C#后端

        public sealed class Base64
        {
                /// <summary>
                /// Base64加密
                /// </summary>
                /// <param name="codeName">加密采用的编码方式</param>
                /// <param name="source">待加密的明文</param>
                /// <returns></returns>
                public static string EncodeBase64(Encoding encode, string source)
                {
                        byte[] bytes = encode.GetBytes(source);
                        try
                        {
                                encode = Convert.ToBase64String(bytes);
                        }
                        catch
                        {
                                encode = source;
                        }
                        return encode;
                }
  
                /// <summary>
                /// Base64加密，采用utf8编码方式加密
                /// </summary>
                /// <param name="source">待加密的明文</param>
                /// <returns>加密后的字符串</returns>
                public static string EncodeBase64(string source)
                {
                        return EncodeBase64(Encoding.UTF8, source);
                }
  
                /// <summary>
                /// Base64解密
                /// </summary>
                /// <param name="codeName">解密采用的编码方式，注意和加密时采用的方式一致</param>
                /// <param name="result">待解密的密文</param>
                /// <returns>解密后的字符串</returns>
                public static string DecodeBase64(Encoding encode, string result)
                {
                        string decode = "";
                        byte[] bytes = Convert.FromBase64String(result);
                        try
                        {
                                decode = encode.GetString(bytes);
                        }
                        catch
                        {
                                decode = result;
                        }
                        return decode;
                }
  
                /// <summary>
                /// Base64解密，采用utf8编码方式解密
                /// </summary>
                /// <param name="result">待解密的密文</param>
                /// <returns>解密后的字符串</returns>
                public static string DecodeBase64(string result)
                {
                        return DecodeBase64(Encoding.UTF8, result);
                }
        }

JS前端

function Base64() {

    // private property
    _keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

    // public method for encoding
    this.encode = function(input) {
        var output = "";
        var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
        var i = 0;
        input = _utf8_encode(input);
        while (i < input.length) {
            chr1 = input.charCodeAt(i++);
            chr2 = input.charCodeAt(i++);
            chr3 = input.charCodeAt(i++);
            enc1 = chr1 >> 2;
            enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
            enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
            enc4 = chr3 & 63;
            if (isNaN(chr2)) {
                enc3 = enc4 = 64;
            } else if (isNaN(chr3)) {
                enc4 = 64;
            }
            output = output +
                            _keyStr.charAt(enc1) + _keyStr.charAt(enc2) +
                            _keyStr.charAt(enc3) + _keyStr.charAt(enc4);
        }
        return output;
    }

    // public method for decoding
    this.decode = function(input) {
        var output = "";
        var chr1, chr2, chr3;
        var enc1, enc2, enc3, enc4;
        var i = 0;
        input = input.replace(/[^A-Za-z0-9\+\/\=]/g, "");
        while (i < input.length) {
            enc1 = _keyStr.indexOf(input.charAt(i++));
            enc2 = _keyStr.indexOf(input.charAt(i++));
            enc3 = _keyStr.indexOf(input.charAt(i++));
            enc4 = _keyStr.indexOf(input.charAt(i++));
            chr1 = (enc1 << 2) | (enc2 >> 4);
            chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
            chr3 = ((enc3 & 3) << 6) | enc4;
            output = output + String.fromCharCode(chr1);
            if (enc3 != 64) {
                output = output + String.fromCharCode(chr2);
            }
            if (enc4 != 64) {
                output = output + String.fromCharCode(chr3);
            }
        }
        output = _utf8_decode(output);
        return output;
    }

    // private method for UTF-8 encoding
    _utf8_encode = function(string) {
        string = string.replace(/\r\n/g, "\n");
        var utftext = "";
        for (var n = 0; n < string.length; n++) {
            var c = string.charCodeAt(n);
            if (c < 128) {
                utftext += String.fromCharCode(c);
            } else if ((c > 127) && (c < 2048)) {
                utftext += String.fromCharCode((c >> 6) | 192);
                utftext += String.fromCharCode((c & 63) | 128);
            } else {
                utftext += String.fromCharCode((c >> 12) | 224);
                utftext += String.fromCharCode(((c >> 6) & 63) | 128);
                utftext += String.fromCharCode((c & 63) | 128);
            }

        }
        return utftext;
    }

    // private method for UTF-8 decoding
    _utf8_decode = function(utftext) {
        var string = "";
        var i = 0;
        var c = c1 = c2 = 0;
        while (i < utftext.length) {
            c = utftext.charCodeAt(i);
            if (c < 128) {
                string += String.fromCharCode(c);
                i++;
            } else if ((c > 191) && (c < 224)) {
                c2 = utftext.charCodeAt(i + 1);
                string += String.fromCharCode(((c & 31) << 6) | (c2 & 63));
                i += 2;
            } else {
                c2 = utftext.charCodeAt(i + 1);
                c3 = utftext.charCodeAt(i + 2);
                string += String.fromCharCode(((c & 15) << 12) | ((c2 & 63) << 6) | (c3 & 63));
                i += 3;
            }
        }
        return string;
    }
}

使用

//前端加密
var a = $("#txtDepart").val();
            var base = new Base64();
            var result = base.encode(a);
            $("#test").val(result);

//后端解密
var a = this.test.Value;
var b = this.Base64Decode(a);