Mark
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
Nokia
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: [email protected]
[mailto:[email protected]] 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 http://dev.w3.org/2006/waf/widgets-reqs/#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 http://dev.w3.org/2006/waf/widgets-reqs/#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 http://dev.w3.org/2006/waf/widgets-reqs/#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 http://dev.w3.org/2006/waf/widgets-reqs/#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
clearer.
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
application.
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] http://dev.w3.org/2006/waf/widgets-reqs/
