(Node.js) Decrypt 256-bit AES GCM Produced by Something Unknown

Demonstrates how to decrypt something produced elsewhere (unknown) with 256-bit AES GCM.

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    // We have the following to decrypt:

    // Key (Base64): 
    var keyBase64 = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";

    // IV (Base64Url):
    var ivBase64Url = "xrvaINMLqotAbWRK";

    // ciphertext (base64url):
    var cipherBase64Url = "RtGNAS-zQOxSB8W0HfqJjCoyt9KgImW_l-HjVC40hOOl-RNfRF3hzDIT1kvFVF8i_KX9XmqAftb6lyq-jLCEc_MSgqt3q1ixv3Ez4SbS3G5e3qGzLwxIMi2sCt00aDNwK2ipsJ4aw8s7ePPnl4oY-y1st9rwCWR0rrgEZwS9jmS4uJWGPn9K3jbKRnMslznDbtFLNJctMVXBTP-cv47JelxLCBOQSlK29rMuEFrhHR_VQrPq6gtZaBVSXZSYT0XOklp7nu9mVhrMCRtBCC5oiu5MPE5JYx4ANo3hUY7_NyQl2bpn9GfRXrdvqRGE-gy2upj-cDkm0t_tV8xmYge9DBQTH3B_4BGl2qTk_o-m7pEmKkS8XSdQhGcuFlykqrkE8SzB5I8esfzWOM0pwxbz0H_VaylKYHY=";

    var crypt = new chilkat.Crypt2();

    crypt.CryptAlgorithm = "aes";
    crypt.CipherMode = "gcm";
    crypt.KeyLength = 256;

    var success = crypt.SetEncodedAad("random","ascii");
    crypt.SetEncodedKey(keyBase64,"base64");
    crypt.SetEncodedIV(ivBase64Url,"base64url");

    // The cipher text contains the 16-byte auth tag at the end.
    // get it separately..
    var bdEncrypted = new chilkat.BinData();
    var bdAuthTag = new chilkat.BinData();
    success = bdEncrypted.AppendEncoded(cipherBase64Url,"base64url");

    var numBytes = bdEncrypted.NumBytes;
    var authTagHex = bdEncrypted.GetEncodedChunk(numBytes - 16,16,"hex");

    console.log("Auth tag in hex: " + authTagHex);

    success = bdAuthTag.AppendEncoded(authTagHex,"hex");
    bdEncrypted.RemoveChunk(numBytes - 16,16);

    // Use this special value to tell Chilkat to ignore the auth tag.
    success = crypt.SetEncodedAuthTag("FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF","hex");

    // Decrypt
    crypt.EncodingMode = "base64";
    var originalText = crypt.DecryptStringENC(bdEncrypted.GetEncoded("base64"));
    if (crypt.LastMethodSuccess == false) {
        console.log(crypt.LastErrorText);
    }
    else {
        console.log(originalText);
        console.log("Success.");
    }

    // Decrypted text

}

chilkatExample();