C# 下base64加密解密实现

using System;
using System.Collections.Generic;
using System.Text;

namespace MyLibs.encrypt
{
    public class Base64
    {
        private static string base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";//编码后的字符集
        private static int[] base64DecodeChars = new int[] { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1 };//对应ASICC字符的位置
        public static string base64encode(string str)
        { //加密
            string Out = "";
            int i = 0, len = str.Length;
            char c1, c2, c3;
            while (i < len)
            {
                c1 = Convert.ToChar(str[i++] & 0xff);
                if (i == len)
                {
                    Out += base64EncodeChars[c1 >> 2];
                    Out += base64EncodeChars[(c1 & 0x3) << 4];
                    Out += "==";
                    break;
                }
                c2 = str[i++];
                if (i == len)
                {
                    Out += base64EncodeChars[c1 >> 2];
                    Out += base64EncodeChars[((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4)];
                    Out += base64EncodeChars[(c2 & 0xF) << 2];
                    Out += "=";
                    break;
                }
                c3 = str[i++];
                Out += base64EncodeChars[c1 >> 2];
                Out += base64EncodeChars[((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4)];
                Out += base64EncodeChars[((c2 & 0xF) << 2) | ((c3 & 0xC0) >> 6)];
                Out += base64EncodeChars[c3 & 0x3F];
            }
            return Out;
        }
        public string utf16to8(string str)
        {
            string Out = "";
            int i, len;
            char c;//char为16位Unicode字符,范围0~0xffff,感谢vczh提醒
            len = str.Length;
            for (i = 0; i < len; i++)
            {//根据字符的不同范围分别转化
                c = str[i];
                if ((c >= 0x0001) && (c <= 0x007F))
                {
                    Out += str[i];
                }
                else if (c > 0x07FF)
                {
                    Out += (char)(0xE0 | ((c >> 12) & 0x0F));
                    Out += (char)(0x80 | ((c >> 6) & 0x3F));
                    Out += (char)(0x80 | ((c >> 0) & 0x3F));
                }
                else
                {
                    Out += (char)(0xC0 | ((c >> 6) & 0x1F));
                    Out += (char)(0x80 | ((c >> 0) & 0x3F));
                }
            }
            return Out;
        }

        public static string base64decode(string str)
        {//解密
            int c1, c2, c3, c4;
            int i, len;
            string Out;
            len = str.Length;
            i = 0; Out = "";
            while (i < len)
            {
                do
                {
                    c1 = base64DecodeChars[str[i++] & 0xff];
                } while (i < len && c1 == -1);
                if (c1 == -1) break;
                do
                {
                    c2 = base64DecodeChars[str[i++] & 0xff];
                } while (i < len && c2 == -1);
                if (c2 == -1) break;
                Out += (char)((c1 << 2) | ((c2 & 0x30) >> 4));
                do
                {
                    c3 = str[i++] & 0xff;
                    if (c3 == 61) return Out;
                    c3 = base64DecodeChars[c3];
                } while (i < len && c3 == -1);
                if (c3 == -1) break;
                Out += (char)(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));
                do
                {
                    c4 = str[i++] & 0xff;
                    if (c4 == 61) return Out;
                    c4 = base64DecodeChars[c4];
                } while (i < len && c4 == -1);
                if (c4 == -1) break;
                Out += (char)(((c3 & 0x03) << 6) | c4);
            }
            return Out;
        }

        public string utf8to16(string str)
        {
            string Out = "";
            int i, len;
            char c, char2, char3;
            len = str.Length;
            i = 0; while (i < len)
            {
                c = str[i++];
                switch (c >> 4)
                {
                    case 0:
                    case 1:
                    case 2:
                    case 3:
                    case 4:
                    case 5:
                    case 6:
                    case 7: Out += str[i - 1]; break;
                    case 12:
                    case 13: char2 = str[i++];
                        Out += (char)(((c & 0x1F) << 6) | (char2 & 0x3F)); break;
                    case 14: char2 = str[i++];
                        char3 = str[i++];
                        Out += (char)(((c & 0x0F) << 12) | ((char2 & 0x3F) << 6) | ((char3 & 0x3F) << 0)); break;
                }
            }
            return Out;
        }

    }
}