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comments on jaye-trust-state-01

From: Dave Kristol <dmk@research.bell-labs.com>
Date: Tue, 19 Aug 97 17:42:00 EDT
Message-Id: <9708192142.AA02980@zp>
To: djaye@engagetech.com
Cc: http-state@lists.research.bell-labs.com, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Following are my comments for Dan Jaye's recently posted
(pre-) draft-ietf-http-jaye-trust-state-01.txt.

General:
1) Be consistent with capitalization for "Trust Authority", "user agent",
"SigBlock".

Specific, interlinear:

[...]
> ABSTRACT
> 
> HTTP TRUST MECHANISM PROPOSAL FOR STATE MANAGEMENT
> March 30, 1997
  ==============  probably not!

[...]
> 
> 2. TERMINOLOGY
> 
> The terms user agent, client, server, and origin are used as in [Kristol].
                                                  ^-- "server"

[...]
> 3.1  Syntax: General
[...]

> trusted-cookie  = 1*Set-cookie2 trust-label
> trust-label     = "PICS-Label:" labellist

This syntax possibly suggests two things that are both false:
1) That the trust label is a continuation of Set-Cookie2; or
2) That the trust label must immediately follow Set-Cookie2.

Although a server may emit PICS-Label immediately after Set-Cookie2, it
may not be in that position at the client, due to proxy header
shuffling.  The only guarantee is that the relative order of
Set-Cookie2 headers will be preserved, as will the relative order of
PICS-Label headers.  But they well may get separated (particularly with
header folding).

There would appear to be a possible danger with using "PICS-Label".
Suppose the origin server wants to emit one PICS-Label to label the
content, and another to label the cookie.  Header folding would group
all PICS-Label headers together, making it harder to pick out which
ones apply to cookies, and which to content.

> 
> "labellist" is as specified in the PICS 1.1 label syntax in [PICS],
> except as extended by the digital signatures working draft [DSIG], some
> options are not used, and we require some of the optional elements, as
> specified below.  The trust-label applies to the immediately preceding
> Set-Cookie2 label.

As I said, the last sentence might be false.

[...]
> label           = labelattr "ratings" "(" privacy-practice ")" [sigblock]

In the PICS spec, "sigblock" -> "SigBlock", as you have it below.

[...]

> authority.  The "gen" boolean indicates whether the label is for only the 
> URL in the "for", or for all URL's for which the specified one is a 
> prefix.  ("True" indicates subdomains included.)  "on" is the date the 
        ^-- (i.e., "generic") -- since you use that term later.

> label was issued.  "exp" is the date the label expires.  SigBlock is the 
> digital signature extension as described in the digital signature working

> Four well-known privacy-practice values are described here to provide 
> recognized values that should be handled by user agents.    
> 
[...]
> The "anonymousexchange 1" rating indicates that the trust authority has 
[...]
> identifying information.  The server may collect IP Addresses but they
> must not be associated with personal information or implicit information
> with personally identifying information to be elegible for this rating.

This last sentence is very confusing; please reword.

[...]
> The "thirdpartyexchange 1" rating indicates that the trust authority has 
[...]
> third parties.  The trust authority should provide information about the 
> purposes for which that information is being used.

How should it do that?  (Via ServiceID?  If so, say so.)

There seems to be the implication that the privacy-practice values
form a partial ordering, and that a more-stringent privacy-practice
in a trust label should be acceptable to a user agent that is set to
accept less-stringent ones, but that ordering and treatment isn't
spelled out explicitly and should be.

> The SigBlock must contain the SigCrypto token within the SigData block.  
> The SigCrypto token must contain the encrypted trust-label-data described
> below.
> 
> trust-label-data = for-URL on-date exp-date privacy-practice
> 
> for-URL          = quotedURL
> exp-date         = quoted-ISO-date
> 
> for-URL is the URL to which the privacy practice applies as listed in the 
> "for" attribute in "labelattr".  Exp-date is the expiration date of the 
> trust label as listed in the "exp" attribute of "labelattr".

The trust-label-data also needs the "gen" attribute to denote the
scope of the label.  Otherwise, a web site could obtain a trust label
for a single URL and try to pass it off as a label for an entire
hierarchy.

A nit:  the on-date in the PICS spec. is the date the signature was
created, not the date the trust label (in our case) becomes effective.
But it would surely be "less that or equal to the current date"
(sect. 3.3.1), rendering that test useless.

[...]
> 3.2	Server Role
> 
> A server communicates its privacy practices by sending an unsigned or 
> signed trust label immediately following the cookie header(s).  The trust 
> label is assumed to apply to all cookies in the response that match the 
> domain and path of the trust label according to the matching rules for 
> matching cookies to request URI's described in [Kristol].

It would help to spell out that "domain and path" are derived from the
"for" URL in the labelattr.  (It took me a couple of readings to realize
that.)

One of the contentious issues with RFC 2109 has been the concept of
"related servers" (although it isn't called that) to which a user agent
can send a cookie.  That's the business with the domain-matching, the
"two-dot rule", etc.  The trust-label mechanism is even *more*
restrictive, since the label applies to a single host.  Won't that
restriction be even more objectionable?

[...]
> 3.3 	User Agent Role
> 
> The user agent receives a cookie headers followed by a trust labels from 

===>					 ^ (delete)		    ^ (delete)

> an origin server.
> 
> 3.3.1 Interpreting the trust-label
> User agents interpret cookies as described in RFC 2109.  In addition 
[...]
> matching rules described in [Kristol].  To help verify the 
> trustworthiness of the server, the user agent may look for a digital 
> signature and use the trust authority's well known public key to 
> decrypt the trust-label-data from the SigCrypto term.

append:  and compare the information there against the plaintext trust
label.

[...]
> The digital signature is valid if the decrypted trust-label-data 
> satisfies the following criteria:

I would have thought the digital signature is valid *if the decrypted
trust-label-data matches the plaintext labelattr*.  The rest of these
tests should apply whether or not there's a SigBlock.  So this spec.
needs to describe how the digital signature gets verified.

> 
> 1) that the domain portion of the URL specified in the for-URL attribute 
> domain matches the domain of the cookie according to the matching 
> rules as sort forth in [Kristol];

	1) That the domain portion of the URL specified in the "for"
	attribute of labelattr domain-matches the Domain attribute of
	the Set-Cookie2 response header, according to the matching
	rules in [Kristol].
> 
> 2) that the path portion of the URL specified in the for-URL attribute is
> compatible with the path of the cookie.  If the trust label is generic, 
> then the for-URL path must be a prefix of the cookie's path.  If the 
> trust label is not generic, then it must match exactly. 

    2) That the path portion of the URL specified in the "for"
    attribute of labelattr is compatible with the Path attribute of the
    Set-Cookie2 response header.  That is, if the trust label is
    generic, then the "for" path must be a prefix of the cookie's Path
    attribute.  If the trust label is not generic, then it must match
    the cookie's Path attribute exactly.
> 
> 3) that the "on-date" attribute of the trust label is less than or equal
> to the current date;

See earlier remark.

> 
> 4) that the "exp-date" attribute of the trust-label-data is greater than
> or equal to the current date.
> 
> If the digital signature is invalid, then the cookie should be rejected.

    If the digital signature is invalid, or if any of the above tests
    fails, then the cookie should be rejected.

> 
> If the user agent is set to accept all cookies then all trust label 
> processing can be skipped.
> 
> 3.3.2	Accepting or rejecting Cookies
[...]

> The User Agent should have a default preference to reject "third-party-
> exchange" cookies from unverifiable transactions.  

Need a definition of "third-party-exchange"; or omit this statement, since
it appears in RFC 2109.

> 
> For example, a user may wish to accept cookies rated anonymousexchange by
> a recognized trust authority, rather than relying on an unsigned trust 
> label or a trust label signed by an unrecognized entity.

I don't understand the point this statement is trying to make.

If a trust label's expiration is sooner than the cookie's, I think the
cookie's expiration should be adjusted to match that of the trust label.
Alternatively, the UA should not send a cookie whose trust label has
expired.

[...]
> 4. EXAMPLES
> 
> 4.1 Example 1
[...]
> 3.  Server -> User Agent
>       HTTP/1.1 200 OK
>       Set-Cookie: Customer="WILE_E_COYOTE"; Max-Age = 94608000; 

        Set-Cookie2: Customer="WILE_E_COYOTE"; Max-Age = 94608000; 

>         Version="1"; Path="/acme" 
[...]
> 
>     A cookie that includes the users identity and an unsigned PICS
>     label header are sent back to the user agent with the request.  The 
		   === -> "is"
>     Cookie is accepted because rating "noshare 1" is acceptable according 
      ====== -> "cookie"
>     to the privacy preferences of the user agent.

... assuming the user agent doesn't require signed trust labels.
Is there really any value in unsigned trust labels?

[...]
> 
> 4.2 Example 2
[...]
> 3.  Server -> User Agent
>       HTTP/1.1 200 OK
>       Set-PICS-Cookie: Customer="0000000123"; Max-Age = 94608000; 

	=============== -> Set-Cookie2

>         Version="1"; Path="/birds" 
>       PICS-Label: (PICS-1.1 "http://www.aaa.org" label 
>         by "paranoid@aaa.org" gen true
>         for "http://www.acme.com/"
>         exp "1997.12.31T23:59-0000" 
>         ratings (noshare 1))
> 
>     A Cookie reflecting the users identity is transmitted with an 
>     Unsigned trust label back to the user agent.  The Cookie is accepted

      ======== -> "unsigned"

>     by user agent because the rating "noshare 1" is compatible with the 
>     user agent privacy preference.
> 
> 4.  User Agent -> Server
> 
>       GET cgi-bin/maps?TERR=deserts&FEAT=cliffs HTTP/1.1
>       Host: roadrunnermaps.com

	GET /cgi-bin/maps?TER=deserts&FE=cliffs HTTP/1.1
	Host: www.roadrunnermaps.com

(Changes to be consistent with step 2, which I omitted.)

> 
>     User requests an image via CGI script from a third party map provider.
>     This is an unverifiable transaction.
> 
> 5.  Server -> User Agent (unverifiable transaction)
>       HTTP/1.1 200 OK
>       Set-PICS-Cookie: Customer="0000000123"; Max-Age = 94608000; 

	=============== -> Set-Cookie2

>         Version="1" 
>       PICS-Label: (PICS-1.1 "http://www.aaa.org" label 
>         by "paranoid@aaa.org" gen true
>         for "http://www.acme.com/"

This "for" does not match the request URL *or* the (implicit) Domain
attribute of the Set-Cookie2!  (www.acme.com vs. www.roadrunnermaps.com)

>         exp "1997.12.31T23:59-0000" 
>         extension 
>           (optional "http://www.w3.org/PICS/DSig/sigblock-1_0.html"
>             ("AttribInfo" 
>               ("http://www.w3.org/PICS/DSig/X509.html" 
>                 "base64-x.509-cert"))
>             ("Signature" "http://www.aaa.org/trust.html" 
>               ("byName" "aaapublickey") 
>               ("SigCrypto" "8E53B19D35A3F19D35A38930E53FD35A7B215B2158")))
>         ratings (anonymousexchange 1))
> 

[...]
> 6. SUMMARY
> 
> This document presents an extension to the state management protocol 
    
    This document presents an extension to the HTTP State Management
    Mechanism

> defined in RFC2109.  It describes only changes to that protocol. Any 
> parts of the state management not explicitly described here are assumed

				^-- mechanism

> to remain as defined in RFC 2109.
> 

[...]
> 8. REFERENCES
[...]
> 
> [Kristol] Kristol, David M., HTTP State Management Mechanism (rev 1). 

								Rev

> Internet Draft <draft-ietf-http-state-man-mec-03.txt>
> ftp://ietf.org/internet-drafts/draft-ietf-http-state-man-mec-03.txt

Don't let Lou Montulli off the hook!  He's a listed co-author.
Received on Tuesday, 19 August 1997 14:47:21 EDT

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