HTTP协议头——Content-MD5

以下为RFC1864的英文主要部分原文：

1. Introduction

Despite all of the mechanisms provided by MIME [1] which attempt to
protect data from being damaged in the course of email transport, it
is still desirable to have a mechanism for verifying that the data,
once decoded, are intact. For this reason, this memo defines the use
of an optional header field, Content-MD5, which may be used as a
message integrity check (MIC), to verify that the decoded data are
the same data that were initially sent. The Content-MD5 header may
also be placed in the encapsulated headers of an object of type
message/external-body, to be used to verify that the retreived and
decoded data are the same data that were initially referenced.

MIC：信息完整性检查，来保证数据传输的完整性。Content-MD5头可能放在一个消息对象内或者外部体内。

MD5 is an algorithm for computing a 128 bit “digest” of arbitrary-
length data, with a high degree of confidence that any alterations in
the data will be reflected in alterations in the digest.

2. Generation of the Content-MD5 Field

The Content-MD5 field is generated by only an originating user agent.
Message relays and gateways are expressly forbidden from generating a
Content-MD5 field.

Use of the Content-MD5 field is completely optional, but its use is
recommended whenever data integrity is desired, but Privacy-Enhanced
The Content-MD5 field may only be added to MIME entities of
a `leaf’ nature, i.e., the Content-MD5 field may be used with any
content type other than multipart or message/rfc822.

To generate the value of the Content-MD5 field, the MD5 algorithm is
computed on the canonical form of the MIME entity’s object. In
particular, this means that the sender applies the MD5 algorithm on
the data immediately after conversion to canonical form, before
applying any content-transfer-encoding, and that the receiver also
applies the MD5 algorithm on the canonical form, after undoing any
content-transfer-encoding. For textual data, this means the MD5
algorithm must be computed on data in which the canonical form for
newlines applies, that is, in which each newline is represented by a
CR-LF pair.
文本数据必须要经过编码，否则编码前后的签名不一致。

The output of the MD5 algorithm is a 128 bit digest. When viewed in
network byte order (big-endian order), this yields a sequence of 16
octets of binary data. These 16 octets are then encoded according to
the base64 algorithm in order to oBTain the value that is placed in
the Content-MD5 field. Thus, if the application of the MD5 algorithm
over the raw data of a MIME entity results in a digest having the
(unlikely) value of “Check Integrity!”, then that MIME entity’s
header could contain the field
编码过后的128bits的摘要一共16Bytes，通过base64编码得到Content-MD5头部的值。
Content-MD5: Q2hlY2sgSW50ZWdyaXR5IQ==

Finally, textual data is regularly
altered in the normal delivery of mail. Because the addition or
deletion of trailing white space will result in a different digest,
either the quoted-printable or base64 algorithm should be employed as
a content-transfer-encoding when the Content-MD5 field is used.
文本数据需要使用content-encoding指定的编码进行转换，否则增加和减少空白字符后的摘要会不同。

3. Processing the Content-MD5 field

If the Content-MD5 field is present, a recipient user agent may
choose to use it to verify that the contents of a MIME entity have
not been modified during transport. Message relays and gateways are
expressly forbidden to alter their processing based on the presence
of the Content-MD5 field. However, a message gateway is allowed to
remove the Content-MD5 field if the corresponding MIME entity is
translated into a different content-type.

4. Security Considerations

This document specifies a data integrity service that protects data
from accidental modification while in transit from the sender to the
recipient.

对于基于HTTP协议进行部分传输的应用，为确保数据完整性，最好需要设置这个扩展头，从而再客户端或者服务端进行完整性验证。