Re: Comments on Widgets 1.0 Security requirements


Some more discussion inline, thanks for taking the time to review.

Do you mind updating the draft with the items we agree?

regards, Frederick

Frederick Hirsch

On Jan 7, 2009, at 11:03 AM, ext Priestley, Mark, VF-Group wrote:

> Hi Frederick,
> Thanks for your comments. As someone who had a hand in some of the
> requirements that you've commented on, please see some responses  
> inline.
> Regards,
> Mark
> -----Original Message-----
> From:
> [] On Behalf Of Frederick Hirsch
> Sent: 05 January 2009 22:22
> To: public-webapps
> Cc: Frederick Hirsch; Thomas Roessler
> Subject: Comments on Widgets 1.0 Security requirements
> I have some comments on requirements section 4.6, Security and DIgital
> Signatures, editors draft [1], and some concrete suggestions for
> changes:
> (1) R44
> This requirement is unclear. Is the intent to say that a signature
> associated with a widget package might be extracted and served to a
> client independently of the package, allowing the package to be
> delivered without the signature inside of it?
> Or is it saying that the certificate chain and/or revocation  
> information
> should be able to be accessed independently of the package?
> In general it might not make sense to validate a signature without
> access the widget content, since that is not meaningful unless it is
> possible to validate the content hashes used to generate and validate
> the signature.
> [MP] Re-reading the requirement I agree we could have been clearer in
> what we were requiring, which is:
> 1. It MUST be possible to extract a _copy_ of the digital signature
> document(s) from the widget package.
> 2. It SHOULD (MUST?) be possible for the widget user agent to complete
> the signature validation processing for a digital signature document
> that is provided independently of a widget package (noting that the
> signature is not validated until the reference validation processing  
> has
> also been successfully completed)
> When we write the specification text to meet this requirement we will
> need to ensure that the error cases are covered, e.g. when the
> independently supplied and packaged digital signature do not match.
> With these clarifications hopefully the requirement and rationale make
> more sense?

Although one can extract a signature XML element from a widget  
package, I'm not sure how meaningful that is if one cannot  
subsequently locate the content that is signed - for example if a  
ds:Reference refers to an item in the widget package, how can an  
extracted signature be validated if that item is no longer available?

Along similar lines, I might expect the URI for a resource to be  
relative if the signature is always enveloped (the signature is within  
the widget package containing the signature and other items) but  
perhaps a full URL for detached, when the signature is stored  
separately from the signed items.

I do not think this requirement is met by the Widgets Signature  
document as it states
"The URI attribute must be a relative path to the root of the widget."

how will this work with detached signatures where the widget content  
is not in the same context as the signature?

> (2) R45
> It would be useful to add a sentence as to why SHA-1 is still  
> required,
> e.g. "Continued SHA-1 support is recommended to enable backward
> compatibility and interoperability".
> On the other hand if the widget specification has not yet been  
> adopted,
> is there a reason not to require SHA-256 (and make SHA-1 optional),
> given the known potential weaknesses with SHA-1?
> Suggestion:  replace "MUST strongly recommend the use of SHA-256" to
> "MUST recommend SHA-256  for new signature generation and must  
> recommend
> SHA-1 and SHA-256 for signature verification" (or explicitly note that
> SHA-1 is optional)
> "strongly recommend" is not a normative phrase according to RFC 2119.
> [MP] I support your suggested changes.

I strongly suggest we require XML Signature 1.1 which should include  
new algorithms and address some practices around their use.

> (3) R46
> Change "and" to "or" in the first sentence and "or" to "and" in the
> second to obtain the intended meaning.
> [MP] Well spotted, this is indeed an error - I support your suggested
> changes
> (4) R49
> The phrase "To provide up-to-date" is misleading, since cached
> information may be less up to date than the result of an online query,
> especially with OCSP.
> Suggest changing  rationale paragraph to
> "To enable a widget to obtain revocation information without having to
> query an online CRL or OSCP server from each device. This is a lot  
> more
> efficient and eases the load on CRL or OCSP servers.  Note, however,
> that the revocation information may not be as up to date as an online
> query. However, if this information is updated at the server in a  
> timely
> manner before widget installations, then an online query would not be
> necessary at the client."
> [MP] I support your suggested changes; more accurate and clearer
> (5) Missing requirement: "A signature should indicate the role of the
> signer."
> Suggested text "A signature may be signed by a widget author as well  
> as
> a widget distributor. The role of the signer should be indicated to
> enable the verifier to understand the role of the signer and  
> associated
> implications."
> [MP] I don't object to this requirement but I would be interested in  
> the
> use case given that we have the author element?

where is the author element?

> (6) Missing requirement: "A signature should indicate a policy
> associated with it, independent of information associated with key or
> certificate information"
> For example, a signature should have a usage (or policy) property
> indicating that it is associated with the W3C Widget Signature
> specification and processing rules. The use of a URL is recommended to
> allow different policies and to enable updated versions.
> [MP] I agree with the intent of this requirement but think we need  
> to be
> clear what we want to specify. What we are really trying to say is  
> that
> the signature may be processed differently depending on the usage
> associated to the signature. However, before we specify this text I
> think we need some further discussion on what the usage properties
> should be and how they should be specified. For example, is it  
> expected
> that for each usage property there will be a section in Widgets 1.0:
> Digital Signatures that defines the processing for that usage property
> on top of the core processing?

I meant something different than what you read, so we may need to be  

I suggest we have a URI that indicates signature conformance to the  
Widgets 1.0 Digital Signature specification.

I believe you are suggesting additional policy that can vary? If so   
that is a different requirement.

> (7) Missing  requirement: "Widget packages only require signature
> validation and certificate and revocation verification upon first
> installation on a device"
> Proposed text:
> "A widget package signature is validated and associated certificates  
> and
> revocation information verified, only when the widget is first  
> installed
> on the device. Signatures and certificate and revocation information  
> may
> be updated over time at the server for subsequent installation on  
> other
> devices, effectively creating a new widget package."
> (8) Missing requirement - "Widget signatures must include counter-
> measures against use of out of date widget packages"
> Since a signature is validated upon widget installation, and this
> signature (and associated certificate and revocation information)  
> can be
> updated before subsequent widget installations, it is important that  
> an
> old signature cannot be re-used (replayed), since that would cause
> updated certificate and revocation information to be ignored.
> Thus a signature should have material to avoid later inappropriate  
> reuse
> - such as a short-lived expiration of the signature.
> Note that a nonce and timestamp, as used for replay attack mitigation,
> may not be suitable since the client may never have installed the  
> widget
> previously and not have access to earlier nonce information.
> [MP] Just so that I am clear, the requirement you are proposing here  
> is
> that the signature format must support the inclusion of signature
> expiration data? This expiration data is associated to the signature  
> and
> not the end-entity certificate used to generate the signature? If this
> is the proposal I am in full agreement. However, in this case I think
> that we re-word the proposed text as it currently implies that we are
> recommending the use of short lived signatures, which is really a
> security policy decision for the signer.

I was trying to capture the use case and associated requirements -  
that signatures are only verified upon widget installation and can be  
short lived (changed between installations). This is different than  
long term signatures used for legal purposes and more a code signing  

Yes, I believe an Expires property could satisfy the requirement but  
the requirement is about verification upon install and not allowing  
inappropriate reuse of widget packages.

> That is all for now, though I may have missed something.
> regards, Frederick
> Frederick Hirsch
> Nokia
> [1]

Received on Wednesday, 7 January 2009 18:37:01 UTC