Example of decoding Ecwid payment request using Web Cryptography API / SublteCrypto. https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto

decode_ecwid_request.js

var bodyData = 'ECWID PAYLOAD'; 
var clientId = 'YOUR CLIENT SECRET'; 

// Added required padding to make the payload a multiple of 4. We can do this using a repeat or a simple while.

// var paddingLength = 4 - (bodyData.length % 4);
// if (paddingLength !== 4) {
//     bodyData + '='.repeat(paddingLength);
// }

while (bodyData.length % 4 !== 0) {
    bodyData += '=';
}

var originalBase64 = bodyData.replace(/-/g, "+").replace(/_/g, "/");
var decodedBase64 = atob(originalBase64);
var iv = str2ab(decodedBase64.substring(0, 16));
var cipherOrder = b642ab(originalBase64);
var keyData = str2ab(clientId.substring(0, 16));

async function decryptOrder() {
	var key = await importKey();
	try {
		return await crypto.subtle.decrypt(
			{ name: "AES-CBC", iv: iv },
			key,
			cipherOrder
		);
	} catch (exception) {
		console.error(
			"An error occurred, Name: ",
			exception.name,
			", Message: ",
			exception.message
		);
	}
}

async function importKey() {
	var key = await crypto.subtle.importKey(
		"raw",
		keyData,
		{ name: "AES-CBC" },
		true,
		["decrypt"]
	);
	return key;
}

// Concept from https://stackoverflow.com/a/11058858. Convert String to Array Buffer
function str2ab(str) {
	const buf = new ArrayBuffer(str.length);
	const bufView = new Uint8Array(buf);
	for (let i = 0, strLen = str.length; i < strLen; i++) {
		bufView[i] = str.charCodeAt(i);
	}
	return buf;
}

// Concept from https://stackoverflow.com/a/21797381/9014097 Convert Base64 to Array Buffer.
function b642ab(base64) {
	var binary_string = window.atob(base64);
	var len = binary_string.length;
	var bytes = new Uint8Array(len);
	for (var i = 0; i < len; i++) {
		bytes[i] = binary_string.charCodeAt(i);
	}
	return bytes.buffer;
}

(async () => {
	var decrypted = await decryptOrder();
	var decryptedStr = new TextDecoder().decode(decrypted);
	console.log(decryptedStr.substring(16));
})();

@parasdaryanani Thats a good solution! I saw the problem with 15/16 bytes but haven't had time to check why it happens.

@parasdaryanani Hey! I’ve modified the gist to include the required padding to make the payload multiple of 4, that should solve the issue with 15/16 bytes.

@manuelfdo brilliant, thank you!