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Re: Content-MD5 and partial responses

From: Mark Nottingham <mnot@mnot.net>
Date: Tue, 30 Jun 2009 13:55:58 +1000
Message-Id: <7337E67E-9CC8-4089-86B3-FC15D6F3739C@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
RFC3230 (n.b., - standards track) seems to agree:
> HTTP/1.1 defines a Content-MD5 header that allows a server to  
> include a digest of the response body. However, this is specifically  
> defined to cover the body of the actual message, not the contents of  
> the full file (which might be quite different, if the response is a  
> Content- Range, or uses a delta encoding).

Now <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/178>.

On 29/06/2009, at 12:00 PM, Mark Nottingham wrote:

> After a quick look, my reading is that a Content-MD5 header on a  
> partial response reflects the bytes in that message, rather than the  
> whole (non-partial) response:
>> The entity-header field "Content-MD5", as defined in [RFC1864], 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" ":" OWS Content-MD5-v
>>  Content-MD5-v = <base64 of 128 bit MD5 digest as per [RFC1864]>
>> The Content-MD5 header field MAY be generated by an origin server or
>> client to function as an integrity check of the entity-body.  Only
>> origin servers or clients 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-coding that has been applied, but not including
>> any transfer-encoding 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.
> Also, note that a multipart message is allowed to have C-MD5 on  
> individual parts;
>> The entity-body for composite types MAY contain many body-parts,  
>> each with its own MIME and HTTP headers (including Content-MD5,  
>> Content-Transfer-Encoding, and Content-Encoding headers).
> For a multipart/byteranges response, this only helps really if they  
> apply to the individual parts...
> However, I'm wondering what a cache should do when combining partial  
> responses that include Content-MD5. This doesn't seem to be  
> addressed in 2616, nor in p5 or p6.
> It looks like there are two options here;
> a) C-MD5 applies to the bytes in the entity-body (as above), and  
> therefore we need to specify what a cache does with it when it  
> combines partial responses (throw it away?).
> b) C-MD5 applies to the *full* response body, avoiding the  
> combination issues, and allowing clients to do a MIC of the full  
> response (assuming they have it), but removing the ability to do a  
> MIC on a partial response on its own.
> Anybody aware of C-MD5 being used with partial responses in the wild  
> (I'm looking at you, Adobe)?
> Cheers,
> --
> Mark Nottingham     http://www.mnot.net/

Mark Nottingham     http://www.mnot.net/
Received on Tuesday, 30 June 2009 03:56:40 UTC

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