The Content-MD5 entity-header field, as defined in RFC 1864 , is an MD5 digest of the entity-body for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: a MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.)
Content-MD5 = "Content-MD5" ":" md5-digest md5-digest = <base64 of 128 bit MD5 digest as per RFC 1864>
The Content-MD5 header field may be generated by an origin server to function as an integrity check of the entity-body. Only origin servers may generate the Content-MD5 header field; proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check. Any recipient of the entity-body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received.
The MD5 digest is computed based on the content of the entity-body, including any Content-Encoding that has been applied, but not including any Transfer-Encoding that may have been applied to the message-body. If the message is received with a Transfer-Encoding, that encoding must be removed prior to checking the Content-MD5 value against the received entity.
This has the result that the digest is computed on the octets of the entity-body exactly as, and in the order that, they would be sent if no Transfer-Encoding were being applied.
HTTP extends RFC 1864 to permit the digest to be computed for MIME composite media-types (e.g., multipart/* and message/rfc822), but this does not change how the digest is computed as defined in the preceding paragraph.
Note: while the definition of Content-MD5 is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways in which the application of Content-MD5 to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does use Transfer-Encoding and Content-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that, in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CRLF. Conversion of all line breaks to CRLF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest.