Digital Signature Label Architecture
WD-DSIG-label-arch-970610
Digital Signature Label Architecture
W3C Working Draft 10-June-97
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Philip DesAutels
Rohit Khare
Status of this memo
Editor's Note: This document represents the general DSig
architecture as envisioned by the DSig Design team. Work in implementing this architecture
along with interaction with other groups working on metadata related efforts (PICS-NG,
XML, DSig Collections, etc. ) has led to substantial changes in the way we now envision
the DSig architecture. The document presented below is incongruous with the current DSig
1.0 Digital Signature specification and the planned direction for DSig 2.0 and thus is
obsolete. A major revision of the DSig Architecture will be available by 1 December, 1997
This is a W3C Working Draft for review by W3C members and other interested parties. It
is a draft document and may be updated, replaced or obsoleted by other documents at any
time. It is inappropriate to use W3C Working Drafts as reference material or to cite them
as other than &work in progress&. A list of current W3C working drafts can be
found at: .
Note: since working drafts are subject to frequent change, you are advised to
reference the above URL, rather than the URLs for working drafts themselves.
Table of Contents
Introduction
Mission Statement
Signatures
Assertions
Putting It All Together
Conclusions
Acknowledgements
This document presents the architecture and design rationale behind DSig's Digital
Signature Label specifications. The overall goal is to use digitally signed labels to make
authenticatable assertions about standalone documents or about manifests of aggregate
objects. The three basic elements, digital signatures, assertions, and manifests, are each
analyzed in terms of its design, operation, data format, and distribution strategy. These
elements can be assembled today within a PICS label to make a signed assertion about an
information resource, or by signing a manifest, making assertions about several resources.
1. Introduction
The Digital Signature Label team is chartered with the design of a signed assertion
format which states 'the keyholder believes assertion(s) about information
resource(s).' This statement format satisfies the twin goals of the DSig project: to identify
and endorse information resources. This team, in turn, has decomposed its goal into
three subtasks:
&The Keyholder believes…& is a cryptographically
authenticated statement encapsulated into a signature block (SigBlock). The functionality
and design requirements for the digital signature cryptography are explained in Section 3,
& Signatures.&
&… assertion(s)…& is a systematic, machine-readable
description which allows for automatable trust decisions. In particular, we propose using
signed PICS ratings in Section 4, &Assertions.&
&… about information resource(s)& is a mapping of the
assertions to several related information resources. Each reference to a resources should
also should include integrity checks that make a secure link from the signed assertion to
the final data stream(s). These requirements are discussed in Section 5,
&Manifests.&
Each of these subtasks will result in interlocking technical specifications:
Signature Block describes the syntax of a generic signature syntax and a series
of cryptosystem-specific formats. The result is a standalone data-signature block. Editor:
Peter Lipp.
Signature Label describes the technical steps for encoding a signature block
within a PICS-1.1 label and its applicability. The latter part describes when signed
PICS-1.1 assertions are appropriate and some inherent risks. Editor: Brian
LaMacchia (how), Paul Lambert (why).
DSig Common Manifest Format (DCMF) describes how manifests can be constructed to
make joint assertions about a package of interrelated referents, as well as a particular
manifest format. Editor: Hemma Prafullchandra
(Please consult the DSig Team pages for detailed updates on the timeline and status of
these specifications.)
Each of these components can be comb this document provides
context on how they fit together:
Each component supports DSig's top-level design goals, such as international
cryptography support, flexible assertion semantics, and so on. Each component has a role
to play in proving the flexibility, portability, and integrity of the DSig system.
Each component's expected uses and supported deployment scenarios are explained
operationally and diagrammed.
Each component has
one of DSig's competitive advantages is its
technological simplicity weighed against the alternatives. Though there are competitors
for each (PKCS-7', freeform assertions, Cryptolopes, etc), each DSig specification
promotes a single format in the end.
Distribution
Each component can be distributed across the Web in several ways. This document explains
which are usable with or with which transmissio
and concrete deployment recommendations (Section 6).
For example, a typical DSig scenario combines the three components in a tree. Here is
how an author might first package up an applet in a manifest declaring his ownership, then
additional certify that the package, taken as a whole, is a safe and useful applet when
used correctly.
Author creates several related resources (the applet, its documentation, and sample
Author creates a manifest that points at each resource, with the assertion &I
created this& for each.
Author creates a separate label pointing to the manifest saying it describes a
&safe applet&.
Author signs the label and embeds the resulting signature block into the label produced
The recipient can reverse the process, verifying the author's signature and
constructing a pathway of three hash values and two assertions between the label and the
eventual applet data proving the author's own identity and endorsement of
that applet. DSig's real power, though, is that a third party can come along and replace
steps 3 and 4 above:
Reviewer comes along and rates the applet for usability and coolness, creating a new
label of the manifest from step #2 above.
Reviewer signs the new &This is cool& label and distributes it separately.
In this scenario, the end-user's trust manager can go seek out a reviewer's endorsement
and make a similar induction chain from the reviewer to the &cool& applet. With
DSig in place, the end-user's trust policy can automate the decision process.
2. Mission Statement
As part of its deliberations, the SigLabel team crafted a mission statement to define
its design envelope.
A Digital Signature Label is a standalone, cryptographically-protected
statement that 'keyholder believes assertion about information
resource(s)'.
Expanding some of the key terms defines the scope of our effort:
Standalone
Unlike traditional security approaches which wrap signed content, SigLabels will be
complete statements separate from the resource itself. This will allow us to leverage the
PICS label-distribution methodology: embedded within content, alongside it (in online
protocols), and from external sources.
Cryptographically-protected
SigLabels will include digital signatures to prove the authenticity and integrity of the
keyholder's statements. SigBlocks will support many different combinations of
cryptographic processes without prejudice.
Mathematically, a digital signature only expresses that at some point, some process had
access to both a secret and the unmodified message text. SigLabels will separate out
mechanisms to deduce principals (keyholders) from those keys -- whether in a certification
system where principals are keys (like SDSI or PGP) through to identity-based
systems (X.509). The central insight is to disintermediate the binding between the
cryptographic calculations and the certification infrastructure.
An assertion defines the meaning of the act of signature, in this case by describing the
content of the information resource. A SigLabel includes assertions according to
machine-readable schema so they are automatable: they can assist users in making
trust decisions. One kind of machine-readability is already provided in PICS-1.1:
numeric-vector rating. Another style of machine-readable encoding is text assertions from
a fixed grammar. These two examples are separate from non-automatable,
non-machine-readable free-form comments or extensions, which can also fit into current
PICS content ratings.
Information Resource
In web usage, any information resource can be indirected through a Universal Resource
Identifier (URI) -- including aggregate objects. Many applications of SigLabels will in
fact be assertions about such sets of resources. A manifest groups several referents
together, including assertions about each of them. Since URI technology can also
incorporate other naming schemes, standalone SigLabels can be applied to any named data.
In this document, we illustrate several of the concepts referred to above with the
following icons:
Key Information
Information Resource
Identity Certificate
Resource Reference Information
Ciphersuite
Digital Signature
Signature Label
Signature Block
3. Signatures
This section describes the design, operation, format, and transmission of the DSig
signature block. Our SigBlock is a general-purpose, simply encoded,
cryptographically-neutral standalone signature -- and it does not rely on any particular
certification/identification scheme, either. It does not have any inherent semantics: the
SigBlock only establishes that the some process had access to the data and a secret (we
will argue later that its semantics are strengthened when it is embedded in a particular
application).
3.1 SigBlock Design
The SigBlock was identified as a separate design task very early on in the project.
Signature systems such as PKCS-7', PGP, etc. traditionally build upon a core data
structure which represents the actual cryptography, and DSig is no exception. On the other
hand, every signature system goes on to add idiosyncratic information to this structure:
crytographic protection (padding, protection of algorithm identifiers),
certification-authority dependence (naming, key serial #s), content-dependent modification
(how is whitespace hashed in?), and mini-assertions (time of signature, whether signed in
hardware/software). The result is then encoded in a particular format (ASN.1, ASCII armor)
and often used to wrap the whole signed data stream. The design goals for the DSig
SigBlock differentiate it from such approaches:
General Purpose
SigBlocks default to signing a fixed data stream. This means any document or label can
conversely, a SigBlock can be used anywhere messages need to be signed (email,
applets, etc). No SigBlock-dependent data modifes the document-signing process. This means
that the only data being signed is the document -- no SigBlock fields go into the
computation by default. The document data is also separate from the SigBlock -- we don't
wrap protected data.
Simple Encoding
SigBlocks are encoded as ASCII text type-value S-expressions. This allows designers of
new ciphersuites to use clean self-describing data structure but does not preclude reuse
of proprietary binary data with an appropriate type identifier. Using S-expressions
sidesteps the complexity potentially affecting low-level type-length-value encoding
problems. This makes it possible to use much simpler tools than traditionally associated
with ASN.1, for example.
Cryptographically-neutral
SigBlocks must be able to incorporate new cryptosystems without prejudice. This includes
'black-box' tools that do not reveal internal steps like hashing. SigBlocks reuse the
well-known concept of 'ciphersuites' to refer to validated combinations of ciphers or
hardware tokens.
Self-contained
The SigBlock can contain all the data it needs to be verified. This implies it needs to
be able to carry associated certificates as needed. It also follows that international
deployment considerations require multiple, parallel signatures so that a standalone
signed assertion can be evaluated against several ciphersuites -- the end-user just
chooses a locally-acceptable variant.
Certification-neutral
Carrying certificates doesn't imply normative dependence, though. There is nothing
inherent in the cryptography of digital signature that requires certification chains, so
SigBlocks, too, should be able to operate with opaque certificates. After all,
certificates and other credentials are only there to establish trust in a key -- which is
a trust management problem, not a digital signature problem. Finally, since the first goal
in the list implies that credentials are not &hashed into& any signatures,
intermediaries can add and subtract credentials from the SigBlock as needed.
3.2 SigBlock Operation
To understand the SigBlock design, consider this picture of the signing process:
[Diagram of document being signed by a ciphersuite and a key to produce a bright, shiny
digital signature.]
[Diagram of the SigBlock itself with its pile of keyholder information, certificates
and digital signature cryptography bits.]
There are three central pieces of data in the diagrams above:
The data to be signed must be fixed to be signable. Typically, a document must
resolve to a unique hash through a computable function. (Though the signature
algorithm need not expose an explicit hash: A hardware token, for example, might not ever
transmit the hash, just the signature. RSAREF software has the same effect through
licensing restrictions.) For the purpose of discussing SigBlock, the Document can be any
data, though DSig overall is only concerned with signed labels and manifests.
Certificate
The credential(s) associated with a signing key are used by a trust management system to
establish the authenticity and validity of the signature. As far as the SigBlock is
concerned, a key must resolve to a keyholder though a certificate. The SigBlock
itself does not rely on this mapping, so it only acts as a carrier for
certificates, not as a user. The attribution-info section of the SigBlock can a
the signature-crypto section may include some keyholder-info
excepted from a certificate to establish the connection from signature block back up to
the keyholder.
The digital signature cryptography itself binds (the hash of) the document and the
signer's private key together. Thus, a SigBlock resolves to a document through a keyed
function. For the user to find the right key, the SigBlock also needs some way to
resolve which public key was used for the signature. Such keyholder-info is used
to1) find the actual signature verification key and 2) to find the keyholder and role so
that the trust policy can evaluate the validity of the signature.
With these concepts in mind, here's the story:
A user controls her own signing key and certificate. Her software
and hardware tokens implements a few ciphersuites. With a document and
choice of ciphersuite in hand, the signature process runs through to completion and
produce a bright, shiny chunk of signature-crypto. Along the way, the signing
key, date, and hash function name may have all been inputs to the
signing process and show up inside the signature-crypto. To make a complete,
formatted SigBlock, though, a few more pieces of information have to be pieced together as
discussed in Section 3.3.
There are a few more operations available to our user. She can proceed to attach
additional parallel signatures of the same document. She may do this herself using several
different algorithms so her recipients in other jurisdictions or in the future can verify
her signatures against whichever ciphersuites they (still) trust. Other like-minded
colleagues can also attach their signatures to the same statement to support policies that
require K-of-N signatories to agree. She can't cascade another signature to her own,
though. The DSIG design team has decided to support SigBlocks with parallel signatures,
but not cascaded statements about a signature. Successive endorsements are still possible,
though, by giving a SigLabel a name a producing another SigLabel about the first (See
Section 4.4).
3.3 SigBlock Format
The SigBlock Format is a concrete representation of the various data elements required
for a standalone signature. Broadly speaking, there are two levels at which we discuss the
SigBlock format: the generic arrangement of attribution-information and signature data,
and the specific level of encoding choices for particular ciphersuites.
[@@ diagram of SigBlock data structure with examples of http://w3.org/dsig/rsa/md5
and /PKCS-7/ --a structured set of bits vs. an opaque ASN.1
At the generic level, a SigBlock needs to flesh out the signature-crypto with
information about the signer. The first step is to pair the signature with attribution-info,
a passive container for certificates and other credentials. This allows the trust
management system to evaluate whether a certain keyholder is trusted and whether the key
is valid. The second step is to make a link from the signature to the keyholder, by adding
some keyholder-info to the signature itself.
There are several kinds of keyholder-info envisioned by DSig: the key itself,
a digested fingerprint of the key, a Distinguished Name, a certifying authority and serial
number, a role, etc. Each type serves to establish a path from the key to a credential
(keyholder). Remember, the whole attribution-info section
the keyholder-info is the only normative link from a signature to a certification
system. It's a critical disintermediating step, because it allows the user to separately
choose a CA infrastructure without changing the signature block format.
That's it for the generic level. By treating the cryptography as opaque, the only
additional information required is keyholder identification. Within the cryptographic
details, though, are a great number of critical, specific formatting decisions.
The ciphersuite is what defines the information found below it. For an
existing format, say PKCS-7' (for RSA), all that's possible is to say &coming up, one
blob of PKCS-7' data&. DSig's preferred modes, though, encourage more trasnparency.
The DSig ciphersuite for RSA, for example, is inspired by SDSI, with separate entries for
the exponent, ciphertext, hash algorithm, signature time, etc burst out into an
S-expression.
Note well the location of the signature-timestamp: per the first goal listed in Section
3.1, the only cryptographically protected data is found within the customizable signature-crypto.
If an implementation of RSA needs to sign its hash algorithm id, it can do so on its own
-- rather than predicating that all DSig-usable ciphersuites must. For example, the US
Government's Digital Signature Algorithm need not, since it is only defined in conjunction
Of course, all this flexibility empowers any organization to issue a new ciphersuite --
how can users trust that they are cryptographically safe combinations? Indeed, how will
the DSig project validate its own recommendations? In the end, users and organizations
will have to choose what makes sense for themselves. DSig will produce a small set of the
most popular ciphersuites and work closely with leading cryptographers and organizations
to vet its work. In particular, the Signature Label Implementation Team will include a
user-driven review team that will cooperate with RSA Labs (holder of the PKCS
specifications), the IETF, and IEEE 1363 in establishing its core set of DSig
ciphersuites.
3.4 SigBlock Distribution
It is worth reiterating that the SigBlock is only a stepping-stone in our work. It is
not deployable on its own for two reasons: 1) technologically, because it does not have
any pointer or connection to the data being signed -- not even a document hash (since some
signature ciphersuites do not expose an intermediate hash value) and 2) philosophically,
because it does not include any assertions about the data being signed. Deploying 'naked'
SigBlocks without associated assertions drags us back to circular question &yes, but
what does this signature mean?&. In short, the DSig project only supports
signed assertions, which translates into SigBlocks embedded in PICS labels or in a
manifest. SigBlocks may later be embedded in other formats, which is why we have made the
effort to separate out its definition -- but even then, not without clear, automatable
semantics in the new embedding.
The first implementation target is an embedding of SigBlocks into PICS 1.x labels, as
discussed in Section 4.4. In this case, signing PICS ratings imbues SigBlocks with
stronger semantics particular to this context: 'keyholder believes ratings are correct'.
4. Assertions
While many signature projects have succeeded at proving the identity or provenance of
an information resource, the DSig project breaks new ground because of its emphasis on endorsement,
in the form of signed assertions which users can rely upon to make trust decisions
automatable. The bedrock of this work is reducing generic &assertions& to
concrete rating labels so we can have clear, machine-readable semantics to sign. In this
section, we discuss the design of a assertion format, inspired largely by PICS-1.x.
Though there are several reasons motivating out adoption of PICS -- its distribution
mechanisms, established user base, rating organizations -- but this section only discusses
the ra ensuring the integrity of the binding to the eventual
information resource is a critical function presented in Section 5.
4.1 Assertion Design
DSig needs an assertion system designed to convey a clear meaning applied to almost any
kind of content. This drives two sets of design criteria, one about the semantics of
'assertable' statements and another about what can be labeled.
Clear Semantics
The ultimate test of an assertion is explaining it to the end user. DSig assertions must
be explained clearly to users. One kind of clear explanation is a value judgement,
a policy statement like &This movie is not for unaccompanied minors&. It may be
subject to interpretation, but it is clear. A more flexible platform is a content
description: a statement that characterizes the content at hand with respect to an
objective scale: &This movie contains adult language and graphic violence& --
giving the end user facts to make a local judgement.
Ratings along well-known axes are a clearly explicable concepts. PICS explicitly adopts
this model for its ratings, and initial deployment reinforces the idea that parents can
set policies in this idiom. The ratings need not be 'x out of y' continuous
values, either: the PICS spec describes how to encode multivalue sets and other variants
Automatable Semantics
Trust management systems can make life easier for users and administrators by acting
upon clearly stated assertions. Automatability requires assertions which are clear and
mechanically interpretable. In another context, automatability requires access to the
rating schema: the system behind the labels. PICS goes above and beyond this
requirement by not only requiring fixed rating systems, but also requiring
machine-readable schema descriptions.
Rating systems will be developed by many organizations for many purposes (Democrats for
political press releases, ISVs for trustworthiness, and so on). The key to broad
acceptance is clearly stated, objective scales, and community support. Clarity is also a
requirement for signable assertions: even more than for PICS rating, digitally signed,
legally enforceable signed assertions need to be very defensible.
Extensible Semantics
At the same time, there are many descriptions that won't fit into any fixed set of axes,
numeric or otherwise. DSig assertions must support extensible, even arbitrary semantics.
PICS' optional and mandatory extensions are a credible escape mechanism for this purpose
-- allowing both signed and unsigned extensions.
DSig needs to apply assertions with those qualities to several different kinds of
documents:
Static Documents
Labeling static documents is trivial using hash functions to prove the integrity of the
link. A document fingerprint in the label can prove that the label and labelled document
are in sync. To support DSig's cryptosystem neutrality, labels must be able to include
several hashes according to different algorithms. Other fields, like a last-modified date
and properties of a document like color, type, size, etc. can be used for the same purpose
(see Section 5)
Dynamic Documents
Many information resources are inherently dynamic: the current temperature in Oslo, chat
rooms, live video cameras. This does not preclude making assertions about them, though. A
service provider may ensure that a chat room is chaperoned and sign an accompanying
&no graphic violence& assertion. In this case, the link from the label to the
resource is the name (URI) itself, perhaps along with some document properties as above.
Novel Documents
New kinds of documents and naming systems are always cropping up. What about an
assertion regarding slides 3-7 of a presentation? How about assertions about geographic
areas? Here, we appeal to the universality of URIs and fragment identifiers which allow
any motivated principal to 1) define new namespaces and 2) new identifiers for subparts of
a document. As a reminder, URIs, unlike URLs, don't have to include hostnames and are
useful for on- and off-line document identification.
Finally, there is a new consideration about the heritability of signed
assertions. While it is technically correct to insist that ratings only apply to the
particular information resource specified, typical Web client usage may trigger a whole
series of actions when visiting a single 'location': loading inline images, embedded
applets, etc. So at one level, assertions need to precisely delineate what they apply to.
Secondly, there is a mathematical question of how far to extend a network of linked
objects. A SigBlock prote the label points indi
the manifest points indirectly to the target
and so on. Even if each
step of the way is verified by a document hash, how large can the tree grow with
confidence that all of the links allow the top-level assertion to be inherited? DSig
design guidelines are clear on this point as well. The conservative rule is that
assertions only apply one level removed by default.
There are a number of additional considerations about working with collections of
different resources, which are discussed at length in Section 5.
4.2 Assertion Operation
[Diagram: An signature label which combines an assertion, information about the
referenced resource, and a digital signature block. If we zoomed in on the SigLabel, a
PICS 1.x label, it would contain a pointer-to-schema (URL), pointer-to-document (URI),
machine-readable rating, extended non-machine-readable rating comments, info-about-rating
(on, by, generic flag), info-about-document (resinfo), and embedded SigBlock.]
A rating assertion is a passive data structure which can be used to answer a series of
questions, whether in the context of a simple content-filtering UI or a full-blown trust
management system:
What language is this rating written in?
To understand the rating, you need to understand its schema, the rating system. The name
of the rating systems is a URL -- the location of the machine-readable rating system
description file(s). A PICS rating system description, for example, identifies the
sponsoring organization, the axes, icons and descriptions of points along each scale, the
kind of scale it is, and so on. This file can be used to construct a user interface on the
fly that presents the whole system to an adminstrator to set limits on acceptable ratings,
for example. Several versions in different languages may be available at the same URL.
What's the rating?
The actual assertion is a rating vector according to some schema plus some optional
extension information. Its meaning can be imputed back from the rating system description
(schema) by presenting the comments, icons, and description of each point on each scale.
The label can include additional metainformation about the rating itself: who made the
whether the rating applies only to the named resources or generically to
any resources with a matching name.
What document does this rating apply to?
Within the label structure, we can extract fields for the name (URI) and optional fields
like the hash, type, etc. Each piece of information about the resource can increase a
user's confidence in the connection: name equivalence, the hash value, file type and
length, and so on.
Of course, an assertion label alone cannot prove that a recipient should believe
that assertion. A Trust Management systems has to investigate several aspects of a signed
label before making that decision. For example, the TM would have to establish the
integrity of the connection of the label to the resource it's labeling. Particular pieces
of resource information can be attacked, but taken together they can prove the integrity
of the association between assertion and the resource. For example, providing the name
alone is vulnerable to attacks on the Domain Name S a cryptographic hash could be
reverse-engineered or 'birthday-attacked'; and a file length can be tampered with, but a
TM system can check each of these.
All of this additional information, or resinfo, has its roots in PICS-1.1 usage,
but expands the range of possible additional data and provides for its own extension
mechanism. More details can be found in the specification for signing PICS 1.x labels by
Brian LaMacchia. Paul Resnick, one of the original designers of PICS, commented that:
I admit that the PICS-1.1 label format does not syntactically separate the components
that describe the information resource (e.g., the URL, hash, and ratings) from those
components that describe the label (e.g., expiration date, signature, by). The PICS
designers' understanding of this separation idea evolved as we wrote the specs: in our
text description of the fields in the spec, we divide them into those that describe the
document and those that describe the label, but we didn't get so far as to make a
syntactic distinction, in the labels themselves. It remains to be seen whether we can
remedy this in PICS-1.2 or 2.0, but it is certainly an important design goal.
4.3 Assertion Format
We are proposing using the PICS rating label format with some expected modifications.
Some are minor, like allowing full S-expressions as the value of an extension field
(instead of only allowing strings). Others are more substantial, but already in discussion
for PICS-1.2, such as string-valued ratings. As DSig implementation continues, we expect a
dialogue with the PICS Working Group which will influence the evolution of both projects.
With respect to signing PICS rating labels, there is a concern about which PICS
extension fields are included under the protection of the SigBlock. For now, we propose
that the entire PICS label and all extension data must be signed together without
exceptions.
Finally, it makes sense to provide default rating systems for basic signature
applications, along the lines of 'This is True' and 'This is Mine'. Just as the SigBlock
is used to prove the identity and integrity of an assertion, these rating systems can be
used to make signed testaments of the provenance and veracity of information resources
themselves.
4.4 Assertion Distribution
One of the primary reasons DSig builds upon PICS for its label syntax is to reuse
PICS's three label distribution mechanisms. Since SigLabels are just PICS labels with
embedded SigBlocks, they can be sent:
Embedded in the information resource itself (e.g. using the HTML META tag)
Attached to the information resource (e.g. using HTTP entity headers)
Detached, possibly from third parties (e.g. using a 'label bureau')
The first mode could be popular for many other trust management applications, such as
embedding SigLabels into applets, fonts, and other protected resources. Especially for the
latter two modes, though, there must be a reliable link to the actual information
resource. As discussed in Section 4.2, labels must accommodate a range of additional
resource information to vouch for the connection. Document hashes are a particularly
effective way of proving the link even when the label and content are separated, but only
for static documents. For dynamic data, such as a chat room or live video camera feed,
other properties might be used.
Note that this assertion distribution strategy also fixes a SigBlock distribution
strategy: embedded in rating labels. SigBlocks are only found embedded in PICS labels in
this scenario. This makes the association of a SigBlock to its signed data extremely
Finally, there is another implicit resource that must be distributed reliably with the
assertions: the rating schema. Since a rating systems can legitimately exist in several
languages and compatible versions, it is not a simple task to protect the integrity of the
reference to the rating system. DSig recommends that applications which are sensitive to
this need should use manifests and include the exact rating system(s). In this case, a
rating system is just one more information resource the overall package depends on, like a
font or a configuration file.
5. Manifests
Many applications which call for the added security and integrity of digital signatures
actually address sets of interrelated content rather than a single information resource.
Several of the organizations participating in the DSig design phase arrived with proposals
including proprietary manifest formats. This section presents common design considerations
for using standalone manifest files and proposes a new intermediate, the DSig Common
Manifest Format (DCMF).
5.1 Manifest Design
The critical difference between a set of singular signed assertions and as collected
into a manifest and signed jointly is the interrelationships between the elements.
Manifests establish relationships by the mere act of se by rating
component and through additional resource information. Each entry in
the manifest also needs to provide enough information about the target resource to
unambiguously identify it. Finally, there can be additional goals for particular manifest
formats, such as optimized data layout, real-time manifest generation, and user interface
The relationships between components can dramatically shade their meaning. A picture
and a caption, for example, are strongly connected, and a different choice among
alternative captions can shade the meaning of the picture dramatically. First, the act of
preparing a manifest alone allows us to make statements about the aggregate as a whole.
Different manifests can associate a picture with different captions. In a legal context,
it can be essential to demonstrate that all parties are referring to the complete
agreement (e.g. a Will and its codicils). Second, the assertions about each entry in the
manifest establish another kind of relationship, specific to the rating system at hand. A
user could select components by label ('please show all the Impressionist pictures').
Third, additional resource information can clarify relationships like
'32-bit-color-version-of' or 'full-screen-sized'.
Beyond clarifying the semantics of a package, a manifest file must unambiguously
identify its components to legitimately stand in for signing each individual part.
Operationally, manifests allow us to seal N resources at once, which is more efficient
than having to execute N potentially expensive and time-consuming signature operations. To
mathematically verify this aggregation, each referent must include verifiable information
about the target resource, like its hash fingerprints. In fact, the entire resinfo
mechanism presented in Section 4.2 actually emerged from manifest discussions. The
PICS-1.1 resinfo extension was created to emphasize the isomorphism between an individual
label and an individual manifest referent.
Finally, there can be more specific design goals for particular manifest formats. Some
applications may integrate the 'user interface' of a package with the manifest itself. An
HTML WebMap file can represent manifest referents and a visual hierarchy,
multimedia descriptions, and more. Another example is signing streaming or dynamic data.
For a real-time video stream, it may be essential to provide a new kind of hash tree for
each video segment rather than a single hash value at the end. A dynamically generated web
page may have several components and use a manifest that sends its table-of-contents
first, then its hash values. Such data layout considerations are discussed in Section 5.3
and in the DCMF specification.
5.2 Manifest Operation
[Diagram: a SigLabel pointing at an aggregate-object-manifest instead of a single
document (and the target resources, in turn, which can also be manifests).]
As far as the signature label is concerned, a manifest is just another type of
information resource. In turn, the manifest is just the collection of
the cryptographic protection is inherited from the SigLabel and doesn't appear directly
within the manifest. The induction chain from the SigBlock to the target resource is
mediated by two assertions, one from the SigLabel that applies to the entire manifest and
an optional assertion from the referent in the manifest.
The manifest itself provides several pieces of data for each referent: name, hash
information, additional resource information, and a rating assertion. This is the same
information provided for a reference in a singular PICS SigLabel, too. The hash
information, in particular, is the crux of the chaining argument that allows the top-level
SigBlock to 'sign' the target data. In fact, the chain can extend further if the target is
another aggregate subcomponent represented by a manifest.
There is a new operational concern about signing several assertions jointly. To protect
the cryptographic hygiene of a signing key, it may be necessary to restrict which kinds of
assertions it can speak for. For example, it could be risky to use the same key to protect
high-value assertions about indemification and low-value assertions about the color
5.3 Manifest Format
The data format for the manifest should be compact and allow efficient access to data
about each referent. In DCMF, 'fixed' information like the selection of hash algorithms
used and the rating schemas are declared once, in a preamble. Then, for each referent
several columns of data are available: the name (URI), ratings, several hash values (if
applicable), and additional resinfo.
This abstract model of a manifest describes many formats: Java JARchives, IBM's
Cryptolope, a PICS labellist, an HTML Web collection (Sitemap). Though a file in any of
these formats could be signed with a SigLabel, and a Trust Management engine could parse
all of them to establish links back to the target resources, DCMF's alleviates
compatiblity problems. DCMF's data layout is designed to be compact, streamable, easily
decoded, extensible, and scalable -- an evolutionary successor to many of these
contenders.
5.4 Manifest Distribution
Since manifests are useful indexes quite separately from SigLabels, several
distribution strategies should be supported. Standalone manifest files can be made
available with the data, from third parties, or embedded directly into a package (e.g. a
ZIP or TAR archive). The complexity is compounded by the additional options for
distributing SigLabels. The SigLabel providing the integrity of a manifest might be
embedded within it, sent with it over HTTP, or come from yet another third party.
As long as a manifest file is static (i.e. can be hashed itself) and can provide
verifiable information about which resources it refers to, any of these distribution
strategies will work. The mathematics can support an induction chain from any SigLabel
through any manifest to any target resource -- even through a cascaded chain of nested
manifests.
6. Putting It All Together
Though we have separated the tasks into understanding how each of these three pieces
work on their own, they are intended to work together. This section explains how the DSig
project implementation will assemble these components to solve users' trust problems.
In general, the user starts with a single object or aggregate object to make an
assertion about. The assertion is prepared as a PICS 1.x rating about the object or
aggregate, then the label is signed with a SigBlock. Indeed, several signatures can be
included in parallel, from multiple endorsers using multiple ciphersuites. In the case of
an aggregate object, the components are listed out in a manifest, optionally including
assertions describing the role of each.
There are many combinations of distributions strategies to reach the end user. A simple
on-line scenario is a signed press release with a SigLabel in its HTTP entity headers. For
an aggregate object distributed over the Web, a user could fetch the manifest over HTTP
and receive the author's SigLabel with it before proceeding to fetch any of its
components. A CD-ROM might include a manifest with an embedded SigLabel. Before running an
applet found on the Internet, a user's trust engine could try to fetch a SigLabel from a
third-party label bureau. Scenarios can be generated for every combination of information
resource, speaker, format, and distribution technique.
7. Conclusions
Digital signature labels are assembled from three interlocking components. Signature
blocks, assertions, and manifests have each been described in terms of their design,
operation, format, and distribution. The initial deployment target is signed PICS rating
labels applied to single i manifests are a particular kind of
information resource which can be used to point in turn at several more resources while
maintaining cryptographic integrity of the signature label.
This architecture document serves as the context for several subsidiary technical
documents specifying the exact syntax and semantics of the SigBlock, SigBlock embedding in
PICS 1.x, and DSig Common Manifest Format (DCMF). Comments on this document should be
directed to the author or any of the specification editors. The SigLabel design team has
its own (closed) mailing list for discussing these issues at w3c-dsig-label@w3.org;
W3C member organizations can send comments there directly. Members are also encouraged to
join the DSig implementation phase to continue developing these specifications in
products, or through our user-organization review teams.
8. Acknowledgments
This document reflects a hard-won consensus among all the various players of the
SigLabel team. Many of the ideas here came from di my role is
primarily to wrap it all together here in this architecture document. Kudos to all!
John Carbajal &carbajal@&
Philip DesAutels &philipd@w3.org&
Rosario Gennaro &rosario@&
Jack Haverty &jhaverty@&
Brian LaMacchia &bal@&
Paul Lambert &palamber@&
Peter Lipp &plipp@iaik.tu-graz.ac.at&
Jim Miller &jmiller@w3.org&
Hemma Prafullchandra &hemma@&
Rob Price &&
Paul Resnick &presnick@&
Pankaj Rohatgi &rohatgi@&
), All Rights Reserved. W3C
rules apply.