base64加密和解码原理和代码

Base64编码,是我们程序开发中经常使用到的编码方法。它是一种基于用64个可打印字符来表示二进制数据的表示方法。它通常用作存储、传输一些二进制数据编码方法!也是MIME(多用途互联网邮件扩展,主要用作电子邮件标准)中一种可打印字符表示二进制数据的常见编码方法!它其实只是定义用可打印字符传输内容一种方法,并不会产生新的字符集!有时候,我们学习转换的思路后,我们其实也可以结合自己的实际需要,构造一些自己接口定义编码方式。好了,我们一起看看,它的转换思路吧!

Base64实现转换原理

 

    它是用64个可打印字符表示二进制所有数据方法。由于2的6次方等于64,所以可以用每6个位元为一个单元,对应某个可打印字符。我们知道三个字节有24个位元,就可以刚好对应于4个Base64单元,即3个字节需要用4个Base64的可打印字符来表示。在Base64中的可打印字符包括字母A-Z、a-z、数字0-9 ,这样共有62个字符,此外两个可打印符号在不同的系统中一般有所不同。但是,我们经常所说的Base64另外2个字符是:“+/”。

这64个字符,所对应表如下。

编号字符 编号字符 编号字符 编号字符
0 A 16 Q 32 g 48 w
1 B 17 R 33 h 49 x
2 C 18 S 34 i 50 y
3 D 19 T 35 j 51 z
4 E 20 U 36 k 52 0
5 F 21 V 37 l 53 1
6 G 22 W 38 m 54 2
7 H 23 X 39 n 55 3
8 I 24 Y 40 o 56 4
9 J 25 Z 41 p 57 5
10 K 26 a 42 q 58 6
11 L 27 b 43 r 59 7
12 M 28 c 44 s 60 8
13 N 29 d 45 t 61 9
14 O 30 e 46 u 62 +
15 P 31 f 47 v 63 /

  转换的时候,将三个byte的数据,先后放入一个24bit的缓冲区中,先来的byte占高位。数据不足3byte的话,于缓冲区中剩下的bit用0补足。然后,每次取出6个bit,按照其值选择ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/中的字符作为编码后的输出。不断进行,直到全部输入数据转换完成。

    如果最后剩下两个输入数据,在编码结果后加1个“=”;如果最后剩下一个输入数据,编码结果后加2个“=”;如果没有剩下任何数据,就什么都不要加,这样才可以保证资料还原的正确性。编码后的数据比原始数据略长,为原来的4/3。无论什么样的字符都会全部被编码,因此不像Quoted-printable 编码,还保留部分可打印字符。所以,它的可读性不如Quoted-printable编码!


文本 M a n
ASCII编码 77 97 110
二进制位 0 1 0 0 1 1 0 1 0 1 1 0 0 0 0 1 0 1 1 0 1 1 1 0
索引 19 22 5 46
Base64编码 T W F u

    M的Ascii码是77,前六位对应值为19,对应base64字符是T,如此类推。其它字符编码就可以自动转换得到!我们看看另外不是刚好是3个字节的情况!

 


文本(1 Byte) A    
二进制位 0 1 0 0 0 0 0 1                                
二进制位(补0) 0 1 0 0 0 0 0 1 0 0 0 0                        
Base64编码 Q Q = =
文本(2 Byte) B C  
二进制位 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 1     x x x x x x
二进制位(补0) 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 1 0 0 x x x x x x
Base64编码 Q k M  =

 

 

下面是js代码实现base64的转换

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/* utf.js - UTF-8 <=> UTF-16 convertion
         *
         * Copyright (C) 1999 Masanao Izumo <iz@onicos.co.jp>
         * Version: 1.0
         * LastModified: Dec 25 1999
         * This library is free. You can redistribute it and/or modify it.
         */
 
        /*
         * Interfaces:
         * utf8 = utf16to8(utf16);
         * utf16 = utf8to16(utf8);
         */
 
        function utf16to8(str) {
            var out, i, len, c;
 
            out = "";
            len = str.length;
            for(i = 0; i < len; i++) {
                c = str.charCodeAt(i);
                if((c >= 0x0001) && (c <= 0x007F)) {
                    out += str.charAt(i);
                } else if(c > 0x07FF) {
                    out += String.fromCharCode(0xE0 | ((c >> 12) & 0x0F));
                    out += String.fromCharCode(0x80 | ((c >> 6) & 0x3F));
                    out += String.fromCharCode(0x80 | ((c >> 0) & 0x3F));
                } else {
                    out += String.fromCharCode(0xC0 | ((c >> 6) & 0x1F));
                    out += String.fromCharCode(0x80 | ((c >> 0) & 0x3F));
                }
            }
            return out;
        }
 
        function utf8to16(str) {
            var out, i, len, c;
            var char2, char3;
 
            out = "";
            len = str.length;
            i = 0;
            while(i < len) {
                c = str.charCodeAt(i++);
                switch(c >> 4) {
                    case 0:
                    case 1:
                    case 2:
                    case 3:
                    case 4:
                    case 5:
                    case 6:
                    case 7:
                        // 0xxxxxxx
                        out += str.charAt(i - 1);
                        break;
                    case 12:
                    case 13:
                        // 110x xxxx 10xx xxxx
                        char2 = str.charCodeAt(i++);
                        out += String.fromCharCode(((c & 0x1F) << 6) | (char2 & 0x3F));
                        break;
                    case 14:
                        // 1110 xxxx 10xx xxxx 10xx xxxx
                        char2 = str.charCodeAt(i++);
                        char3 = str.charCodeAt(i++);
                        out += String.fromCharCode(((c & 0x0F) << 12) |
                            ((char2 & 0x3F) << 6) |
                            ((char3 & 0x3F) << 0));
                        break;
                }
            }
 
            return out;
        }
 
        //上面的这段代码中定义了两个函数,utf16to8()用于将utf-16转成utf-8,utf8to16用于将utf-8转成utf-16。
        //
        //下面才是真正用于base64编码的函数。
 
        /* Copyright (C) 1999 Masanao Izumo <iz@onicos.co.jp>
         * Version: 1.0
         * LastModified: Dec 25 1999
         * This library is free. You can redistribute it and/or modify it.
         */
 
        /*
         * Interfaces:
         * b64 = base64encode(data);
         * data = base64decode(b64);
         */
 
        var base64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
        var base64DecodeChars = new Array(-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);
 
        function base64encode(str) {
            var out, i, len;
            var c1, c2, c3;
 
            len = str.length;
            i = 0;
            out = "";
            while(i < len) {
                c1 = str.charCodeAt(i++) & 0xff;
                if(i == len) {
                    out += base64EncodeChars.charAt(c1 >> 2);
                    out += base64EncodeChars.charAt((c1 & 0x3) << 4);
                    out += "==";
                    break;
                }
                c2 = str.charCodeAt(i++);
                if(i == len) {
                    out += base64EncodeChars.charAt(c1 >> 2);
                    out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
                    out += base64EncodeChars.charAt((c2 & 0xF) << 2);
                    out += "=";
                    break;
                }
                c3 = str.charCodeAt(i++);
                out += base64EncodeChars.charAt(c1 >> 2);
                out += base64EncodeChars.charAt(((c1 & 0x3) << 4) | ((c2 & 0xF0) >> 4));
                out += base64EncodeChars.charAt(((c2 & 0xF) << 2) | ((c3 & 0xC0) >> 6));
                out += base64EncodeChars.charAt(c3 & 0x3F);
            }
            return out;
        }
 
        function base64decode(str) {
            var c1, c2, c3, c4;
            var i, len, out;
 
            len = str.length;
            i = 0;
            out = "";
            while(i < len) {
                /* c1 */
                do {
                    c1 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
                } while (i < len && c1 == -1);
                if(c1 == -1)
                    break;
 
                /* c2 */
                do {
                    c2 = base64DecodeChars[str.charCodeAt(i++) & 0xff];
                } while (i < len && c2 == -1);
                if(c2 == -1)
                    break;
 
                out += String.fromCharCode((c1 << 2) | ((c2 & 0x30) >> 4));
 
                /* c3 */
                do {
                    c3 = str.charCodeAt(i++) & 0xff;
                    if(c3 == 61)
                        return out;
                    c3 = base64DecodeChars[c3];
                } while (i < len && c3 == -1);
                if(c3 == -1)
                    break;
 
                out += String.fromCharCode(((c2 & 0XF) << 4) | ((c3 & 0x3C) >> 2));
 
                /* c4 */
                do {
                    c4 = str.charCodeAt(i++) & 0xff;
                    if(c4 == 61)
                        return out;
                    c4 = base64DecodeChars[c4];
                } while (i < len && c4 == -1);
                if(c4 == -1)
                    break;
                out += String.fromCharCode(((c3 & 0x03) << 6) | c4);
            }
            return out;
        }
 
        //上面代码中的base64encode()用于编码,base64decode()用于解码。
         
        //因此,对utf-8字符进行编码要这样写:
        var str = "hello world"
        sEncoded = base64encode(utf16to8(str));
        console.log(sEncoded);//aGVsbG8gd29ybGQ=
        //然后,解码要这样写:
 
        sDecoded = utf8to16(base64decode(sEncoded));
        console.log(sDecoded);//hello world

 

posted @   SultanST  阅读(2419)  评论(0编辑  收藏  举报
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