How to load a PKCS#12 Digital Certificate with Javascript WebCrypto API

Web cryptography api does not support PKCS # 12. You can use a third party library to decode the p12 as forge https://github.com/digitalbazaar/forge#pkcs12 and load privateKey in webcrypto

Reading the PKCS#12 certificate

PKCS#12 is stored in DER, so first read it from a File or use a pre-stored base64

//Reading certificate from a 'file' form field
var reader = new FileReader();
reader.onload = function(e) {               
    var contents = e.target.result;
    var pkcs12Der = arrayBufferToString(contents)
    var pkcs12B64 = forge.util.encode64(pkcs12Der);     
    //do something else...

}   
reader.readAsArrayBuffer(file);

function arrayBufferToString( buffer ) {
    var binary = '';
    var bytes = new Uint8Array( buffer );
    var len = bytes.byteLength;
    for (var i = 0; i < len; i++) {
        binary += String.fromCharCode( bytes[ i ] );
    }
    return binary;
}

//p12 certificate stored in Base64 format
var pkcs12Der= forge.util.decode64(pkcs12B64);

Decode PKCS#12 with forge and extract private key

Then decode DER format to ASN1, and let forge reads the content

var pkcs12Asn1 = forge.asn1.fromDer(pkcs12Der);
var pkcs12 = forge.pkcs12.pkcs12FromAsn1(pkcs12Asn1, false, password);

Then get the private key from pkcs12 of the desired certificate (see forge doc) and convert to PKCS # 8 to be imported with webcrypto

// load keypair and cert chain from safe content(s) 
for(var sci = 0; sci < pkcs12.safeContents.length; ++sci) {
    var safeContents = pkcs12.safeContents[sci];

    for(var sbi = 0; sbi < safeContents.safeBags.length; ++sbi) {
        var safeBag = safeContents.safeBags[sbi];

        // this bag has a private key
        if(safeBag.type === forge.pki.oids.keyBag) {
            //Found plain private key
            privateKey = safeBag.key;
        } else if(safeBag.type === forge.pki.oids.pkcs8ShroudedKeyBag) {
            // found encrypted private key
            privateKey = safeBag.key;
        } else if(safeBag.type === forge.pki.oids.certBag) {
            // this bag has a certificate...        
        }   
    }
}

Convert to PKCS#8

function _privateKeyToPkcs8(privateKey) {
     var rsaPrivateKey = forge.pki.privateKeyToAsn1(privateKey);
     var privateKeyInfo = forge.pki.wrapRsaPrivateKey(rsaPrivateKey);
     var privateKeyInfoDer = forge.asn1.toDer(privateKeyInfo).getBytes();
     var privateKeyInfoDerBuff = stringToArrayBuffer(privateKeyInfoDer);
     return privateKeyInfoDerBuff;
 }
 function stringToArrayBuffer(data){
     var arrBuff = new ArrayBuffer(data.length);
     var writer = new Uint8Array(arrBuff);
     for (var i = 0, len = data.length; i < len; i++) {
         writer[i] = data.charCodeAt(i);
     }
     return arrBuff;
  }

Import key in Webcrypto

And finally import the key in webcrypto

function _importCryptoKeyPkcs8(privateKey,extractable) {
    var privateKeyInfoDerBuff = _privateKeyToPkcs8(privateKey);

    //Import the webcrypto key
    return crypto.subtle.importKey(
            'pkcs8', 
            privateKeyInfoDerBuff, 
            { name: "RSASSA-PKCS1-v1_5", hash:{name:"SHA-256"}},
            extractable, 
            ["sign"]);        

}
_importCryptoKeyPkcs8(entry.privateKey,extractable).    
        then(function(cryptoKey) {
            //your cryptokey is here!!!
        });

Digital signature

With the imported cryptoKey returned from the above method you can sign with webcrypto.

var digestToSign = forge.util.decode64(digestToSignB64);
var digestToSignBuf = stringToArrayBuffer(digestToSign);

crypto.subtle.sign(
            {name: "RSASSA-PKCS1-v1_5"},
            cryptoKey,
            digestToSignBuf)
.then(function(signature){
    signatureB64 = forge.util.encode64(arrayBufferToString(signature))
});

I include coding from base64 because data conversions are not trivial

In pkc12 you also have the certification chain if you need to build advanced formats like AdES