为什么不看看当前定义Base64的RFC呢?

Base encoding of data is used in many situations to store or transfer data in environments that, perhaps for legacy reasons, are restricted to US-ASCII [1] data.Base encoding can also be used in new applications that do not have legacy restrictions, simply because it makes it possible to manipulate objects with text editors. In the past, different applications have had different requirements and thus sometimes implemented base encodings in slightly different ways. Today, protocol specifications sometimes use base encodings in general, and "base64" in particular, without a precise description or reference. Multipurpose Internet Mail Extensions (MIME) [4] is often used as a reference for base64 without considering the consequences for line-wrapping or non-alphabet characters. The purpose of this specification is to establish common alphabet and encoding considerations. This will hopefully reduce ambiguity in other documents, leading to better interoperability.

Base64最初被设计为一种允许二进制数据附加到电子邮件的方式，作为多用途互联网邮件扩展的一部分。

而不是转义特殊字符

我将给您一个非常不同但真实的例子:我编写javascript代码以在浏览器中运行。HTML标记有ID值，但是在ID中哪些字符有效是有限制的。

但是我希望我的ID无损地引用文件系统中的文件。现实中的文件中可以有各种各样奇怪而奇妙的字符，从感叹号、重音字符、波浪号，甚至是表情符号!我不能这样做:

<div id="/path/to/my_strangely_named_file!@().jpg">
    <img src="http://myserver.com/path/to/my_strangely_named_file!@().jpg">
    Here's a pic I took in Moscow.
</div>

假设我想运行这样的代码:

# ERROR
document.getElementById("/path/to/my_strangely_named_file!@().jpg");

我认为这段代码在执行时会失败。

使用Base64，我可以引用一些复杂的东西，而不用担心哪种语言允许哪些特殊字符以及哪些需要转义:

document.getElementById("18GerPD8fY4iTbNpC9hHNXNHyrDMampPLA");

与使用MD5或其他哈希函数不同，您可以反向编码以找出真正有用的数据。

我希望我64年前就知道Base64。如果使用' encodeURIComponent '和str.replace(' \n '， ' \\n ')，我就不会把我的头发扯掉了。

SSH传输文本:

如果你试图通过ssh传递复杂的数据(例如，一个dotfile，这样你就可以得到你的shell个性化)，在没有base64的情况下，祝你好运。这是你用64进制来做的(我知道你可以使用SCP，但这需要多个命令-这使得ssh到服务器的键绑定复杂化):

https://superuser.com/a/1376076/114723

为什么/如何使用Base64编码?

Base64 is one of the binary-to-text encoding scheme having 75% efficiency. It is used so that typical binary data (such as images) may be safely sent over legacy "not 8-bit clean" channels. In earlier email networks (till early 1990s), most email messages were plain text in the 7-bit US-ASCII character set. So many early comm protocol standards were designed to work over "7-bit" comm links "not 8-bit clean". Scheme efficiency is the ratio between number of bits in the input and the number of bits in the encoded output. Hexadecimal (Base16) is also one of the binary-to-text encoding scheme with 50% efficiency.

Base64编码步骤(简化):

Binary data is arranged in continuous chunks of 24 bits (3 bytes) each. Each 24 bits chunk is grouped in to four parts of 6 bit each. Each 6 bit group is converted into their corresponding Base64 character values, i.e. Base64 encoding converts three octets into four encoded characters. The ratio of output bytes to input bytes is 4:3 (33% overhead). Interestingly, the same characters will be encoded differently depending on their position within the three-octet group which is encoded to produce the four characters. The receiver will have to reverse this process to recover the original message.

“设计用于处理文本数据的媒体”是什么意思?

在ASCII码统治世界的时候，处理非ASCII码的值是一件令人头痛的事情。为了在不丢失信息的情况下将这些信息通过网络传输，人们经历了各种各样的考验。

为什么不看看当前定义Base64的RFC呢?

Base encoding of data is used in many situations to store or transfer data in environments that, perhaps for legacy reasons, are restricted to US-ASCII [1] data.Base encoding can also be used in new applications that do not have legacy restrictions, simply because it makes it possible to manipulate objects with text editors. In the past, different applications have had different requirements and thus sometimes implemented base encodings in slightly different ways. Today, protocol specifications sometimes use base encodings in general, and "base64" in particular, without a precise description or reference. Multipurpose Internet Mail Extensions (MIME) [4] is often used as a reference for base64 without considering the consequences for line-wrapping or non-alphabet characters. The purpose of this specification is to establish common alphabet and encoding considerations. This will hopefully reduce ambiguity in other documents, leading to better interoperability.

Base64最初被设计为一种允许二进制数据附加到电子邮件的方式，作为多用途互联网邮件扩展的一部分。

您的第一个错误是认为ASCII编码和Base64编码是可互换的。事实并非如此。它们有不同的用途。

当您用ASCII编码文本时，您从文本字符串开始，并将其转换为字节序列。 在Base64中编码数据时，从一个字节序列开始，并将其转换为文本字符串。

为了理解为什么Base64首先是必要的，我们需要一点计算的历史。

计算机以二进制(0和1)进行通信，但人们通常希望使用更丰富的表单数据(如文本或图像)进行通信。为了在计算机之间传输这些数据，首先必须将其编码为0和1，然后发送，然后再次解码。以文本为例，有许多不同的方法来执行这种编码。如果我们都能同意一种编码，那就简单多了，但遗憾的是，事实并非如此。

Originally a lot of different encodings were created (e.g. Baudot code) which used a different number of bits per character until eventually ASCII became a standard with 7 bits per character. However most computers store binary data in bytes consisting of 8 bits each so ASCII is unsuitable for tranferring this type of data. Some systems would even wipe the most significant bit. Furthermore the difference in line ending encodings across systems mean that the ASCII character 10 and 13 were also sometimes modified.

为了解决这些问题，引入了Base64编码。这允许您将任意字节编码为已知可以安全发送而不会被损坏的字节(ASCII字母数字字符和一对符号)。缺点是使用Base64编码消息会增加它的长度——每3个字节的数据被编码为4个ASCII字符。

为了可靠地发送文本，您可以首先使用您选择的文本编码(例如UTF-8)将其编码为字节，然后使用Base64将结果二进制数据编码为可以安全地以ASCII编码发送的文本字符串。接收方将不得不反转此过程以恢复原始消息。当然，这需要接收方知道使用了哪些编码，而这些信息通常需要单独发送。

历史上，它被用于编码电子邮件消息中的二进制数据，其中电子邮件服务器可能修改行结束符。一个更现代的例子是使用Base64编码直接在HTML源代码中嵌入图像数据。这里需要对数据进行编码，以避免'<'和'>'这样的字符被解释为标记。

下面是一个工作示例:

我想发一条短信，两行字:

Hello
world!

如果我把它作为ASCII(或UTF-8)发送，它会像这样:

72 101 108 108 111 10 119 111 114 108 100 33

字节10在某些系统中被损坏，因此我们可以将这些字节以Base64编码为Base64字符串:

SGVsbG8Kd29ybGQh

当使用ASCII编码时，看起来是这样的:

83 71 86 115 98 71 56 75 100 50 57 121 98 71 81 104

这里的所有字节都是已知的安全字节，因此任何系统破坏此消息的可能性很小。我可以发送这个而不是我的原始消息，并让接收者反向处理以恢复原始消息。