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Re: [widgets] Comments on Widget Signature update (was RE: Widget Signature update)

From: Frederick Hirsch <Frederick.Hirsch@nokia.com>
Date: Fri, 13 Mar 2009 10:50:03 -0400
Cc: Frederick Hirsch <Frederick.Hirsch@nokia.com>, ext Marcos Caceres <marcosc@opera.com>, WebApps WG <public-webapps@w3.org>, Thomas Roessler <tlr@w3.org>
Message-Id: <D071BE13-874B-4572-B51E-CD1EBCB9D88C@nokia.com>
To: "ext Priestley, Mark, VF-Group" <Mark.Priestley@vodafone.com>
Mark

Thanks for your review, I have some comments inline. Thomas, can you  
please review my proposed change to the security considerations text  
Mark mentioned?

Thanks

regards, Frederick

Frederick Hirsch
Nokia

On Mar 12, 2009, at 12:53 PM, ext Priestley, Mark, VF-Group wrote:

> Hi Frederick, All,
>
> Some comments on the updated specification but first let me again say
> thanks for doing a great job making all the changes!
>
> ---------------------------
> Substantive comments
> ---------------------------
>
> 3
>
> "Implementers are encouraged to provide mechanisms to enable end-users
> to install additional root certificates. Trust in a root certificate  
> is
> established through a security critical mechanism implemented by the
> widget platform that is out of scope for this specification"
>
> [Comment] I know this was discussed before, and while I agree with the
> overall sentiment of the text, if we are encouraging implementers to  
> do
> this then I wonder if we should also add some warning text to the
> security considerations section, eg mechanisms to install new root
> certificates should be subject to security critical mechanisms, for
> example it end-users should be made aware of what they are doing and  
> why
> when installing a new root certificate.

sounds reasonable to add text to security considerations, will do.
>
>
> 4
>
> "5 Process the digital signatures in the signatures list in descending
> order, with distributor signatures first.
>
>   a. Only the first distributor signature MUST be processed."
>
> [Comment] Why is it required to always process the first distributor
> signature? What if the widget user agents security policy is only
> concerned with the author signature?  I think 5a should be removed.

ok, but where do we say that only one need be processed in the set of  
specifications?
Do we need to clarify that even if more than one is present, not all  
need be processed? This seems to be important assumption/decision that  
will get lost.

>
>
> 6.1
>
> "Required for signature verification, optional for generation:
> DSAwithSHA1"
>
> [Comment] When we discussed this before I think we agreed that it  
> might
> be necessary to support DSAwithSHA1 (and RSAwithSHA1?) for the
> verification of signatures in certificate chains but we ruled out the
> use of DSAwithSHA1 (and RSAwithSHA1) for widget signature generation
> (and therefore verification) as they are already considered too weak.
> Did I miss something?

What is the status of Requirement R47? looks like the algorithm MUSTs  
etc and requirement both need adjustment.


>
>
> 7.1
>
> Constraint 3b
>
> "The Algorithm attribute of the ds:digestMethod MUST be set to a  
> Digest
> method specified in the Algorithms section of this document."
>
> Constraint 5b
>
> "The ds:SignatureValue element MUST contain a signature generated  
> using
> a Signature method specified in the Algorithms section of this  
> document
> and MUST use a key that is of the length of a recommended key length."
>
> [Comment] These constraints are "MUST"s however the sections where we
> describe Digest Algorithms, Signature Algorithms and recommended key
> lengths the text currently allow the use of undefined other algorithms
> and key lengths. This seems inconsistent. I think we need to allow for
> the use of other algorithms and key lengths but at the same time we  
> have
> to somehow state that a widget user agent MUST support the base set
> defined in the specification, and authors should use these if they  
> want
> to ensure interoperability. As such, perhaps 3b and 5b would be better
> included as authoring guidelines?

how about replacing:

The ds:Signature MUST meet the following requirements:

	 The Algorithm attribute of the ds:CanonicalizationMethod element  
MUST be set to a Canonicalization method specified in the Algorithms  
section of this document.
	 The ds:SignatureValue element MUST contain a signature generated  
using a Signature method specified in the Algorithms section of this  
document and MUST use a key that is of the length of a recommended key  
length.
with



The ds:Signature MUST meet the following requirements:

	* Algorithms MUST conform to requirements given in the algorithms  
section of this specification.

re is really no need to say this except to make sure it is not missed,  
since the algorithms section outlines algorithm requirements. I'm also  
ok with removing this entirely. What do you think?)

>
>
> 8
>
> "Implementations that store the content of widget archives to the file
> system during signature verification MUST NOT trust any path  
> components
> of file names present in the archive, to avoid overwriting of  
> arbitrary
> files during signature verification."
>


> {Comment] I don't understand this sentence - which may well be a  
> problem
> with my understanding rather than the sentence - please can you
> enlighten me, thanks.
>
>


I think this is better worded as:

Implementations MUST NOT overwrite <widget files> during signature  
verification, as this could open the possibility of an attack based on  
substituting content for files due to malformed ds:Reference URIs in a  
signature that has been replaced.

(Thomas, can you please verify that I got that right?)

> ---------------------------
> Editorial comments
> ---------------------------
>
> General Terminology
>
> "Widget agent", "widget platform", "application"? -> "widget user
> agent"?
>
> "signature", "digital signature(s)" -> "widget signature(s)"

some instances refer to the widget signature file (widget signature)  
other to an XML Signature itself, some to generic signature  
operations. I'll take a look to clarify.

>
>
> "Policy" -> "Security policy"

ok
>
> "author widget signature" -> author signature (or vice versa)

ok

>
>
> "distributor widget signature" -> distributor signature (or vice  
> versa)

ok

>
>
> "Digest method" -> "Digest Algorithm"
>
> Also, as a general comment, not all defined terms are linked  
> throughout
> the document.
>

doesn't it get to be a bit much to link every instance?  If a term is  
linked once in a paragraph/list, I suggest not linking it again in  
that same paragraph/list.

>
> 1.4
>
> "Example of a distributor signature document, named signature.xml:"
>
> [Change] "signature.xml" -> "signature1.xml"

yes

>
>
> 4
>
> [Comment] Has it been decided to move this processing to the Digital
> Signatures specification rather than the Packaging and Configuration
> specification? FWIW I think it's cleaner to have it in the Packaging  
> and
> Configuration specification but I don't have strong feelings either  
> way.
>

it was decided by the WG I believe.

>
> 5.2
>
> "The author signature can be used to determine the author of a widget,
> that the widget is as the author intended, and whether two widgets  
> came
> from the same author."
>
> [Comment] The author signature _may_ be used to determine whether two
> widgets came from the same author, ie it depends whether the same
> private key was used.
> [Change] "and whether two widgets came from the same author" -> "and  
> may
> be used to determine whether two widgets came from the same author"

isn't this saying the same thing with more words?

>
>
> "An author signature need not be present in a widget resource, but at
> most one author signature may be present. A widget resource  MAY  
> contain
> zero or one author signatures, as defined by this specification."
>
> [Comment] Sentence contains redundant text.
> [Delete] "An author signature need not be present in a widget  
> resource,
> but at most one author signature may be present."

ok

>
>
> 7.3
>
> "If Widget Signature Validation fails for any reason then the
> application MUST be informed of the failure. In this case the
> application might choose not to install the widget."
>
> [Comment] by "application" do you mean "widget user agent"?
>

yes

>
> Thanks again!
>
> Mark
>
>
>
>> -----Original Message-----
>> From: public-webapps-request@w3.org
>> [mailto:public-webapps-request@w3.org] On Behalf Of Frederick Hirsch
>> Sent: 09 March 2009 20:51
>> To: ext Marcos Caceres
>> Cc: Frederick Hirsch; WebApps WG
>> Subject: Re: Widget Signature update
>>
>> I updated section 4 to correspond to  this:
>>
>> "If the signatures list is not empty, sort the list of
>> signatures by the file name field in ascending numerical order
>> (e.g.signature1.xml followed by signature2.xml followed by
>> signature3.xml etc)."
>>
>>
>> regards, Frederick
>>
>> Frederick Hirsch
>> Nokia
>>
>>
>>
>> On Mar 6, 2009, at 10:07 AM, ext Marcos Caceres wrote:
>>
>>> Hi Frederick,
>>>
>>> On 3/6/09 3:59 PM, Frederick Hirsch wrote:
>>>> I've updated the widget signature document distributor file naming
>>>> convention to the following after discussing this with Josh (thanks
>>>> Josh):
>>>>
>>>> Naming convention for a distributor signature:
>>>>   |"signature" [1-9][0-9]* ".xml"|
>>>>
>>>>       *
>>>>
>>>>         Each distributor signature /MUST/ have a name consisting of
>>>>         the string "signature" followed by a digit in the range 1-9
>>>>         inclusive, followed by zero or more digits in the range 0-9
>>>>         inclusive and then ".xml", as stated by the BNF.
>> An example
>>>> is
>>>>         "signature20.xml".
>>>>
>>>>       *
>>>>
>>>>         Leading zeros are disallowed in the numbers.
>>>>
>>>>       *
>>>>
>>>>         Any file name that does not match this BNF /MUST/ be
>>>> ignored.
>>>>         Thus a file named "signature01.xml" will be ignored. A
>>>> warning
>>>>         /MAY/ be generated.
>>>>
>>>>       *
>>>>
>>>>         There is no requirement that all the signature file names
>>>> form
>>>>         a contiguous set of numeric values.
>>>>
>>>>       *
>>>>
>>>>         These signatures /MUST/ be sorted numerically based on the
>>>>         numeric portion of the name. Thus signature2.xml preceeds
>>>>         signature11.xml, for example.
>>>>
>>>>
>>>> See draft
>>>> http://dev.w3.org/2006/waf/widgets-digsig/#distributor-signatures
>>>>
>>>> I also updated the notation section, changed the code format to be
>>>> italic (without color), and updated the body style to not
>> be quite so
>>>> large.
>>>>
>>>> Please indicate any comment or corrections on the list.
>>>>
>>>
>>> The changes look good to me! thank you.
>>>
>>> Kind regards,
>>> Marcos
>>
>>
>>

regards, Frederick

Frederick Hirsch
Nokia
Received on Friday, 13 March 2009 14:51:38 GMT

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