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[widget] [widget-digsig] Comment on WD of Widgets 1.0: Digital Signatures - use of Created property

From: Priestley, Mark, VF-Group <Mark.Priestley@vodafone.com>
Date: Wed, 15 Apr 2009 11:45:42 +0200
Message-ID: <0BE18111593D8A419BE79891F6C4690902C95D06@EITO-MBX01.internal.vodafone.com>
To: "public-webapps" <public-webapps@w3.org>
Dear All,
I have a number of comments against the Created property.
As previously communicated on conference calls (although I can't find
the relevant minutes) Vodafone objects to the mandatory use of the
Created property. The main objection is that on mobile devices the user
often does not set the correct time (or more usually the correct year)
which means the device defaults to the time/year of manufacture. As a
result many signatures will contain Created property values that, as far
as the device is concerned, happen in the future. Without a requirement
on a reliable and accurate timesource, which we are not proposing to
introduce, the Created property cannot be relied on. This combined with
the fact that the use of the Created property is down to the signer, or
the signing scheme within which the signer is operating, mean we think
its use should be optional.
This general comment translates to the specific comments below.   

"Each signature file MUST contain a dsp:Created signature properties
element compliant with XML Signature Properties [XMLDSIG-Properties] and
this specification."  
We would like to see the above changed to a MAY.

"As an example of use, assume a distributor's signing process is found
to have been compromised. Thus, it is not practical to exchange the
signature key. Being able to invalidate all signatures made before a
particular date would be important in such a scenario."
I'm not sure the above is a good example? If the signing process has
been compromised then I may want to invalidate signatures before this
date, but wouldn't I also change my key at this time to stop creating
new compromised signatures? In this case the end-entity cert should be
revoked. My understanding of timestamps was that their main reason for
being is to confirm that a signature was created at a particular
instance in time. This information can then be used for non-repudiation
and/or proof of existence of the signed object at a particular time in
the past. The above use case seems to be suggesting something else which
I do not fully understand.  
As previously communicated I think there is a case for an Expires
property, which could be used to state a point in time after which a
Signature is no longer valid (to allow for Signature with shorter lives
than the keys used to create them), but this is different from the
Created property. 
My suggestion is to rework the example. 
The sentence:
"A wall clock timestamp SHOULD be placed" 
is inconsistent with the text in 5.1 which states the element as a MUST.
If the text in 5.1 is changed to a MAY then the text in 7.2 would be OK
but we would prefer to make this a MAY as well.

"The Created Signature Property value SHOULD represent a wall clock
timestamp earlier than the current time, to the nearest minute. "
It's not clear what the user agent should do to respect this
requirement? We think that this should be left to the signer or signing
scheme to reflect use of the Created property through the UA's security
policy. The text on the Created property could then be deleted from this
"Upon signature generation, if this property is used, the time value is
set to a reference time, as defined by the application. "
Again, this is inconsistent with the text in 5.1 in which the Created
property is mandatory, unless the intention of the text is to be if the
property is used by the UA?
We think it should be made clear that Validation of the Created property
is optional. 
Mark Priestley 

Mobile: +44 (0)7717512838
E-mail: mark.priestley@vodafone.com <mailto:mark.priestley@vodafone.com>

Vodafone Group Services Limited 
Registered Office: Vodafone House, The Connection, Newbury, Berkshire
RG14 2FN Registered in England No 3802001

Received on Wednesday, 15 April 2009 09:46:29 UTC

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