从http://en.wikipedia.org/wiki/Base64

The term Base64 refers to a specific MIME content transfer encoding. It is also used as a generic term for any similar encoding scheme that encodes binary data by treating it numerically and translating it into a base 64 representation. The particular choice of base is due to the history of character set encoding: one can choose a set of 64 characters that is both part of the subset common to most encodings, and also printable. This combination leaves the data unlikely to be modified in transit through systems, such as email, which were traditionally not 8-bit clean. Base64 can be used in a variety of contexts: Evolution and Thunderbird use Base64 to obfuscate e-mail passwords[1] Base64 can be used to transmit and store text that might otherwise cause delimiter collision Base64 is often used as a quick but insecure shortcut to obscure secrets without incurring the overhead of cryptographic key management Spammers use Base64 to evade basic anti-spamming tools, which often do not decode Base64 and therefore cannot detect keywords in encoded messages. Base64 is used to encode character strings in LDIF files Base64 is sometimes used to embed binary data in an XML file, using a syntax similar to ...... e.g. Firefox's bookmarks.html. Base64 is also used when communicating with government Fiscal Signature printing devices (usually, over serial or parallel ports) to minimize the delay when transferring receipt characters for signing. Base64 is used to encode binary files such as images within scripts, to avoid depending on external files. Can be used to embed raw image data into a CSS property such as background-image.

一个十六进制数字是一个啃位(4位)。两个小字节构成8位，也称为1字节。

MD5生成128位输出，该输出使用32个十六进制数字序列表示，这些十六进制数字依次为32*4=128位。128位等于16字节(因为1字节等于8位)。

每个Base64字符编码6位(除了最后一个非填充字符可以编码2,4或6位);以及最后的填充字符(如果有的话)。因此，根据Base64编码，128位哈希至少需要⌈128/6⌉= 22个字符，如果有的话再加上pad。

使用base64，我们可以生成所需长度(6、8或10)的编码输出。 如果我们选择8字符长的输出，它只占用8个字节，而128位哈希输出则占用16个字节。

因此，除了安全性之外，还使用base64编码来减少空间消耗。

有些传输协议只允许传输字母数字字符。想象一下这样一种情况:控制字符用于触发特殊操作，或者每个字符只支持有限的位宽。Base64将任何输入转换为只使用字母数字字符、+、/和=作为填充字符的编码。

几年前，当邮件功能被引入时，它完全是基于文本的，随着时间的推移，对图像和媒体(音频、视频等)等附件的需求出现了。当这些附件通过互联网发送时(基本上是以二进制数据的形式)，原始形式的二进制数据损坏的概率很高。因此，为了解决这个问题，BASE64出现了。

二进制数据的问题是它包含null字符，在一些语言中，如C, c++表示字符串的结束，因此以包含null字节的原始形式发送二进制数据将阻止文件被完全读取并导致损坏的数据。

例如:

在C和c++中，这个“null”字符表示字符串的结束。所以"HELLO"是这样存储的:

H e l l o

72 69 76 76 79 00

00表示“停在这里”。

现在让我们深入研究BASE64编码是如何工作的。

注意:字符串的长度应该是3的倍数。

例1:

要编码的字符串:" ace "，长度=3

将每个字符转换为十进制。

A = 97, c= 99, e= 101

将每个小数改为8位二进制表示。

97= 01100001, 99= 01100011, 101= 01100101

合并:01100001 01100011 01100101

在一组6位中分离。

011000 010110 001101 100101

从二进制到十进制计算

011000= 24, 010110= 22, 001101= 13, 100101= 37

使用base64 chart将十进制字符转换为base64。

24= Y, 22= W, 13= N, 37= l

“ace”=>“YWNl”

例2:

要编码的字符串:" abcd "长度=4，不是3的倍数。因此，要使字符串长度为3的倍数，我们必须添加2位填充使length= 6。填充位用“=”符号表示。

需要注意的是:一个填充位等于两个000，所以两个填充位等于四个0 0000。

所以让我们开始这个过程:-

将每个字符转换为十进制。

A = 97, b= 98, c= 99, d= 100

将每个小数改为8位二进制表示。

97= 01100001, 98= 01100010, 99= 01100011, 100= 01100100

在一组6位中分离。

011000, 010110, 001001, 100011, 011001, 00

所以最后6位是不完整的，所以我们插入两个填充位，等于4个零“0000”。

011000, 010110, 001001, 100011, 011001, 000000 ==

现在，它是相等的。末尾的两个等号表示添加了4个零(有助于解码)。

将二进制计算为十进制。

011000= 24, 010110= 22, 001001= 9, 100011= 35, 011001= 25, 000000=0 ==

使用base64 chart将十进制字符转换为base64。

24= Y, 22= W, 9= j, 35= j, 25= Z, 0= A ==

“abcd”=>“YWJjZA==”

从http://en.wikipedia.org/wiki/Base64

The term Base64 refers to a specific MIME content transfer encoding. It is also used as a generic term for any similar encoding scheme that encodes binary data by treating it numerically and translating it into a base 64 representation. The particular choice of base is due to the history of character set encoding: one can choose a set of 64 characters that is both part of the subset common to most encodings, and also printable. This combination leaves the data unlikely to be modified in transit through systems, such as email, which were traditionally not 8-bit clean. Base64 can be used in a variety of contexts: Evolution and Thunderbird use Base64 to obfuscate e-mail passwords[1] Base64 can be used to transmit and store text that might otherwise cause delimiter collision Base64 is often used as a quick but insecure shortcut to obscure secrets without incurring the overhead of cryptographic key management Spammers use Base64 to evade basic anti-spamming tools, which often do not decode Base64 and therefore cannot detect keywords in encoded messages. Base64 is used to encode character strings in LDIF files Base64 is sometimes used to embed binary data in an XML file, using a syntax similar to ...... e.g. Firefox's bookmarks.html. Base64 is also used when communicating with government Fiscal Signature printing devices (usually, over serial or parallel ports) to minimize the delay when transferring receipt characters for signing. Base64 is used to encode binary files such as images within scripts, to avoid depending on external files. Can be used to embed raw image data into a CSS property such as background-image.

它基本上是一种用ASCII文本编码任意二进制数据的方法。每3个字节的数据需要4个字符，最后可能还会有一些填充。

基本上，输入的每6位都用64个字符的字母表进行编码。“标准”字母表使用a-z, a-z, 0-9和+和/，用=作为填充字符。有url安全的变体。

维基百科是一个相当好的信息来源。