JavaScript實(shí)現(xiàn)的encode64加密算法實(shí)例分析

    這段JavaScript代碼可實(shí)現(xiàn)encode64加密算法，速度還是相當(dāng)不錯(cuò)的。
    //encode64編解碼
    (function() {
    var codeChar = "PaAwO65goUf7IK2vi9-xq8cFTEXLCDY1Hd3tV0ryzjbpN_BlnSs4mGRkQWMZJeuh";
    window.encode64 = function(str) {
    var s = "";
    var a = strToBytes(str);
    //取得字串的字節(jié)數(shù)組, 數(shù)組長(zhǎng)度是字串長(zhǎng)度的2倍.
    var res = a.length % 3;
    //3個(gè)字節(jié)一組進(jìn)行處理, 余下特殊處理
    var i = 2, v;
    for (; i < a.length; i += 3) {
    //每3個(gè)字節(jié)用4個(gè)字符表示,
    //相當(dāng)于3個(gè)字符(實(shí)際上是6個(gè)字節(jié))用8個(gè)字符編碼(實(shí)際為16個(gè)字節(jié))
    //看起來(lái)容量膨脹了很多, 但是在啟用壓縮的情況下, 這些又被抵消掉了
    v = a[i - 2] + (a[i - 1] << 8) + (a[i] << 16);
    s += codeChar.charAt(v & 0x3f);
    s += codeChar.charAt((v >> 6) & 0x3f);
    s += codeChar.charAt((v >> 12) & 0x3f);
    s += codeChar.charAt((v >> 18));
    }
    if (res == 1) {//字節(jié)余一位時(shí)候, 補(bǔ)2個(gè)字符, 64*64>256
    v = a[i - 2];
    s += codeChar.charAt(v & 0x3f);
    s += codeChar.charAt((v >> 6) & 0x3f);
    } else if (res == 2) {
    //字節(jié)余2位的時(shí)候, 補(bǔ)3個(gè)字節(jié), 64*64*64>256*256, 所以是可行的
    v = a[i - 2] + (a[i - 1] << 8);
    s += codeChar.charAt(v & 0x3f);
    s += codeChar.charAt((v >> 6) & 0x3f);
    s += codeChar.charAt((v >> 12) & 0x3f);
    }
    return s;
    };
    window.decode64 = function(codeStr) {
    var dic = [];
    for (var i = 0; i < codeChar.length; i++) {
    dic[codeChar.charAt(i)] = i;
    }
    var code = [];
    var res = codeStr.length % 4;
    var i = 3, v;
    for (; i < codeStr.length; i += 4) {
    v = dic[codeStr.charAt(i - 3)];
    v += dic[codeStr.charAt(i - 2)] << 6;
    v += dic[codeStr.charAt(i - 1)] << 12;
    v += dic[codeStr.charAt(i)] << 18;
    code.push(v & 0xff, (v >> 8) & 0xff, (v >> 16) & 0xff);
    }
    if (res == 2) {
    //正確的字節(jié)數(shù)肯定是余2或3, 沒(méi)有1的情況, 如果出現(xiàn), 舍棄.
    v = dic[codeStr.charAt(i - 3)];
    v += dic[codeStr.charAt(i - 2)] << 6;
    code.push(v & 0xff);
    } else if (res == 3) {
    v = dic[codeStr.charAt(i - 3)];
    v += dic[codeStr.charAt(i - 2)] << 6;
    v += dic[codeStr.charAt(i - 1)] << 12;
    code.push(v & 0xff, (v >> 8) & 0xff);
    }
    return strFromBytes(code);
    };
    })();