One document matched: draft-ietf-xmldsig-core-01.txt
Differences from draft-ietf-xmldsig-core-00.txt
XML Digital Signatures Working Group D. Eastlake,
INTERNET-DRAFT IBM
draft-ietf-xmldsig-core-01.txt J. Reagle,
Expires April 22, 2000 W3C/MIT
D. Solo,
Citigroup
XML-Signature Core Syntax and Processing
Copyright Notice
Copyright (c) 1999 The Internet Society & W3C (MIT, INRIA, Keio), All
Rights Reserved.
IETF Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
W3C Status of this document
This document is a production of the joint IETF/W3C XML Signature
Working Group.
http://www.w3.org/Signature
The comparable html draft of this version may be found at
http://www.w3.org/TR/1999/WD-xmldsig-core-19991022
The latest version of this draft series may be found at:
http://www.w3.org/TR/xmldsig-core
This is the second (and rough) public draft of this specification.
This draft covers most of the topics the final specification will
cover, however parts of the text and syntax within this specification
are subject to change (and may be incorrect or inconsistent.)
Please send comments to the editors and cc: the list
Eastlake, Reagle, Solo [Page 1]
Internet Draft XML-Signature Core Syntax October 1999
<w3c-ietf-xmldsig@w3.org>. Publication as a Working Draft does not
imply endorsement by the W3C membership or IESG. This is a draft
document and may be updated, replaced or obsoleted by other documents
at any time. It is inappropriate to cite W3C Drafts as other than
"work in progress." A list of current W3C working drafts can be found
at http://www.w3.org/TR
Patent disclosures relevant to this specification may be found on the
WG's patent disclosure page.
Abstract
This document specifies the syntax and processing rules for the
encoding of digital signatures using XML. Such signatures can provide
integrity, message authentication, and/or signer authentication
services for data of any type, whether located within the XML that
includes the signature or locatable elsewhere.
Table of Contents
1. Introduction
1.1 Editorial Conventions
1.2 Design Philosophy
1.3 Versions
2. Signature Overview
2.1 The Signature Element
2.2 The SignedInfo Element
2.3 The ObjectReference Element
2.4 The Manifest and Package Elements
3. Core Signature Syntax
3.1 The Signature element
3.2 The SignatureValue Element
3.3 The SignedInfo Element
3.4 The KeyInfo Element
3.5 The Object Element
3.6 The Parameter Element
4. Additional Signature Syntax
4.1 The Manifest and Package Elements
4.2 The Properties Element
4.3 Processing Instructions
4.4 Comments in dsig Elements
5. Algorithms
5.1 Algorithm Identifiers and Requirements
5.2 Message Digests
5.3 Message Authentication Codes
5.4 Signature Algorithms
5.5 Canonicalization Algorithms
5.6 Transform Algorithms
5.7. Algorithm References
6. Processing rules
6.1 Generation
6.2 Signature Validation
7. Security Considerations
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7.1 Only What is Signed is Secure
7.2 Only What is "Seen" Should be Signed
7.3 Check the Security Model
7.4 Algorithms, Key Lengths, Etc.
8. Example syntax
9. DTD
10. Open Issues
11. Acknowledgements
12. Other Useful Types
13. References
1 Introduction
This document describes the proposed syntax and processing rules for
the XML Digital Signature specification. This specification provides a
mechanism for applying digital signatures to XML documents and other
Internet resources and encoding those signatures as XML.
The structure allows for both embedded and detached signatures. An
embedded signature can include the signature within the signed object
or embed the signed object within the signature. A detached signature
allows the signature to be independent of the object. The processing
structure allows for switching between embedded and detached
signatures without necessailry invalidating the signature.
In addition to the basic signature type, this document also defines
other useful types including methods of referencing multiple resources
and key management and algorithm definitions.
1.1 Editorial Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The XML namespace [XML-namespace] URI that MUST be used by
experimental implementations of this dated specification is:
xmlns="http://www.w3.org/1999/10/signature-core"
While applications MUST support XML-namespaces, the use of our "dsig"
XML namespace prefix and defaulting/scoping conventions are OPTIONAL
-- we use these facilities so as to provide compact and readable
examples.
The URI in the namespace declaration above is also used as a prefix
for URIs which identify resources, algorithms, or semantics under
control of this specification. We use MIME types to identify
algorithms, resources, or their characteristics under the control of
IANA. Otherwise we define a URN Namespace Identifiers [RFC2141] for
other organizations, for example: urn:ietf-org:hmac-sha1
This document includes the following abbreviations for long words.
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(The acronyms are generated by wrapping the word_length-2 in the first
and last letter):
* c14n: canonicalization
* i18n: internationalization
Finally, this document includes a list of open issues which are still
being addressed by the working group.
Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's
"Declaring Elements and Attributes in an XML DTD."
1.2 Design Philosophy
The design philosophy and requirements of this specification are
addressed in the XML-Signature Requirements document
[XML-Signature-RD].
1.3 Versions
No provision is made for an explicit version number in this syntax. If
a future version is needed, it is expected to use a different
Namespace.
2 Signature Overview
This section provides a general top down overview of XML digital
signature syntax and processing. The formal specification is provided
in later sections. General familiarity with digital signature concepts
and XML syntax is assumed.
2.1 The Signature Element
XML digital signatures are very flexible and may be used to apply
signatures to any type of resource. The object(s) being signed may be
included within the signature, outside the signature in the same
document, or completely outside of the document.
XML digital signatures are represented by the Signature element which
has the following structure:
<Signature>
(SignedInfo)
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The required SignedInfo element is the information which is actually
signed. SignedInfo includes a digest calculated over each of the data
objects being signed. The core signature verification includes the
verification of these digests. The algorithms used in calculating the
SignatureValue are also included in the signed information. The
signature can not cover itself so the SignatureValue element is
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outside SignedInfo.
KeyInfo indicates what key was used to create the signature. It is
optional because in some applications the key is implied by the
circumstances. A wide variety of KeyInfo forms are available including
certificates, key names, key agreement algorithms and information,
etc. The keying information is outside of the signed information so
that it need not be signed. KeyInfo might contain auxiliary
information it is not desired to reveal to all signature verifiers. If
KeyInfo were signed, it would be necessary to pass all of it to all
verifiers. On the other hand, if it is desired to bind the keying
information in to the signature, its digest and a pointer to it can
easily be included in the signed information.
Object is an optional element for carrying the signed data. A
signature can be applied to a mix of external and embedded objects.
The data can be optionally typed and/or encoded. While Object elements
can appear inside a signature as shown above, they can also appear
outside of the Signature element in the same document or in other
documents.
There is no explicit provision for additional signature properties
such as time of signing. (These are traditional called "attributes"
although that term in that context has no relationship to the XML term
"attribute".) Signature properties can be included as a type of Object
and thus can easily be signed or not as appropriate.
2.2 The SignedInfo Element
The SignedInfo element has the structure indicated below.
<Signature>
<SignedInfo>
(CanonicalizationMethod)
(SignatureMethod)
(ObjectReference)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The CanonicalizationMethod is the algorithm which is used to
canonicalize the SignedInfo element before it is digested as part of
the signature operation. [The working group is considering specifying
a fixed CanonicalizationMethod which would eliminate the need for this
element.]
The SignatureMethod is the algorithm used to convert the canonicalized
SignedInfo into the SignatureValue. It is a combination of a digest
algorithm and a key dependent algorithm such as RSA-SHA1 or HMAC-SHA1.
The algorithm names are signed to resist attacks based on substituting
a weaker algorithm.
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To promote interoperability, there are mandatory to implement
canonicalization and signature algorithms. Additional standard
algorithms are specified as Recommended or Optional and user defined
algorithms are permitted.
The ObjectReference elements specify the things secured by the
signature. As discussed in more detail below, they point to the thing,
specify any transformations, specify the digest algorithm, and include
the digest value itself. It is the signing of this digest value and
its verification as part of the signature verification that secures
the thing pointed to.
The indirect reference to secured things via the ObjectReference means
that it is possible to change a Signature from one where the data in
enclosed as an Object within the Signature to one where the Object
appears elsewhere or to move a secure item between locations outside a
Signature without invalidating the signature provided the secured data
can still be located from the same ObjectReference.
2.3 The ObjectReference Element
The ObjectReference element has the structure indicated below.
...
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
<ObjectReference Location=? Type=? >
(Transforms)?
(DigestMethod)
(DigestValue)
</ObjectReference>+
</SignedInfo>
...
The optional Location attribute says where the secured thing is.
The optional Type attribute provides information about the content of
the thing at Location. In particular, it can indicate that the thing
consists of signature Properties or is a Manifest or Package (see
below).
Transforms is an optional ordered list of processing steps that are
applied to the thing at Location before it is digested. These
transforms can include any number of canonicalizations, encoding and
decoding including compression and inflation, and XPath based and
other transforms. XPath based transforms permit parts of an XML thing
to be omitted. For example, if a thing being secured encloses the
signature itself, such a transform must be used to exclude the
signature from the data covered. If no Transforms element is present,
the data pointed at by Location is digested directly.
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To promote interoperability, there are mandatory to implement
canonicalization and decoding algorithms. Additional standard
canonicalization, coding, and XPath based transform algorithms are
specified as Recommended or Optional and user defined transform
algorithms are permitted.
DigestMethod is the algorithm which, when applied to the thing at
Location after Transforms is applied, results in DigestValue. The
signing of the DigestValue is what secures the thing pointed to.
2.4 The Manifest and Package Elements
There are cases where it is efficient to have one signature covering
many items. One approach is to include multiple object references
within SignedInfo. Since the core verification behavior of this
specification includes verifying the digests of objects referenced
within SignedInfo, some applications may need an alternative approach
which allows pushing the validation decision to the application. This
allows more complex processing to be defined on an application
specific basis; for example, it may be sufficient if the signature's
validity for n out of m of the items can be verified or there may be a
large number of items that it is desired to sign with multiple
signature algorithms and / or keys where listing all of the items
within the SignedInfo element of each Signature is too bulky.
To answer these requirements, additional object types have been
defined which may be referenced by SignedInfo. The Manifest element is
provided which similarly contains a collection of references and
objects (like SignedInfo), but leaves it entirely up to the
application which digest or digests it will verify. Multiple
signatures over the possibly large number of items in a Manifest need
only point to the Manifest from one ObjectReference in each
signature's SignedInfo.
The structure of Manifest, which reuses the ObjectReference and Object
elements described above, is as follows:
<Manifest>
(ObjectReference)+
(Object)*
</Manifest>
A Package is syntactically identical to a Manifest but asserts the
identity of each of its ObjectReference elements after Transforms
application.
Manifest and Package appear as the content of Objects.
3 Core Signature Syntax
The general structure of an XML signature is described in section 2
above. This section provides detailed syntax of the core signature
features and actual exampes (TBD). Syntax in this section is mandatory
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to implement unless othewise indicated.
3.1 The Signature element
<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*)>
<!ATTLIST SignedInfo
Id ID #IMPLIED>
A simple example follows:
<Signature xmlns="http://www.w3.org/1999/10/signature-core">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/.../xml-c14n"/>
<SignatureMethod Algorithm="dsig:dsaWithSHA-1"/>
<ObjectReference Location="http://www.ietf.org">
<DigestMethod Algorithm="urn:nist-gov:sha1"/>
<DigestValue
encoding="urn:ietf-org:base64">a23bcd43</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue
encoding="urn:ietf-org:base64">dd2323dd</SignatureValue>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
Note: this example will be revised to ensure hash/signature validate.
3.2 The SignatureValue Element
The SignatureValue element contains the actual value of the digital
signature. The ability to define a SignatureMethod and SignatureValue
pair which includes multiple distinct signatures is explicitly
permitted (e.g. "rsawithsha-1 and ecdsawithsha-1").
<!ELEMENT SignatureValue CDATA >
<!-- base64 encoded signature value -->
<!ATTLIST SignatureValue
encoding CDATA "urn:ietf-org:base64" >
3.3 The SignedInfo Element
The structure of SignedInfo includes a canonicalization algorithm, a
signature algorithm, and one or more references to objects. The
SignedInfo element may contain an optional ID attribute that will
allow it to be referenced by other signatures and objects.
<!ELEMENT SignedInfo(CanonicalizationMethod, SignatureMethod,
ObjectReference+ ) >
<!ATTLIST SignedInfo
Id ID #IMPLIED >
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SignedInfo does not include explicit signature properties. If an
application needs to associate properties (such as signing time,
signing device, etc.) with the signature, it may add an additional
Object that includes that data and reference that Object via an
ObjectReference. See the Properties element below.
3.3.1 The CanonicalizationMethodElement
CanonicalizationMethod is a mandatory element which specifies the
canonicalization algorithm applied to the SignedInfo element prior to
performing signature calculations. This element uses the general
structure here for algorithms in which a URI is included as an
attribute naming the algorithm and optional Parameter contents of the
element contain any parameter, value, or other information for the
algorithm. Possible options may include a minimal algorithm (CRLF and
charset normalization), or more extensive operations such as
[XML-C14N]. An expected default for this value will be defined once
the specification of XML aware canonicalization algorithms are
finalized.
<!ELEMENT CanonicalizationMethod Parameter* >
<!ATTLIST CanonicalizationMethod
Algorithm CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI] -->
[The working group is considering specifying a fixed
CanonicalizationMethod which would eliminate the need for this
element.]
3.3.2 The SignatureMethod Element
SignatureMethod is a required element which specifies the algorithm
used for signature generation and validation. This algorithm
identifies all cryptographic functions involved in the signature
operation (e.g. hashing, public key algorithms, MACs, etc.). This
element uses the general structure here for algorithms in which a URI
is included as an attribute naming the algorithm and optional
Parameter contents of the element contain any parameter, value, or
other information for the algorithm. While there is a single
identifier, that identifier may specify a format containing multiple
distinct signature values.
<!ELEMENT SignatureMethod Parameter* >
<!ATTLIST SignatureMethod
Algorithm CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI] -->
3.3.3 The ObjectReference Element
ObjectReference is an element that may occur one or more times. It
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includes a pointer to the object being signed, the type of the object,
an optional list of transforms to be applied prior to digesting, a
digest algorithm and digest value.
<!ELEMENT ObjectReference ( Transforms?, DigestMethod, DigestValue ) >
<!ATTLIST ObjectReference
Id ID #IMPLIED
Location CDATA #IMPLIED>
Type CDATA #IMPLIED>
<!-- The values of Location and Type conform
to the productions specified by [URI] -->
There is an optional ID attribute so that it can be referenced from
elsewhere.
The optional Location attribute identifies where to find the Object
using a URI. As the terms are defined in RFC2396 [URI], some URIs are
used in conjunction with a fragment identifier by use of a separating
hash (#), but the URI does not include the fragment identifier.
Location only permits a URI, and fragment identification is covered
under Transforms. Note that a null URI is proper an indicates the
current document (Location=""). If this attribute is omitted, then the
receiving application is expected to be able to determine the object
to which the signature applies. For example, this attribute might be
omitted for a signature in a lightweight protocol data unit. The
location may be omitted only if there is a single object reference. If
there are multiple object references, they each must contain an
explicit location. Note, it is the content yielded after the URI is
dereferenced, decoded, and transformed that the digest algorithm is
applied to. If the URI indicates an XML document, the document is
assumed to be unparsed prior to the application of Transforms. If
there are no Transforms, then the indicated resource is passed to the
digest algorithm unmodified.
The optional Type attribute contains information about the type of
object being signed (e.g. manifest, package, document, SignedInfo.
This is represented as a URI.
Examples follow:
Type="http://www.w3.org/1999/10/signature-core/manifest"
Type="urn:ietf-org:hmac-sha1"
3.3.3.1 The Transforms Element
Transforms is an optional element that contains one or more operations
to be performed on an indicated resource prior to digest calculation.
(These operations are different from those specified in the Signature;
those are are applied over SignedInfo.) If the Transforms element is
omitted, the exact data referenced is digested byte for byte.
The Transforms element contains an ordered list of Transform elements.
The output of each Transform serves as input to the next Transform.
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The input to the first Transform is the raw data result of obtaining
the resource given by the Location attrbiute or as determined by the
application if no Location is meaningful for that application. The
output from the last Transform is the input for the digest algorithm.
<!ELEMENT Transforms (Transformation+) >
Each Transform consists of an Algorithm attribute, optional Encoding,
Type, and Charset attributes, and content appropriate for the given
algorithm. The Algorithm attribute value specifies the name of the
algorithm to be performed, and the Transform content is provided as
additional data to govern the algorithm's processing of the input
resource. If the Encoding attribute indicates that the Transform
content is base 64 encoded, then the Transform content will be base-64
decoded before it is presented to the transformation algorithm.
The optional Type and Charset attributes are made available to
algorithms which need and are otherwise unable to deduce that
inforamtion about the data they are processing.
<!ELEMENT Transform ANY>
<!ATTLIST Transform
Algorithm CDATA #REQUIRED
Encoding CDATA #IMPLIED
Type CDATA #IMPLIED
Charset CDATA #IMPLIED >
<!-- The CDATA for Algoirhtm and Encoding conforms
to the productions specified by [URI] -->
Examples of resource transforms include but are not limited to base-64
decoding, canonicalization, XPath filtering, and XSLT. The generic
definition of the Transform element also allows application-specific
transform algorithms. For example, the transform could be a
decompression routine given by a base-64 encoded Java class appearing
in the Transform content. However, applications should refrain from
using application-specific transforms whenever possible since the
resulting signature will not necessarily be verifiable outside of the
application domain. The section Transform Algorithms defines the list
of standard transformations.
Implementation Comment: When transformations are applied the signer is
not signing the native (original) document but the resulting
(transformed) document that is not captured explicitly in the
signature syntax.
3.3.3.2 The DigestMethod Element
DigestMethod is a required element which identifies the digest
algorithm to be applied to the signed object. This element uses the
general structure here for algorithms in which a URI is included as an
attribute naming the algorithm and optional Parameter contents of the
element contain any parameter, value, or other information for the
algorithm.
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<!ELEMENT DigestMethod Parameter* >
<!ATTLIST DigestMethod
Algorithm CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI] -->
3.3.3.3 The DigestValue Element
DigestValue is an element which contains the base64 encoded value of
the digest.
<!ELEMENT DigestValue CDATA>
<!-- base64 encoded signature value -->
<!ATTLIST DigestValue
Encoding CDATA "urn:ietf-org:base64">
3.4 The KeyInfo Element
KeyInfo may contain keys, names, certificates and other public key
management information (such as inband key distribution or agreement
data or data supporting any other method.) This specification defines
a few simple types but applications may place (embed) their own key
identification and exchange semantics within this element through the
XML-namespace facility. [XML-namespace]
<!ELEMENT KeyInfo (#PCDATA | (KeyName | KeyValue |
SubjectName | RetrievalMethod | x509Data |
PGPData | MgmtData)* )>
KeyInfo is an optional element which enables the recipient(s) to
obtain the key(s) needed to validate the signature. If omitted, the
recipient is expected to be able to identify the key based on
application context information. This element contains one KeyInfo
data element providing information for the recipient(s). Applications
may define and use any mechanism they choose through inclusion of
elements from a different namespace.
Compliant versions implementing KeyInfo MUST implement KeyValue, and
SHOULD implement RetrievalMethod.
* KeyName contains an identifier for the key which may be useful to
the recipient. This may be a name, index, etc.
* KeyValue contains the actual key(s) used to validate the
signature. If the key is sent in protected form, the MgmtData
element should be used. Specific types must be defined for each
algorithm type (see algorithms).
* SubjectName contains one or more names for the sender. Forms to be
supported include a simple name string, encoded DN, email address,
etc.
* RetrievalMethod is a URI which may be used to obtain key and/or
certificate information. The URI should contain the complete
string for retrieving the key needed for this message (rather than
a generic URI).
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* X509Data contains an identifier of the key/cert used for
validation (either an IssuerSerial value, a subject name, or a
subjectkeyID) and an optional collection of certificates and
revocation/status information which may be used by the recipient.
IssuerSerial contains the encoded issuer name (RFC 2253) along
with the serial number.
* PGPData data associated with a PGP key.
* MgmtData contains in-band key distribution or agreement data.
Examples may include DH key exchange, RSA key encryption etc.
<!ELEMENT KeyValue ANY>
<!ELEMENT KeyName (#PCDATA) >
<!ELEMENT SubjectName (#PCDATA) >
<!ELEMENT RetrievalMethod (#PCDATA) >
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509Name),
(X509Certificate | X509CRL | ANY)* ) >
<!ELEMENT MgmtData (#PCDATA)>
<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?)>
<!ELEMENT X509IssuerSerial (X509Name, X509SerialNumber)>
<!ELEMENT X509Name (#PCDATA)>
<!-- Where the name is encoded accroding to RFC 2253 -->
<!ELEMENT X509SerialNumber (#PCDATA)>
<!-- Where the data is the serial number encoded as a decimal integer
-->
<!ELEMENT X509SKI (#PCDATA)>
<!-- Where the data consists of the SKI base64 encoded -->
<!ELEMENT X509Certificate (#PCDATA)>
<!-- Where the data conists of the base64 encoded certificate -->
<!ELEMENT X509CRL (#PCDATA)>
<!-- Where the data consists of the base64 encoded CRL -->
<!ELEMENT PGPKeyID (#PCDATA)>
<!-- Where the data conists of the hex encoding of the key ID. -->
<!ELEMENT PGPKeyPacket (#PCDATA)>
<!-- Where the data consists of the base64 encoded key packet --->
Note: This section is preliminary. A more detailed version will be
included in a subsequent version of this specification.
3.5 The Object Element
Object is an optional element which may occur one or more times. When
present this element may contain any item and specifies the encoding.
The digest is calculated over the entire Object element including
start and end tags. If the application wishes to exclude the <Object>
tags from the digest calculation, then a transform must be used.
Exclusion of the object tags may be desired for cases where the
signature is intended to survive a change between embedded and
detached objects or where the content of the Object is an encoding of
an original binary document and it is desired to extract and decode so
as to sign the original as closely as possible.
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<!ELEMENT Object ANY>
<!ATTLIST Object
Id ID #IMPLIED
Type CDATA #IMPLIED
Encoding CDATA #IMPLIED >
<!-- Where type and encoding CDATA conforms to the
productions specified by [URI] -->
The Object's ID is referenced from the ObjectReference in SignedInfo.
This element is used for embedded signatures where the object being
signed is to be included in the signature document. The Object element
may include optional type, ID, and encoding attributes.
3.6 The Parameter Element
Algorithms are provided with parameters and input data, when
necessary, by having Parameter elements in the content of the
algorithm element. Algorithms also have an implicit input such as the
canonicalized SignedInfo for SignatureMethod and the tranformed data
pointed to for DigestMethod.
Where more than one Parameter appears, they are passed to the
algorithm as an ordered vector corresponding to the order they appear
in the algorithm element content.
<!ELEMENT Parameter #PCDATA>
<!ATTLIST Parameter
Encoding CDATA #IMPLIED >
<!-- Encoding CDATA conforms to the productions
specified by [URI] -->
4 Additional Signature Syntax
This section describes the optional to implement Manifest and Package
elements and describes the handling of XML Processing Instructions and
Comments.
4.1 The Manifest and Package Elements
The Manifest element provides a list of ObjectReferences. The
difference from the list in SignedInfo is that it is application
defined which, if any, of the digests are actually checked against the
objects referenced and what to do if the object is inaccessible or the
digest compare fails. If a Manifest is pointed to from SignedInfo, the
digest over the Manifest itself will be checked by the core signture
verifciation behavior. The digests within such a Manifest are checked
at application discretion. If a Manifest is referenced from another
Manifest, even the overall digest of this two level deep Manifest
might not be checked.
A Package has the same syntax as a Manifest but also asserts the
equality of each of its referenced objects, after any transforms. The
testing of this equality and action if it fails is also entirely at
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the discretion of the applicaiton.
<!ELEMENT Manifest ( ObjectReference+, Object* ) >
<!ATTLIST Manifest
Id ID #IMPLIED >
<!ELEMENT Package ( ObjectReference+, Object* ) >
<!ATTLIST Package
Id ID #IMPLIED >
4.2 The Properties Element
Additional information concerning the signature overall or as it
applied to particular ObjectReferences can be placed in a Properties
element inside an Object. This should be such information as signing
time or the serial number of crypto hardware used. Additional
information concerning data being signed should be with that data.
<!ELEMENT Property ANY >
<!ATTLIST Property
Target IDREF #REQUIRED >
4.3 Processing Instructions
TDB - will specify the use, if any, of XML processing instructions by
this specification and the handling of PIs appearing within elements
specified in this document.
4.4 Comments in dsig Elements
TDB - will specify the use, if any, and handling of XML comments
appearing within elements specified in this document.
5 Algorithms
This section identifies algorithms used with the XML digital signature
standard. Entries contain the identifier to be used in signature
documents, a reference to the formal specification, and definitions,
where applicable, for the representation of keys and the results of
cryptographic operations.
5.1 Algorithm Identifiers and Requirements
The specification defines a set of algorithms, their URIs, and
requirements for implementation. Requirements are specified over
implementation, not over requirements for signature use. Furthermore,
the mechanism is extensible, alternative algorithms may be used by
signature applications.
Algorithm Type Algorithm Requirements Algorithm URI URN Derivation
Digest
SHA1 REQUIRED urn:nist-gov:sha1 IOTP
Encoding
Base64 REQUIRED urn:ietf-org:base64 suggested
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MAC
HMAC-SHA1 REQUIRED urn:ietf-org:hmac-sha1 extrapolated from IOTP
Signature
DSAwithSHA1 (DSS) REQUIRED urn:nist-gov:dsa IOTP
RSAwithSHA1 RECOMMENDED urn:rsasdi-com:rsa-sha1 extrapolated from
IOTP
ECDSA OPTIONAL urn:nist-gov:ecdsa extrapolated from IOTP
Canonicalization
minimal REQUIRED http://www.w3.org/1999/10/signature-core/minimal
suggested W3C
XML-Canonicalization RECOMMENDED
http://www.w3.org/1999/07/WD-xml-c14n-19990729 W3C
Transform
XSLT RECOMMENDED http://www.w3.org/TR/1999/PR-xslt-19991008 W3C
XPath RECOMMENDED http://www.w3.org/TR/1999/PR-xpath-19991008 W3C
XPointer RECOMMENDED http://www.w3.org/1999/07/WD-xptr-19990709 W3C
5.2 Message Digests
5.2.1 SHA-1
The SHA-1 algorithm identifier is urn:nist-gov:sha1. The SHA-1
algorithm takes no parameters. An example of an SHA-1 DigestAlg
element is
<DigestMethod Algorithm="urn:nist-gov:sha1"/>
An SHA-1 digest is a 160-bit string. The content of the DigestValue
element shall be the base64 encoding of this bit string viewed as a
20-octet octet stream. Example: the DigestValue element for the
message digest
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
5.3 Message Authentication Codes
5.3.1 HMAC
The HMAC algorithm identifiers are urn:ietf-org:hmac-sha1 and
urn:ietf-org:hmac-md5. The HMAC algorithm takes the truncation length
in bits as a parameter (parameter identifier
urn:ietf-org:hmac-outputlength). An example of an HMAC
SignatureMethod element:
<SignatureMethod Algorithm="urn:ietf-org:hmac-sha1">
<Parameter type="urn:ietf-org:hmac-outputlength">
128
</Parameter>
</SignatureMethod>
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The output of the HMAC algorithm is ultimately the output (possibly
truncated) of the chosen digest algorithm. This value shall be base64
encoded in the same straightforward fashion as the output of the
digest algorithms. Example: the SignatureValue element for the
HMAC-MD5 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [RFC 2104] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
5.4 Signature Algorithms
5.4.1 DSA
The DSA algorithm identifier is urn:nist-gov:dsa. The DSA algorithm
takes no parameters. An example of a DSA SignatureMethod element is
<SignatureMethod Algorithm="urn:nist-gov:dsa"/>
The output of the DSA algorithm consists of a pair of integers usually
referred by the pair (r, s). The signature value shall consist of the
base64 encoding of the concatenation of two octet-streams that
respectively result from the octet-encoding of the values r and s.
Integer to octet-stream conversion shall be done according to the
I2OSP operation defined in the PKCS #1 specification with a k
parameter equal to 20. Example: the SignatureValue element for a DSA
signature (r, s) with values specified in hexadecimal
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
<SignatureValue>i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA
==</SignatureValue>
DSA key values have the following set of fields: P, Q, G and Y are
mandatory when appearing as a key value, J, seed and pgenCounter are
optional but SHOULD be present. (The seed and pgenCounter fields MUST
both either appear or be absent). All parameters are encoded as base64
values.
<!ELEMENT DssKeyValue (P, Q, G, Y, J?, (seed, pgenCounter)?) >
<!-- Each of these fields consists a CDATA where the data is base64
encoded -->
5.4.2 RSA
The expression "RSA algorithm" as used in this document refers to the
RSASSA-PKCS1-v1_5 algorithm described in RFC 2437.
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The RSA algorithm identifiers are urn:rsasdi-com:rsa-sha1 and
urn:rsasdi-com:rsa-md5. The RSA algorithm takes no parameters. An
example of an RSA SignatureMethod element is
<SignatureMethod name="urn:rsasdi-com:rsa-sha1"/>
The output of the RSA algorithm is an octet string. The SignatureValue
content for an RSA signature shall be the base64 encoding of this
octet string. Example: TBD
RSA key values have two fields: Modulus and Exponent.
<!ELEMENT RSAKeyValue ( Modulus, Exponent ) > <!-- Each field contains
a CDATA which is the value for that item base64 encoded -->
5.4.3 ECDSA
The expression ECDSA as used in this document refers to the signature
algorithms specified in ANSI X9.62. Additional details are to be
provided.
5.5 Canonicalization Algorithms
5.5.1 Null Canonicalization
Null canonicalization, i.e., no modification whatsoever, can be
achieved for signed data by simply not putting any canonicalization in
the Transforms element (omitting it entirely if no other tranforms are
needed).
5.5.2 Minimal Canonicalization
The algorithm identifier for the minimal canonicalization is
http://www.w3.org/1999/10/signature-core/minimal. An example of a
minimal canonicalization CanonicalizationAlg element is
<CanonicalizationMethod
name="http://www.w3.org/1999/10/signature-core/minimal"/>
The minimal canonicalization algorithm:
* converts the character encoding to UTF-8, removing the encoding
pseudo-attribute
* normalizes line endings
This algorithm is only applicable to XML resources.
5.5.3 Canonical XML
The algorithm identifier for XML canonicalization is
http://www.w3.org/1999/07/WD-xml-c14n-19990729. An example of an XML
canonicalization CanonicalizationAlg element is
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<CanonicalizationMethod
name="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
See the Canonical XML specification.
5.6 Transform Algorithms
Application developers are strongly encouraged to support all
transforms listed in this section as RECOMMENDED unless the
application environment has severe resource constraints that would
make such support impractical. The working group goal is to maximize
application interoperability on XML signatures, and the working group
expects ubiquitous availability of software to support these
transforms that can be incorporated into applications without
extensive development.
5.6.1 Canonicalization
The Algorithm value for canonicalization is defined above.
The Transform element content MUST include a Canonicalization element,
which specifies the canonicalization algorithm that will be applied to
the input of the Transform element.
5.6.2 Base-64 Decoding
The Algorithm value for the base 64 decoding transform is
urn:ietf-org:base64.
The base-64 decoding algorithm identifier is urn:ietf-org:base64.
The base-64 Transform element has no content. The input (from the
Location or from the previous Transform) is base-64 decoded by this
algorithm. This transform is useful if an application needs to sign
the raw data associated with base-64 encoded content of an element.
5.6.3 XPath Filtering
The Algorithm value for the XPath filtering transform is
"http://www.w3.org/TR/1999/PR-xpath-19991008"
The Transform element content MUST conform to the XML Path Language
(XPath) syntax.
XPath assumes that an XML processor has processed the input resource.
So, for example, entity reference expansion, normalization of
linefeeds and attribute values are normalized, and CDATA section
replacement are expected. As well, XPath joins all consecutive
characters into a single text node.
The input resource MUST be a well-formed XML document. The result of
applying the XPath to the input resource MUST be a node-set (as
defined in XPath). The output of this transform is a new XML document
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with the following characteristics:
1. The output document has the XML declaration of the input resource
(see rule 23 XMLDecl in XML specification). If the encoding is
UTF-16, the output document has the same byte order mark as the
input resource.
2. The output document contains the nodes in the node-set identified
by the XPath, and excludes the nodes of the input resource that
are not not in the node-set identified by the XPath.
3. The nodes in the output document appear in the document order (as
defined in XPath) of the input resource.
4. The output document has all of the input resource's entity
references expanded, except that characters corresponding to
illegal XML are reencoded as character references (XML rule 66)
except the ampersand and less than symbol, which are encoded using
& and <, respectively.
5. Attribute values are normalized in accordance with the rules for a
validating XML processor (even if the implementation did not use a
validating XML processor to parse the input resource).
It is RECOMMENDED that the XPath be constructed such that the result
of this operation is a well-formed XML document. This should be the
case if root element of the input resource is included by the XPath
(even if a number of its descendant elements and attributes are
omitted by the XPath).
5.6.4 XPointer Filtering
The Algorithm value for the XPointer filtering transform is
"http://www.w3.org/1999/07/WD-xptr-19990709".
The Transform element content MUST conform to the XML Pointer Language
(XPointer) syntax.
The processing rules for XPointer filtering are identical to those for
XPath filtering (stated above), except that the additional
functionality offered by XPointer can be utilized in constructing the
output node-set.
The XPointer filter is particularly important if the input resource is
processed by a validating XML processor since the XPointer barename
shortcut could then be used to implement the well-known fragment
identification by ID attribute.
NOTE: In application environments with severe resource limitations,
applications MAY constrain XPointer support to barename processing and
also to determination of the ID attribute by means other than a
validating XML processor. In fact, the use of an XML processor for
barename resolution is OPTIONAL. However, the output expectations of
this transform MUST be supported by the application.
5.6.5 XSLT Transform
The Algorithm value for the XSLT transform is
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"http://www.w3.org/TR/1999/PR-xslt-19991008"
The Transform element content MUST conform to the XSL Transforms
(XSLT) language syntax.
The processing rules for the XSLT transform are stated in the XSLT
specification.
5.6.6 Java Transform
The Algorithm value for the Java transform is urn:ECMA-org:java.
Details to be determined.
Although the Algorithm attribute of a Transform can take
application-specific values, having a Java transform seems to be the
most reasonable way to allow application-specific transforms that can
be processed outside of the application domain.
5.7 Algorithm References
Base64
RFC 2045. Multipurpose Internet Mail Extensions (MIME) Part
One: Format of Internet Message Bodies. N. Freed & N.
Borenstein. DRAFT STANDARD.
http://www.ietf.org/rfc/rfc2045.txt
DSS
FIPS PUB 186-1. Digital Signature Standard (DSS). U.S.
Department of Commerce/National Institute of Standards and
Technology.
http://www.ietf.org/rfc/rfc2104.txt
HMAC
RFC 2104. HMAC: Keyed-Hashing for Message Authentication. H.
Krawczyk, M. Bellare, R. Canetti. INFORMATIONAL.
MD5
RFC 1321. The MD5 Message-Digest Algorithm. R. Rivest.
INFORMATIONAL.
http://www.ietf.org/rfc/rfc1321.
RSA
RFC 2437. PKCS #1: RSA Cryptography Specifications Version 2.0.
B. Kaliski, J. Staddon. INFORMATIONAL.
http://www.ietf.org/rfc/rfc2432.txt
SHA-1
FIPS PUB 180-1. Secure Hash Standard. U.S. Department of
Commerce/National Institute of Standards and Technology.
http://csrc.nist.gov/fips/fip180-1.pdf
URNs
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RFC 2141. URN Syntax. R. Moats. PROPOSED STANDARD.
ftp://ftp.isi.edu/in-notes/rfc2141.txt
RFC 2611. URN Namespace Definition Mechanisms. L. Daigle, D.
van Gulik, R. Iannella, P. Falstrom. BEST CURRENT PRACTICE.
ftp://ftp.isi.edu/in-notes/rfc2611.txt
XML-Canonicalization
Canonical XML. W3C Working Draft
http://www.w3.org/1999/07/WD-xml-c14n-19990729
XPath
XML Path Language (XPath)Version 1.0. W3C Proposed
Recommendation
http://www.w3.org/TR/1999/PR-xpath-19991008
XPointer
XML Pointer Language (XPointer). W3C Working Draft.
http://www.w3.org/1999/07/WD-xptr-19990709
XSL
Extensible Stylesheet Language (XSL) W3C Working Draft
http://www.w3.org/TR/1999/WD-xsl-19990421
XSLT
XSL Transforms (XSLT) Version 1.0. W3C Proposed Recommendation
http://www.w3.org/TR/1999/PR-xslt-19991008
6 Processing rules
These sections describe the operations to be performed as part of
signature generation and validation. The description is of a logical
behavior and does not specify an order of execution, nor specify
discrete steps.
6.1 Generation
1. apply Transforms determined by application to each object being
signed.
2. calculate digest over each transformed object (including start and
end tags)
3. create ObjectReference element(s) including location of object,
digest, digest algorithm, and transform elements, if required.
4. create SignedInfo element with SignatureMethod,
CanonicalizationMethod, and ObjectReference(s).
5. canonicalize and calculate signature over SignedInfo based on
algorithms in step 4.
6. construct signature document with SignedInfo, Object (s) (if
desired, encoding may be different than that used for signing),
KeyInfo (if required), and SignatureValue.
6.2 Signature Validation
1. locate object and apply Transforms to the specified resource
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based on each ObjectReference(s) in the SignedInfo element. Each
transform is applied in order from left to right to the object
with the output of each transform being the input to the next.
2. calculate digest over each transformed signed object(s) (including
start and end tags) based on the algorithm in ObjectReference(s).
3. compare value against DigestValue in SignedInfo for each reference
(if any mismatch, validation fails).
4. canonicalize the SignedInfo element based on the
CanonicalizationMethod in SignedInfo.
5. obtain the validation keying info from KeyInfo or externally.
6. validate the SignatureValue based on the SignatureMethod in the
SignedInfo element, the key obtained in step 5, and the results of
step 4. - Digest calculation is performed over the SignedInfo
element including start and end tags.
Any processing beyond cryptographic validation (e.g. certificate
validation, applicability decisions, time related processing) is
outside the scope of this specification.
7 Security Considerations
The XML digital signature standard provides a very flexible mechanism.
In designing a system to make use of it, due consideration should be
given to the threat model being defended against and to the factors
covered in the subsections below.
7.1 Only What is Signed is Secure
The flexible Transforms mechanism, including canonicalization and
explicit filtering and extraction, permit securing only a subset of
data in an object. This is good for many applications where a limited
portion of an object must change after the signature or different
signatures secure different parts or the application modifies aspects
of the object that are not significant and can be omitted from
signature coverage or the like. Keep in mind that whenever this is
done, those aspects that are not signed can be arbitrarily modified
and the signature will still validate.
7.2 Only What is "Seen" Should be Signed
If signing is intended to convey the judgment or consent of an
automated mechanism or person concerning some information, then it is
normally necessary to secure as exactly as possible the information
that was presented to that mechanism or person. Note that this can be
accomplished by literally signing what was presented, for example the
screen images shown a user. However, this may result in data which it
is difficult for subsequent software to manipulate. It can be
effective instead to secure the full data along with whatever filters,
style sheets, or the like were used to control the part of the
information that was presented.
7.3 Check the Security Model
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This standard specifies public key signatures and secret key keyed
hash authentication codes. These have substantially different security
models. Furthermore, it permits user specified additions which may
have other models.
With public key signatures, any number of parties can hold the public
key and verify signatures while only the parties with the secret key
can create signatures. The number of holders of the secret key should
be minimized and preferably be one. Confidence by verifiers in the
public key they are using and its binding to the entity or
capabilities represented by the corresponding secret key is an
important issue, usually addressed by certificate or on line authority
systems.
Keyed hash authentication codes, based on secret keys, are typically
much more efficient in terms of the computational effort required but
have the characteristic that all verifiers need to have possession of
the same key as the signer. Thus any verifier can forge signatures.
This standard permits user provided signature algorithms and keying
information designators. Such user provided algorithms may have
further different security models. For example, methods involving
biometrics usually depend on a "key" which is a physical
characteristic of the user and thus can not be changed the way public
or secret keys can be and may have other security model differences.
7.4 7.4 Algorithms, Key Lengths, Etc.
The strength of a particular signature depends on all links in the
security chain. This includes the signature and digest algorithms
used, the strength of the key generation [RFC 1750] and the size of
the key, the security of key and certificate authentication and
distribution mechanisms, protection of all cryptographic processing
from hostile observation and tampering, etc. The security of an
overall system would also depend on the security and integrity of its
operating procedures, its personnel, and on the administrative
enforcement of those procedures. The factors listed in this paragraph,
while critical to the overall security of a system, are mostly beyond
the scope of this document.
8 Example syntax
<Signature xmlns="http://www.w3.org/1999/10/signature-core">
<SignedInfo Id="5">
<CanonicalizationMethod
Algorithm="http://www.w3.org/.../xml-c14n"/>
<SignatureMethod Algorithm="urn:nist-gov:dsa"/>
<ObjectReference Location="...">
<!-- pointer to external signedobject -->
<Transforms>
<Transform
Algorithm="http://www.w3.org/1999/10/signature-core/null">
<Encoding Algorithm="urn:ietf-org:base64"/>
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</Transforms>
<DigestMethod Algorithm="urn:nist-gov:sha1"/>
<DigestValue>a23bcd43"</DigestValue>
</ObjectReference>
<ObjectReference Locations="#timestamp"
Type="http://www.w3.org/1999/10/signature-core/signatureattributes">
<!-- points to Object below -->
<Transforms>
<CanonicalizationMethod name="http://..."/>
</Transforms>
<DigestMethod Algorithm="urn:nist-gov:sha1"/>
<DigestValue>a53uud43"</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue
encoding="urn:ietf-org:base64">dd2323dd</SignatureValue>
<Object ID="timestamp"
type="http://www.w3.org/1999/10/signature-core/signatureattributes " >
<timestamp about="#5" xmlns="http://www.ietf.org/rfc/1234">
<date>19990908</date>
<time>14:34:34:34</time>
</timestamp>
</Object>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
9 DTD
[TBD: Combined DTD]
10 Open Issues
1. Additional review is required on use of the dsig namespace; other
use of namespaces; specification of types; etc.
2. Need to review Default CanonicalizationMethod algorithms for
SignedInfo and for objects. Other defaults. Mandatory to implement
cryptographic algorithms.
3. Much more detail for KeyInfo types.
4. Make sure we are consistent with respect to types, algorithm IDs,
URIs, etc.
5. The signature data structures specified in this document are not
yet associated with a data model.
11 Acknowledgements
* Milton Anderson, FSTC
* Mark Bartel, JetForm Corporation (Author)
* John Boyer, UWI.com (Author)
* Richard Brown, Globeset
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* Donald Eastlake 3rd, IBM (Chair, Editor)
* Barb Fox, Microsoft (Author)
* Phillip Hallam-Baker, VeriSign Inc
* Richard Himes, US Courts
* Joseph Reagle, W3C (Chair, Editor)
* Ed Simon , Entrust Technologies Inc.
* Chris Smithies, PenOp
* David Solo, Citigroup (Editor)
* Winchel Todd Vincent III, GSU
* Greg Whitehead, Signio Inc.
12 12.0 Other Useful Types
We define the following types for use in identifying XML resources
that include Signture semantics.
http://www.w3.org/1999/10/signature-core/signatureproperties
designates that the referenced resource is a statement about
the referring signature.
http://www.w3.org/1999/10/signature-core/manifest
designates that the referenced resource is a collection of
other resources.
http://www.w3.org/1999/10/signature-core/package
designates that the referenced resources is a collection of
other resources and the creator of that collection asserts that
the specified resources, when transformed as specified, yield
the same exact content.
13 References
Other references can be found in section 5.7.
DOMHASH
Internet Draft. Digest Values for DOM (DOMHASH)
http://search.ietf.org/internet-drafts/draft-hiroshi-dom-hash-0
1.txt .
RDF
RDF Schema
http://www.w3.org/TR/1999/PR-rdf-schema-19990303
RDF Model and Syntax
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222
[RFC1750]
RFC1750 -- Randomness Recommendations for Security.
http://www.ietf.org/rfc/rfc1750.txt
[RFC2119]
RFC2119 -- Key words for use in RFCs to Indicate Requirement
Levels.
http://www.ietf.org/rfc/rfc2119.txt
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URI
Uniform Resource Identifiers (URI): Generic Syntax
http://www.ietf.org/rfc/rfc2396.txt
XLink
XML Linking Language
http://www.w3.org/1999/07/WD-xlink-19990726
XML
Extensible Markup Language (XML) Recommendation.
http://www.w3.org/TR/1998/REC-xml-19980210
XML-namespace
Namespaces in XML
http://www.w3.org/TR/1999/REC-xml-names-19990114
XML-schema
XML Schema Part 1: Structures
http://www.w3.org/1999/05/06-xmlschema-1/
XML Schema Part 2: Datatypes
http://www.w3.org/1999/05/06-xmlschema-2/
XML-Signature-RD
XML-Signature Requirements
http://www.w3.org/1999/08/WD-xmldsig-requirements-990820
WebData
Web Architecture: Describing and Exchanging Data.
http://www.w3.org/1999/04/WebData
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