01;
sample_data[2] = 0x02;
sample_data[3] = 0x03;
sample_data[4] = 0x04;

cms_signed_simpl = new org.pkijs.simpl.CMS_SIGNED_DATA({
    digestAlgorithms: [
        new org.pkijs.simpl.ALGORITHM_IDENTIFIER({ algorithm_id: "" }) // SHA-1
    encapContentInfo: new org.pkijs.simpl.cms.EncapsulatedContentInfo({
        eContentType: "1.2.840.113549.1.7.1", // "data" content type
        eContent: new org.pkijs.asn1.OCTETSTRING({ value_hex: sample_data })
    signerInfos: [
        new org.pkijs.simpl.CMS_SIGNER_INFO({
            sid: new org.pkijs.simpl.cms.IssuerAndSerialNumber({
                issuer: cert_simpl.issuer,
                serialNumber: cert_simpl.serialNumber
            digestAlgorithm: new org.pkijs.simpl.ALGORITHM_IDENTIFIER({ algorithm_id: "" }), // SHA-1
            signatureAlgorithm: new org.pkijs.simpl.ALGORITHM_IDENTIFIER({ algorithm_id: "1.2.840.113549.1.1.5" }), // RSA + SHA-1
    certificates: [cert_simpl]
// #endregion 

return cms_signed_simpl.sign(privateKey, 0);

In this sample you can see we are putting our content to be signed within the SignedData message and then signing it with RSA and SHA1, this is in-the exact same thing that is needed to implement what is called opaque signed email in S/MIME.


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