base64

base64是很基本的加密算法，常用于郵件和迅雷的下載下傳位址，下面是js版的算法代碼和網頁示例

編碼：

//分割成每6位，最大是111111，即63，是以共需要64個字元

var base64EncodeChars = [

    "A", "B", "C", "D", "E", "F", "G", "H",

    "I", "J", "K", "L", "M", "N", "O", "P",

    "Q", "R", "S", "T", "U", "V", "W", "X",

    "Y", "Z", "a", "b", "c", "d", "e", "f",

    "g", "h", "i", "j", "k", "l", "m", "n",

    "o", "p", "q", "r", "s", "t", "u", "v",

    "w", "x", "y", "z", "0", "1", "2", "3",

    "4", "5", "6", "7", "8", "9", "+", "/"

];

function base64encode(str) {

    var out, i, j, len;

    var c1, c2, c3;//讀取待加密的三個字元

    len = str.length;

    i = j = 0;

    out = [];

    while (i < len) {

        c1 = str.charCodeAt(i++) & 0xff;

        if (i == len)//最後一組隻有一個字元

        {

            out[j++] = base64EncodeChars[c1 >> 2];

            out[j++] = base64EncodeChars[(c1 & 0x3) << 4];

            out[j++] = "==";

            break;

        }

        c2 = str.charCodeAt(i++) & 0xff;

        if (i == len)最後一組隻有倆字元

            out[j++] = base64EncodeChars[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];

            out[j++] = base64EncodeChars[(c2 & 0x0f) << 2];

            out[j++] = "=";

        c3 = str.charCodeAt(i++) & 0xff;

        out[j++] = base64EncodeChars[c1 >> 2];//右移2位，即取高6位，8-2=6，唔識翻去讀國小

        out[j++] = base64EncodeChars[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];

        //c1 & 0x03是高6位置0留下低2位，再左移4位變成高5,6位，再用邏輯或與c2的高四位合并，即c2

          的高4位充當第二個已加密字元的低四位

        out[j++] = base64EncodeChars[((c2 & 0x0f) << 2) | ((c3 & 0xc0) >> 6)];

               //c2的低4位與c3高2位合并

        out[j++] = base64EncodeChars[c3 & 0x3f];

         //高2位置0

    //我是在解釋代碼還是在講數學呢？？？？？

    }

    return out.join('');

}

解碼：

//11111111=127，是以要128個字元，但密文是由64種字元組成的（好似咩！@￥%*^哩d咯，都唔系密文就無所謂解密啦），是以不是base64加密的記作-1，并且無法解密

var base64DecodeChars = [

    -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

function base64decode(str) {

    var c1, c2, c3, c4;//讀取待解密的四個字元

    var i, j, len, out;

        /* c1 */

        do {

            c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];

        } while (i < len && c1 == -1);//當遇到非法密文繼續讀取，直到讀到一個正常的密文

        if (c1 == -1) break;

        /* c2 */

            c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];

        } while (i < len && c2 == -1);

        if (c2 == -1) break;

        out[j++] = String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));//解開第一個

base64編碼加密

原碼

base64 encode

base64 decode