/**
 * convert RFC 1342-like base64 strings to array buffer
 * @param {mixed} obj
 * @returns {undefined}
 */
function recursiveBase64StrToArrayBuffer(obj) {
    let prefix = '=?BINARY?B?';
    let suffix = '?=';
    if (typeof obj === 'object') {
        for (let key in obj) {
            if (typeof obj[key] === 'string') {
                let str = obj[key];
                if (str.substring(0, prefix.length) === prefix && str.substring(str.length - suffix.length) === suffix) {
                    str = str.substring(prefix.length, str.length - suffix.length);

                    let binary_string = window.atob(str);
                    let len = binary_string.length;
                    let bytes = new Uint8Array(len);
                    for (var i = 0; i < len; i++)        {
                        bytes[i] = binary_string.charCodeAt(i);
                    }
                    obj[key] = bytes.buffer;
                }
            } else {
                recursiveBase64StrToArrayBuffer(obj[key]);
            }
        }
    }
}

/**
 * Convert a ArrayBuffer to Base64
 * @param {ArrayBuffer} buffer
 * @returns {String}
 */
function arrayBufferToBase64(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 window.btoa(binary);
}