base64加密和解码原理和代码

/* 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