gistfile1.txt

//Object DetectSubtleCrypto()
//Establish the availability of various crypto.subtle API's necessary for keygen, signing, verification, hashing
function DetectSubtleCrypto(){
  var supportedApis = {};
  var baselineSupport = (
                            (crypto && crypto.subtle)
                            && (
                                (location.protocol === "https:" || "chrome-extension:" || "chrome:") 
                                || (location.hostname === "localhost" || "127.0.0.1") 
                               )
                        ) ? true : false ;
  if (!baselineSupport) {
    supportedApis = {}
  } else {
    try {
      crypto.subtle.generateKey(
      { name: "RSASSA-PKCS1-v1_5", modulusLength: 2048, hash:{name:"SHA-256"}, publicExponent: new Uint8Array([0x01, 0x00, 0x01]) },
        true, //exportable;
        ["sign", "verify"]).then(function(result){
          if (result.publicKey && result.privateKey) {
            supportedApis.generateRSASSAKey = true;
            try{
             crypto.subtle.exportKey("pkcs8",key.privateKey).then(function(result){
                supportedApis.exportPrivatePKCS8 = true;
             });
            } catch (e) {
             supportedApis.exportPrivatePKCS8 = false;
            }
            try {
             crypto.subtle.exportKey("raw",key.privateKey).then(function(result){
                supportedApis.exportPrivateRAW = true;
             });
            } catch (e) {
             supportedApis.exportPrivateRAW = false;
            }
            try {
             crypto.subtle.exportKey("jwk",key.privateKey).then(function(result){
                supportedApis.exportPrivateJWK = true;
             })
            } catch (e) {
             supportedApis.exportPrivateJWK = false;
            }
          } else {
            supportedApis.generateRSASSAKey = false
          }
      });
    } catch (e){
      supportedApis.sign = false;
      supportedApis.verify = false;
    }
    
    var testDigest = new Uint8Array(1000)
    try{
      crypto.subtle.digest({name:"SHA-256"}, testDigest.buffer).then(function(result){\
        supportedApis.digestSHA256 = true;
      });
    } catch (e) {
      supportedAPIs.digestSHA256 = false
    }
  }
  
  return function (desiredApisArray){
    //what I'm thinking here is that KeyChain could supply desired api's based on default/developer needs for encrypt/decrypt, key formats, storage formats, etc. please advise. 
    //The most important thing about returning this closure is that it allows us to empirically test the crypto capabilities of the browser asyncronously at the beginning of runtime, and get only use the features we're sure are implimented.
    return desiredApis.fulfilledBy(supportedApis);
  }
}

module.exports = DetectSubtleCrypto();