Copyright ©2001 The Internet Society & W3C® (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This document specifies XML digital signature processing rules and syntax. XML Signatures provide integrity, message authentication, and/or signer authentication services for data of any type, whether located within the XML that includes the signature or elsewhere.
This document is a Proposed Recommendations (PR) of the W3C. The IETF/W3C XML Signature Working Group(W3C Activity Statement) believes this specification addresses all issues raised during Candidate Recommendation and its specification is sufficient for the creation of independent interopable implementations (see the Interoperability Report).
W3C Advisory Committee Members are invited to send formal review comments to the W3C Team until 17 September 2001 at xml-dsig-review@w3.org.The public is invited to send comments to the editors and the public mailing list w3c-ietf-xmldsig@w3.org (archive ).
After the review the Director will announce the document's disposition. This announcement should not be expected sooner than 14 days after the end of the review. Advancement (or even the announcement) will not occur until the Director is confident of its advancement to Draft Standard in the IETF.
Patent disclosures relevant to this specification may be found on the Working Group's patent disclosure page, in conformance with W3C policy, and the IETF Page of Intellectual Property Rights Notices, in conformance with IETF policy.
Publication as a Proposed Recommendation does not imply endorsement by the W3C membership. This is still a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite a W3C Proposed Recommendations as other than a "work in progress." A list of current W3C Recommendations and other technical documents can be found at http://www.w3.org/TR/.
This document specifies XML syntax and processing rules for creating and representing digital signatures. XML Signatures can be applied to any digital content (data object), including XML. An XML Signature may be applied to the content of one or more resources. Enveloped or enveloping signatures are over data within the same XML document as the signature; detached signatures are over data external to the signature element. More specifically, this specification defines an XML signature element type and an XML signature application; conformance requirements for each are specified by way of schema definitions and prose respectively. This specification also includes other useful types that identify methods for referencing collections of resources, algorithms, and keying and management information.
The XML Signature is a method of associating a key with referenced data (octets); it does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed. Consequently, while this specification is an important component of secure XML applications, it itself is not sufficient to address all application security/trust concerns, particularly with respect to using signed XML (or other data formats) as a basis of human-to-human communication and agreement. Such an application must specify additional key, algorithm, processing and rendering requirements. For further information, please see Security Considerations (section 8).
For readability, brevity, and historic reasons this document uses the term "signature" to generally refer to digital authentication values of all types. Obviously, the term is also strictly used to refer to authentication values that are based on public keys and that provide signer authentication. When specifically discussing authentication values based on symmetric secret key codes we use the terms authenticators or authentication codes. (See Check the Security Model, section 8.3.)
This specification provides a XML Schema [XML-schema] and DTD [XML]. The schema definition is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in RFC2119 [KEYWORDS]:
"they MUST only be used where it is actually required for interoperation or to limit behavior which has potential for causing harm (e.g., limiting retransmissions)"
Consequently, we use these capitalized keywords to unambiguously specify requirements over protocol and application features and behavior that affect the interoperability and security of implementations. These key words are not used (capitalized) to describe XML grammar; schema definitions unambiguously describe such requirements and we wish to reserve the prominence of these terms for the natural language descriptions of protocols and features. For instance, an XML attribute might be described as being "optional." Compliance with the XML-namespace specification [XML-ns] is described as "REQUIRED."
The design philosophy and requirements of this specification are addressed in the XML-Signature Requirements document [XML-Signature-RD].
No provision is made for an explicit version number in this syntax. If a future version is needed, it will use a different namespace. The XML namespace [XML-ns] URI that MUST be used by implementations of this (dated) specification is:
xmlns="http://www.w3.org/2000/09/xmldsig#"
This namespace is also used as the prefix for algorithm identifiers used by this specification. While applications MUST support XML and XML namespaces, the use of internal entities [XML] or our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL; we use these facilities to provide compact and readable examples.
This specification uses Uniform Resource Identifiers [URI] to identify resources, algorithms, and semantics. The URI in the namespace declaration above is also used as a prefix for URIs under the control of this specification. For resources not under the control of this specification, we use the designated Uniform Resource Names [URN] or Uniform Resource Locators [URL] defined by its normative external specification. If an external specification has not allocated itself a Uniform Resource Identifier we allocate an identifier under our own namespace. For instance:
SignatureProperties
is identified and defined by
this specification's namespaceFinally, in order to provide for terse namespace declarations we sometimes use XML internal entities [XML] within URIs. For instance:
<?xml version='1.0'?> <!DOCTYPE Signature SYSTEM "xmldsig-core-schema.dtd" [ <!ENTITY dsig "http://www.w3.org/2000/09/xmldsig#"> ]> <Signature xmlns="&dsig;" Id="MyFirstSignature"> <SignedInfo> ...
The contributions of the following working group members to this specification are gratefully acknowledged:
As are the last call comments from the following:
This section provides an overview and examples of XML digital signature syntax. The specific processing is given in Processing Rules (section 3). The formal syntax is found in Core Signature Syntax (section 4) and Additional Signature Syntax (section 5).
In this section, an informal representation and examples are used to describe the structure of the XML signature syntax. This representation and examples may omit attributes, details and potential features that are fully explained later.
XML Signatures are applied to arbitrary digital content (data
objects) via an indirection. Data objects are digested, the
resulting value is placed in an element (with other information)
and that element is then digested and cryptographically signed. XML
digital signatures are represented by the Signature
element which has the following structure (where "?" denotes zero
or one occurrence; "+" denotes one or more occurrences; and "*"
denotes zero or more occurrences):
<Signature> <SignedInfo> (CanonicalizationMethod) (SignatureMethod) (<Reference (URI=)? > (Transforms)? (DigestMethod) (DigestValue) </Reference>)+ </SignedInfo> (SignatureValue) (KeyInfo)? (Object)* </Signature>
Signatures are related to data objects via URIs [URI]. Within an XML document, signatures are
related to local data objects via fragment identifiers. Such local
data can be included within an enveloping signature or can enclose an enveloped
signature. Detached signatures are over external network
resources or local data objects that reside within the same XML
document as sibling elements; in this case, the signature is
neither enveloping (signature is parent) nor enveloped (signature
is child). Since a Signature
element (and its
Id
attribute value/name) may co-exist or be combined
with other elements (and their IDs) within a single XML document,
care should be taken in choosing names such that there are no
subsequent collisions that violate the ID uniqueness validity
constraint [XML].
Signature
, SignedInfo
,
Methods
, and References
)The following example is a detached signature of the content of the HTML4 in XML specification.
[s01] <Signature Id="MyFirstSignature" xmlns="http://www.w3.org/2000/09/xmldsig#"> [s02] <SignedInfo> [s03] <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s04] <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/> [s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> [s14] <KeyInfo> [s15a] <KeyValue> [s15b] <DSAKeyValue> [s15c] <P>...</P><Q>...</Q><G>...</G><Y>...</Y> [s15d] </DSAKeyValue> [s15e] </KeyValue> [s16] </KeyInfo> [s17] </Signature>
[s02-12]
The required SignedInfo
element is the information that is actually signed. Core validation of
SignedInfo
consists of two mandatory processes: validation of the
signature over SignedInfo
and validation of each
Reference
digest within SignedInfo
.
Note that the algorithms used in calculating the
SignatureValue
are also included in the signed
information while the SignatureValue
element is
outside SignedInfo
.
[s03]
The CanonicalizationMethod
is
the algorithm that is used to canonicalize the
SignedInfo
element before it is digested as part of
the signature operation. Note that this example, and all examples
in this specification, are not in canonical form.
[s04]
The SignatureMethod
is the
algorithm that is used to convert the canonicalized
SignedInfo
into the SignatureValue
. It is
a combination of a digest algorithm and a key dependent algorithm
and possibly other algorithms such as padding, for example
RSA-SHA1. The algorithm names are signed to resist attacks based on
substituting a weaker algorithm. To promote application
interoperability we specify a set of signature algorithms that MUST
be implemented, though their use is at the discretion of the
signature creator. We specify additional algorithms as RECOMMENDED
or OPTIONAL for implementation; the design also permits arbitrary
user specified algorithms.
[s05-11]
Each Reference
element
includes the digest method and resulting digest value calculated
over the identified data object. It also may include
transformations that produced the input to the digest operation. A
data object is signed by computing its digest value and a signature
over that value. The signature is later checked via reference and
signature
validation.
[s14-16]
KeyInfo
indicates the key to
be used to validate the signature. Possible forms for
identification include certificates, key names, and key agreement
algorithms and information -- we define only a few.
KeyInfo
is optional for two reasons. First, the signer
may not wish to reveal key information to all document processing
parties. Second, the information may be known within the
application's context and need not be represented explicitly. Since
KeyInfo
is outside of SignedInfo
, if the
signer wishes to bind the keying information to the signature, a
Reference
can easily identify and include the
KeyInfo
as part of the signature.
Reference
[s05] <Reference URI="http://www.w3.org/TR/2000/REC-xhtml1-20000126/"> [s06] <Transforms> [s07] <Transform Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [s11] </Reference>
[s05]
The optional URI
attribute of
Reference
identifies the data object to be signed.
This attribute may be omitted on at most one Reference
in a Signature
. (This limitation is imposed in order
to ensure that references and objects may be matched
unambiguously.)
[s05-08]
This identification, along with the
transforms, is a description provided by the signer on how they
obtained the signed data object in the form it was digested (i.e.
the digested content). The verifier may obtain the digested content
in another method so long as the digest verifies. In particular,
the verifier may obtain the content from a different location such
as a local store than that specified in the URI
.
[s06-08] Transforms
is an optional ordered list of
processing steps that were applied to the resource's content before
it was digested. Transforms can include operations such as
canonicalization, encoding/decoding (including
compression/inflation), XSLT, XPath, XML Schema validation, or
XInclude. XPath transforms permit the signer to derive an XML
document that omits portions of the source document. Consequently
those excluded portions can change without affecting signature
validity. For example, if the resource being signed encloses the
signature itself, such a transform must be used to exclude the
signature value from its own computation. If no
Transforms
element is present, the resource's content
is digested directly. While the Working Group has specified
mandatory (and optional) canonicalization and decoding algorithms,
user specified transforms are permitted.
[s09-10] DigestMethod
is the algorithm applied to
the data after Transforms
is applied (if specified) to
yield the DigestValue
. The signing of the
DigestValue
is what binds a resources content to the
signer's key.
Object
and SignatureProperty
)This specification does not address mechanisms for making
statements or assertions. Instead, this document defines what it
means for something to be signed by an XML Signature (integrity, message
authentication, and/or signer authentication). Applications that wish
to represent other semantics must rely upon other technologies,
such as [XML, RDF].
For instance, an application might use a foo:assuredby
attribute within its own markup to reference a
Signature
element. Consequently, it's the application
that must understand and know how to make trust decisions given the
validity of the signature and the meaning of assuredby
syntax. We also define a SignatureProperties
element
type for the inclusion of assertions about the signature itself
(e.g., signature semantics, the time of signing or the serial
number of hardware used in cryptographic processes). Such
assertions may be signed by including a Reference
for
the SignatureProperties
in SignedInfo
.
While the signing application should be very careful about what it
signs (it should understand what is in the
SignatureProperty
) a receiving application has no
obligation to understand that semantic (though its parent trust
engine may wish to). Any content about the signature generation may
be located within the SignatureProperty
element. The
mandatory Target
attribute references the
Signature
element to which the property applies.
Consider the preceding example with an additional reference to a
local Object
that includes a
SignatureProperty
element. (Such a signature would not
only be detached [p02]
but enveloping
[p03]
.)
[ ] <Signature Id="MySecondSignature" ...> [p01] <SignedInfo> [ ] ... [p02] <Reference URI="http://www.w3.org/TR/xml-stylesheet/"> [ ] ... [p03] <Reference URI="#AMadeUpTimeStamp" [p04] Type="http://www.w3.org/2000/09/xmldsig#SignatureProperties"> [p05] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [p06] <DigestValue>k3453rvEPO0vKtMup4NbeVu8nk=</DigestValue> [p07] </Reference> [p08] </SignedInfo> [p09] ... [p10] <Object> [p11] <SignatureProperties> [p12] <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> [p13] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p14] <date>19990908</date> [p15] <time>14:34:34:34</time> [p16] </timestamp> [p17] </SignatureProperty> [p18] </SignatureProperties> [p19] </Object> [p20]</Signature>
[p04]
The optional Type
attribute of
Reference
provides information about the resource
identified by the URI
. In particular, it can indicate
that it is an Object
, SignatureProperty
,
or Manifest
element. This can be used by applications
to initiate special processing of some Reference
elements. References to an XML data element within an
Object
element SHOULD identify the actual element
pointed to. Where the element content is not XML (perhaps it is
binary or encoded data) the reference should identify the
Object
and the Reference
Type
, if given, SHOULD indicate Object
.
Note that Type
is advisory and no action based on it
or checking of its correctness is required by core behavior.
[p10]
Object
is an optional element
for including data objects within the signature element or
elsewhere. The Object
can be optionally typed and/or
encoded.
[p11-18]
Signature properties, such as time of
signing, can be optionally signed by identifying them from within a
Reference
. (These properties are traditionally called
signature "attributes" although that term has no relationship to
the XML term "attribute".)
Object
and Manifest
)The Manifest
element is provided to meet additional
requirements not directly addressed by the mandatory parts of this
specification. Two requirements and the way the
Manifest
satisfies them follow.
First, applications frequently need to efficiently sign multiple
data objects even where the signature operation itself is an
expensive public key signature. This requirement can be met by
including multiple Reference
elements within
SignedInfo
since the inclusion of each digest secures
the data digested. However, some applications may not want the core validation
behavior associated with this approach because it requires every
Reference
within SignedInfo
to undergo reference
validation -- the DigestValue
elements are
checked. These applications may wish to reserve reference
validation decision logic to themselves. For example, an
application might receive a signature valid SignedInfo
element that includes three Reference
elements. If a
single Reference
fails (the identified data object
when digested does not yield the specified
DigestValue
) the signature would fail core validation.
However, the application may wish to treat the signature over the
two valid Reference
elements as valid or take
different actions depending on which fails. To accomplish
this, SignedInfo
would reference a
Manifest
element that contains one or more
Reference
elements (with the same structure as those
in SignedInfo
). Then, reference validation of the
Manifest
is under application control.
Second, consider an application where many signatures (using
different keys) are applied to a large number of documents. An
inefficient solution is to have a separate signature (per key)
repeatedly applied to a large SignedInfo
element (with
many Reference
s); this is wasteful and redundant. A
more efficient solution is to include many references in a single
Manifest
that is then referenced from multiple
Signature
elements.
The example below includes a Reference
that signs a
Manifest
found within the Object
element.
[ ] ... [m01] <Reference URI="#MyFirstManifest" [m02] Type="http://www.w3.org/2000/09/xmldsig#Manifest"> [m03] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [m04] <DigestValue>345x3rvEPO0vKtMup4NbeVu8nk=</DigestValue> [m05] </Reference> [ ] ... [m06] <Object> [m07] <Manifest Id="MyFirstManifest"> [m08] <Reference> [m09] ... [m10] </Reference> [m11] <Reference> [m12] ... [m13] </Reference> [m14] </Manifest> [m15] </Object>
The sections below describe the operations to be performed as part of signature generation and validation.
The REQUIRED steps include the generation of
Reference
elements and the SignatureValue
over SignedInfo
.
For each data object being signed:
Transforms
, as determined by the
application, to the data object.Reference
element, including the
(optional) identification of the data object, any (optional)
transform elements, the digest algorithm and the
DigestValue
. (Note, it is the canonical form of these
references that are signed in 3.1.2 and validated in 3.2.1 .)SignedInfo
element with
SignatureMethod
, CanonicalizationMethod
and Reference
(s).SignatureValue
over SignedInfo
based on algorithms specified in
SignedInfo
.Signature
element that includes
SignedInfo
, Object
(s) (if desired,
encoding may be different than that used for signing),
KeyInfo
(if required), and
SignatureValue
.The REQUIRED steps of core validation include (1) reference
validation, the verification of the digest contained in each
Reference
in SignedInfo
, and (2) the
cryptographic signature validation of the signature
calculated over SignedInfo
.
Note, there may be valid signatures that some signature applications are unable to validate. Reasons for this include failure to implement optional parts of this specification, inability or unwillingness to execute specified algorithms, or inability or unwillingness to dereference specified URIs (some URI schemes may cause undesirable side effects), etc.
Comparison of values in reference and signature validation are over the numeric (e.g., integer) or decoded octet sequence of the value. Different implementations may produce different encoded digest and signature values when processing the same resources because of variances in their encoding, such as accidental white space. But if one uses numeric or octet comparison (choose one) on both the stated and computed values these problems are eliminated.
SignedInfo
element based on the
CanonicalizationMethod
in SignedInfo
.
For each Reference
in SignedInfo
:
URI
and
execute Transforms
provided by the signer in the
Reference
element, or it may obtain the content
through other means such as a local cache.)DigestMethod
specified in its Reference
specification.DigestValue
in the SignedInfo
Reference
; if there is any mismatch, validation
fails.Note, SignedInfo
is canonicalized in step 1. The
application must ensure that the CanonicalizationMethod has no
dangerous side affects, such as rewriting URIs, (see CanonicalizationMethod
(section 4.3)) and that it Sees What is
Signed, which is the canonical form.
KeyInfo
or from an external
source.SignatureMethod
using the CanonicalizationMethod
and use
the result (and previously obtained KeyInfo
) to
confirm the SignatureValue
over the
SignedInfo
element.Note, KeyInfo
(or some
transformed version thereof) may be signed via a
Reference
element. Transformation and validation of
this reference (3.2.1) is orthogonal to Signature Validation which
uses the KeyInfo
as parsed.
Additionally, the SignatureMethod
URI may have been
altered by the canonicalization of SignedInfo
(e.g.,
absolutization of relative URIs) and it is the canonical form that
MUST be used. However, the required canonicalization [XML-C14N] of this specification does not
change URIs.
The general structure of an XML signature is described in Signature Overview (section 2). This section provides detailed syntax of the core signature features. Features described in this section are mandatory to implement unless otherwise indicated. The syntax is defined via DTDs and [XML-Schema] with the following XML preamble, declaration, and internal entity.
Schema Definition: <?xml version="1.0" encoding="utf-8"?> <!DOCTYPE schema PUBLIC "-//W3C//DTD XMLSchema 200102//EN" "http://www.w3.org/2001/XMLSchema.dtd" [ <!ATTLIST schema xmlns:ds CDATA #FIXED "http://www.w3.org/2000/09/xmldsig#"> <!ENTITY dsig 'http://www.w3.org/2000/09/xmldsig#'> <!ENTITY % p ''> <!ENTITY % s ''> ]> <schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" targetNamespace="http://www.w3.org/2000/09/xmldsig#" version="0.1" elementFormDefault="qualified">
DTD: <!-- The following entity declarations enable external/flexible content in the Signature content model. #PCDATA emulates schema:string; when combined with element types it emulates schema mixed="true". %foo.ANY permits the user to include their own element types from other namespaces, for example: <!ENTITY % KeyValue.ANY '| ecds:ECDSAKeyValue'> ... <!ELEMENT ecds:ECDSAKeyValue (#PCDATA) > --> <!ENTITY % Object.ANY ''> <!ENTITY % Method.ANY ''> <!ENTITY % Transform.ANY ''> <!ENTITY % SignatureProperty.ANY ''> <!ENTITY % KeyInfo.ANY ''> <!ENTITY % KeyValue.ANY ''> <!ENTITY % PGPData.ANY ''> <!ENTITY % X509Data.ANY ''> <!ENTITY % SPKIData.ANY ''>
This specification defines the ds:CryptoBinary
simple type for representing arbitrary-length integers (e.g.
"bignums") in XML as octet strings. The integer value is first
converted to a "big endian" bitstring. The bitstring is then padded
with leading zero bits so that the total number of bits == 0 mod 8
(so that there are an integral number of octets). If the bitstring
contains entire leading octets that are zero, these are removed (so
the high-order octet is always non-zero). This octet string is then
base64 [MIME] encoded. (The conversion from
integer to octet string is equivalent to IEEE 1363's I2OSP [1363] with minimal length).
This type is used by "bignum" values such as
RSAKeyValue
and DSAKeyValue
. If a value
can be of type base64Binary
or
ds:CryptoBinary
they are defined as base64Binary
.
For example, if the signature algorithm is RSA or DSA then
SignatureValue
represents a bignum and could be
ds:CryptoBinary
. However, if HMAC-SHA1 is the
signature algorithm then SignatureValue
could have
leading zero octets that must be preserved. Thus
SignatureValue
is generically defined as of type
base64Binary
.
Schema Definition: <simpleType name="CryptoBinary"> <restriction base="base64Binary"> </restriction> </simpleType>
Signature
elementThe Signature
element is the root element of an XML
Signature. Implementation MUST generate
laxly schema valid [XML-schema]
Signature
elements as specified by the following
schema:
Schema Definition: <element name="Signature" type="ds:SignatureType"/> <complexType name="SignatureType"> <sequence> <element ref="ds:SignedInfo"/> <element ref="ds:SignatureValue"/> <element ref="ds:KeyInfo" minOccurs="0"/> <element ref="ds:Object" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
DTD: <!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?, Object*) > <!ATTLIST Signature xmlns CDATA #FIXED 'http://www.w3.org/2000/09/xmldsig#' Id ID #IMPLIED >
SignatureValue
ElementThe SignatureValue
element contains the actual
value of the digital signature; it is always encoded using base64
[MIME]. While we identify two
SignatureMethod
algorithms, one mandatory and one
optional to implement, user specified algorithms may be used as
well.
Schema Definition: <element name="SignatureValue" type="ds:SignatureValueType"/> <complexType name="SignatureValueType"> <simpleContent> <extension base="base64Binary"> <attribute name="Id" type="ID" use="optional"/> </extension> </simpleContent> </complexType>
DTD: <!ELEMENT SignatureValue (#PCDATA) > <!ATTLIST SignatureValue Id ID #IMPLIED>
SignedInfo
ElementThe structure of SignedInfo
includes the
canonicalization algorithm, a signature algorithm, and one or more
references. The SignedInfo
element may contain an
optional ID attribute that will allow it to be referenced by other
signatures and objects.
SignedInfo
does not include explicit signature or
digest properties (such as calculation time, cryptographic device
serial number, etc.). If an application needs to associate
properties with the signature or digest, it may include such
information in a SignatureProperties
element within an
Object
element.
Schema Definition: <element name="SignedInfo" type="ds:SignedInfoType"/> <complexType name="SignedInfoType"> <sequence> <element ref="ds:CanonicalizationMethod"/> <element ref="ds:SignatureMethod"/> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
DTD: <!ELEMENT SignedInfo (CanonicalizationMethod, SignatureMethod, Reference+) > <!ATTLIST SignedInfo Id ID #IMPLIED
CanonicalizationMethod
ElementCanonicalizationMethod
is a required element that
specifies the canonicalization algorithm applied to the
SignedInfo
element prior to performing signature
calculations. This element uses the general structure for
algorithms described in Algorithm Identifiers
and Implementation Requirements (section 6.1). Implementations
MUST support the REQUIRED canonicalization
algorithms.
Alternatives to the REQUIRED canonicalization algorithms (section 6.5), such as Canonical XML with Comments (section 6.5.1) or a minimal canonicalization (such as CRLF and charset normalization), may be explicitly specified but are NOT REQUIRED. Consequently, their use may not interoperate with other applications that do not support the specified algorithm (see XML Canonicalization and Syntax Constraint Considerations, section 7). Security issues may also arise in the treatment of entity processing and comments if non-XML aware canonicalization algorithms are not properly constrained (see section 8.2: Only What is "Seen" Should be Signed).
The way in which the SignedInfo
element is
presented to the canonicalization method is dependent on that
method. The following applies to algorithms which process XML as
nodes or characters:
SignedInfo
and
currently indicating the SignedInfo
, its descendants,
and the attribute and namespace nodes of SignedInfo
and its descendant elements.We recommend that resource constrained applications that do not implement XML based canonicalization and instead choose a text based canonicalization be implemented to generate canonicalized XML as their output serialization so as to mitigate interoperability and security concerns. For instance, such an implementation SHOULD (at least) generate standalone XML instances [XML].
NOTE: The signature
application must exercise great care in accepting and executing an
arbitrary CanonicalizationMethod
. For example, the
canonicalization method could rewrite the URIs of the
References
being validated. Or, the method could
massively transform SignedInfo
so that validation
would always succeed (i.e., converting it to a trivial signature
with a known key over trivial data). Since
CanonicalizationMethod
is inside
SignedInfo
, in the resulting canonical form it could
erase itself from SignedInfo
or modify the
SignedInfo
element so that it appears that a different
canonicalization function was used! Thus a Signature
which appears to authenticate the desired data with the desired
key, DigestMethod
, and SignatureMethod
,
can be meaningless if a capricious
CanonicalizationMethod
is used.
Schema Definition: <element name="CanonicalizationMethod" type="ds:CanonicalizationMethodType"/> <complexType name="CanonicalizationMethodType" mixed="true"> <sequence> <any namespace="##any" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
DTD: <!ELEMENT CanonicalizationMethod (#PCDATA %Method.ANY;)* > <!ATTLIST CanonicalizationMethod Algorithm CDATA #REQUIRED >
SignatureMethod
ElementSignatureMethod
is a required element that
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, padding, etc.). This element uses the general
structure here for algorithms described in section 6.1: Algorithm Identifiers and Implementation
Requirements. While there is a single identifier, that
identifier may specify a format containing multiple distinct
signature values.
Schema Definition: <element name="SignatureMethod" type="ds:SignatureMethodType"/> <complexType name="SignatureMethodType" mixed="true"> <sequence> <element name="HMACOutputLength" minOccurs="0" type="ds:HMACOutputLengthType"/> <any namespace="##other" minOccurs="0" maxOccurs="unbounded"/> <!-- (0,unbounded) elements from (1,1) external namespace --> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
DTD: <!ELEMENT SignatureMethod (#PCDATA|HMACOutputLength %Method.ANY;)* > <!ATTLIST SignatureMethod Algorithm CDATA #REQUIRED >
Reference
ElementReference
is an element that may occur one or more
times. It specifies a digest algorithm and digest value, and
optionally an identifier of the object being signed, the type of
the object, and/or a list of transforms to be applied prior to
digesting. The identification (URI) and transforms describe how the
digested content (i.e., the input to the digest method) was
created. The Type
attribute facilitates the processing
of referenced data. For example, while this specification makes no
requirements over external data, an application may wish to signal
that the referent is a Manifest
. An optional ID
attribute permits a Reference
to be referenced from
elsewhere.
Schema Definition: <element name="Reference" type="ds:ReferenceType"/> <complexType name="ReferenceType"> <sequence> <element ref="ds:Transforms" minOccurs="0"/> <element ref="ds:DigestMethod"/> <element ref="ds:DigestValue"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="URI" type="anyURI" use="optional"/> <attribute name="Type" type="anyURI" use="optional"/> </complexType>
DTD: <!ELEMENT Reference (Transforms?, DigestMethod, DigestValue) > <!ATTLIST Reference Id ID #IMPLIED URI CDATA #IMPLIED Type CDATA #IMPLIED>
URI
AttributeThe URI
attribute identifies a data object using a
URI-Reference, as specified by RFC2396 [URI]. The set of allowed characters for
URI
attributes is the same as for XML, namely [Unicode]. However, some Unicode characters
are disallowed from URI references including all non-ASCII
characters and the excluded characters listed in RFC2396 [URI, section 2.4]. However, the number sign
(#), percent sign (%), and square bracket characters re-allowed in
RFC 2732 [URI-Literal] are
permitted. Disallowed characters must be escaped as follows:
XML signature applications MUST be able to parse URI syntax. We RECOMMEND they be able to dereference URIs in the HTTP scheme. Dereferencing a URI in the HTTP scheme MUST comply with the Status Code Definitions of [HTTP] (e.g., 302, 305 and 307 redirects are followed to obtain the entity-body of a 200 status code response). Applications should also be cognizant of the fact that protocol parameter and state information, (such as HTTP cookies, HTML device profiles or content negotiation), may affect the content yielded by dereferencing a URI.
If a resource is identified by more than one URI, the most specific should be used (e.g. http://www.w3.org/2000/06/interop-pressrelease.html.en instead of http://www.w3.org/2000/06/interop-pressrelease). (See the Reference Validation (section 3.2.1) for a further information on reference processing.)
If the URI
attribute is omitted altogether, the
receiving application is expected to know the identity of the
object. For example, a lightweight data protocol might omit this
attribute given the identity of the object is part of the
application context. This attribute may be omitted from at most one
Reference
in any particular SignedInfo
,
or Manifest
.
The optional Type attribute contains information about the type of object being signed. This is represented as a URI. For example:
Type="http://www.w3.org/2000/09/xmldsig#Object"
Type="http://www.w3.org/2000/09/xmldsig#Manifest"
The Type attribute applies to the item being pointed at, not its
contents. For example, a reference that identifies an
Object
element containing a
SignatureProperties
element is still of type
#Object
. The type attribute is advisory. No validation
of the type information is required by this specification.
Note: XPath is RECOMMENDED. Signature applications need not conform to [XPath] specification in order to conform to this specification. However, the XPath data model, definitions (e.g., node-sets) and syntax is used within this document in order to describe functionality for those that want to process XML-as-XML (instead of octets) as part of signature generation. For those that want to use these features, a conformant [XPath] implementation is one way to implement these features, but it is not required. Such applications could use a sufficiently functional replacement to a node-set and implement only those XPath expression behaviors REQUIRED by this specification. However, for simplicity we generally will use XPath terminology without including this qualification on every point. Requirements over "XPath node-sets" can include a node-set functional equivalent. Requirements over XPath processing can include application behaviors that are equivalent to the corresponding XPath behavior.
The data-type of the result of URI dereferencing or subsequent Transforms is either an octet stream or an XPath node-set.
The Transforms specified in this document are defined with respect to the input they require. The following is the default signature application behavior:
Users may specify alternative transforms that override these
defaults in transitions between Transforms that expect different
inputs. The final octet stream contains the data octets being
secured. The digest algorithm specified by
DigestMethod
is then applied to these data octets,
resulting in the DigestValue
.
Unless the URI-Reference is a 'same-document' reference as defined in [URI, Section 4.2], the result of dereferencing the URI-Reference MUST be an octet stream. In particular, an XML document identified by URI is not parsed by the signature application unless the URI is a same-document reference or unless a transform that requires XML parsing is applied (See Transforms (section 4.3.3.1).)
When a fragment is preceded by an absolute or relative URI in
the URI-Reference, the meaning of the fragment is defined by the
resource's MIME type. Even for XML documents, URI dereferencing
(including the fragment processing) might be done for the signature
application by a proxy. Therefore, reference validation might fail
if fragment processing is not performed in a standard way (as
defined in the following section for same-document references).
Consequently, we RECOMMEND that the URI
attribute not include fragment identifiers and that such processing
be specified as an additional XPath
Transform.
When a fragment is not preceded by a URI in the URI-Reference, XML signature applications MUST support the null URI and barename XPointer. We RECOMMEND support for the same-document XPointers '#xpointer(/)' and '#xpointer(id('ID'))' if the application also intends to support any canonicalization that preserves comments. (Otherwise URI="#foo" will automatically remove comments before the canonicalization can even be invoked.) All other support for XPointers is OPTIONAL, especially all support for barename and other XPointers in external resources since the application may not have control over how the fragment is generated (leading to interoperability problems and validation failures).
The following examples demonstrate what the URI attribute identifies and how it is dereferenced:
URI="http://example.com/bar.xml"
URI="http://example.com/bar.xml#chapter1"
URI=""
URI="#chapter1"
Dereferencing a same-document reference MUST result in an XPath
node-set suitable for use by Canonical XML. Specifically,
dereferencing a null URI (URI=""
) MUST result in an
XPath node-set that includes every non-comment node of the XML
document containing the URI
attribute. In a fragment
URI, the characters after the number sign ('#') character conform
to the XPointer syntax [Xptr]. When
processing an XPointer, the application MUST behave as if the root
node of the XML document containing the URI
attribute
were used to initialize the XPointer evaluation context. The
application MUST behave as if the result of XPointer processing
were a node-set derived from the resultant location-set as
follows:
The second to last replacement is necessary because XPointer typically indicates a subtree of an XML document's parse tree using just the element node at the root of the subtree, whereas Canonical XML treats a node-set as a set of nodes in which absence of descendant nodes results in absence of their representative text from the canonical form.
The last step is performed for null URIs, barename XPointers and
child sequence XPointers. It's necessary because when [XML-C14N] is passed a node-set, it
processes the node-set as is: with or without comments. Only when
it's called with an octet stream does it invoke it's own XPath
expressions (default or without comments). Therefore to retain the
default behavior of stripping comments when passed a node-set, they
are removed in the last step if the URI is not a full XPointer. To
retain comments while selecting an element by an identifier
ID, use the following full XPointer:
URI='#xpointer(id('ID'))'
. To retain comments while
selecting the entire document, use the following full XPointer:
URI='#xpointer(/)'
. This XPointer contains a simple
XPath expression that includes the root node, which the second to
last step above replaces with all nodes of the parse tree (all
descendants, plus all attributes, plus all namespaces nodes).
Transforms
ElementThe optional Transforms
element contains an ordered
list of Transform
elements; these describe how the
signer obtained the data object that was digested. The output of
each Transform
serves as input to the next
Transform
. The input to the first
Transform
is the result of dereferencing the
URI
attribute of the Reference
element.
The output from the last Transform
is the input for
the DigestMethod
algorithm. When transforms are
applied the signer is not signing the native (original) document
but the resulting (transformed) document. (See Only What is Signed is Secure (section
8.1).)
Each Transform
consists of an
Algorithm
attribute and content parameters, if any,
appropriate for the given algorithm. The Algorithm
attribute value specifies the name of the algorithm to be
performed, and the Transform
content provides
additional data to govern the algorithm's processing of the
transform input. (Seee Algorithm Identifiers
and Implementation Requirements (section 6).)
As described in The Reference Processing Model (section 4.3.3.2), some transforms take an XPath node-set as input, while others require an octet stream. If the actual input matches the input needs of the transform, then the transform operates on the unaltered input. If the transform input requirement differs from the format of the actual input, then the input must be converted.
Some Transform
s may require explicit MIME type,
charset (IANA registered "character set"), or other such
information concerning the data they are receiving from an earlier
Transform
or the source data, although no
Transform
algorithm specified in this document needs
such explicit information. Such data characteristics are provided
as parameters to the Transform
algorithm and should be
described in the specification for the algorithm.
Examples of transforms include but are not limited to base64
decoding [MIME], canonicalization [XML-C14N], XPath filtering [XPath], and XSLT [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 Java class appearing as a base64
encoded parameter to a Java Transform
algorithm.
However, applications should refrain from using
application-specific transforms if they wish their signatures to be
verifiable outside of their application domain. Transform Algorithms (section 6.6)
defines the list of standard transformations.
Schema Definition: <element name="Transforms" type="ds:TransformsType"/> <complexType name="TransformsType"> <sequence> <element ref="ds:Transform" maxOccurs="unbounded"/> </sequence> </complexType> <element name="Transform" type="ds:TransformType"/> <complexType name="TransformType" mixed="true"> <choice minOccurs="0" maxOccurs="unbounded"> <any namespace="##other" processContents="lax"/> <!-- (1,1) elements from (0,unbounded) namespaces --> <element name="XPath" type="string"/> </choice> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
DTD: <!ELEMENT Transforms (Transform+)> <!ELEMENT Transform (#PCDATA|XPath %Transform.ANY;)* > <!ATTLIST Transform Algorithm CDATA #REQUIRED > <!ELEMENT XPath (#PCDATA) >
DigestMethod
ElementDigestMethod
is a required element that identifies
the digest algorithm to be applied to the signed object. This
element uses the general structure here for algorithms specified in
Algorithm Identifiers and Implementation
Requirements (section 6.1).
If the result of the URI dereference and application of Transforms is an XPath node-set (or sufficiently functional replacement implemented by the application) then it must be converted as described in the Reference Processing Model (section 4.3.3.2). If the result of URI dereference and application of Transforms is an octet stream, then no conversion occurs (comments might be present if the Canonical XML with Comments was specified in the Transforms). The digest algorithm is applied to the data octets of the resulting octet stream.
Schema Definition: <element name="DigestMethod" type="ds:DigestMethodType"/> <complexType name="DigestMethodType" mixed="true"> <sequence> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> <attribute name="Algorithm" type="anyURI" use="required"/> </complexType>
DTD: <!ELEMENT DigestMethod (#PCDATA %Method.ANY;)* > <!ATTLIST DigestMethod Algorithm CDATA #REQUIRED >
DigestValue
ElementDigestValue is an element that contains the encoded value of the digest. The digest is always encoded using base64 [MIME].
Schema Definition: <element name="DigestValue" type="ds:DigestValueType"/> <simpleType name="DigestValueType"> <restriction base="base64Binary"/> </simpleType>
DTD:
<!ELEMENT DigestValue (#PCDATA) >
<!-- base64 encoded digest value -->
KeyInfo
ElementKeyInfo
is an optional element that enables the
recipient(s) to obtain the key needed to validate the
signature. KeyInfo
may contain keys, names,
certificates and other public key management information, such as
in-band key distribution or key agreement data. This specification
defines a few simple types but applications may extend those types
or all-together replace them with their own key identification and
exchange semantics using the XML namespace facility. [XML-ns] However, questions of trust of such
key information (e.g., its authenticity or strength) are out
of scope of this specification and left to the application.
If KeyInfo
is omitted, the recipient is expected to
be able to identify the key based on application context. Multiple
declarations within KeyInfo
refer to the same key.
While applications may define and use any mechanism they choose
through inclusion of elements from a different namespace, compliant
versions MUST implement KeyValue
(section 4.4.2) and
SHOULD implement RetrievalMethod
(section
4.4.3).
The schema/DTD specifications of many of KeyInfo
's
children (e.g., PGPData
, SPKIData
,
X509Data
) permit their content to be
extended/complemented with elements from another namespace. This
may be done only if it is safe to ignore these extension elements
while claiming support for the types defined in this specification.
Otherwise, external elements, including alternative
structures to those defined by this specification, MUST be a child
of KeyInfo
. For example, should a complete XML-PGP
standard be defined, its root element MUST be a child of
KeyInfo
. (Of course, new structures from external
namespaces can incorporate elements from the
&dsig;
namespace via features of the type
definition language. For instance, they can create a DTD that mixes
their own and dsig qualified elements, or a schema that permits,
includes, imports, or derives new types based on
&dsig;
elements.)
The following list summarizes the KeyInfo
types
that are allocated an identifier in the &dsig;
namespace; these can be used within the
RetrievalMethod
Type
attribute to
describe a remote KeyInfo
structure.
In addition to the types above for which we define an XML structure, we specify one additional type to indicate a binary (ASN.1 DER) X.509 Certificate.
Schema Definition: <element name="KeyInfo" type="ds:KeyInfoType"/> <complexType name="KeyInfoType" mixed="true"> <choice maxOccurs="unbounded"> <element ref="ds:KeyName"/> <element ref="ds:KeyValue"/> <element ref="ds:RetrievalMethod"/> <element ref="ds:X509Data"/> <element ref="ds:PGPData"/> <element ref="ds:SPKIData"/> <element ref="ds:MgmtData"/> <any processContents="lax" namespace="##other"/> <!-- (1,1) elements from (0,unbounded) namespaces --> </choice> <attribute name="Id" type="ID" use="optional"/> </complexType>
DTD: <!ELEMENT KeyInfo (#PCDATA|KeyName|KeyValue|RetrievalMethod| X509Data|PGPData|SPKIData|MgmtData %KeyInfo.ANY;)* > <!ATTLIST KeyInfo Id ID #IMPLIED >
KeyName
ElementThe KeyName
element contains a string value (in
which white space is significant) which may be used by the signer
to communicate a key identifier to the recipient. Typically,
KeyName
contains an identifier related to the key pair
used to sign the message, but it may contain other protocol-related
information that indirectly identifies a key pair. (Common uses of
KeyName
include simple string names for keys, a key
index, a distinguished name (DN), an email address, etc.)
Schema Definition: <element name="KeyName" type="string"/>
DTD: <!ELEMENT KeyName (#PCDATA) >
KeyValue
ElementThe KeyValue
element contains a single public key
that may be useful in validating the signature. Structured formats
for defining DSA (REQUIRED) and RSA (RECOMMENDED) public keys are
defined in Signature Algorithms
(section 6.4). The KeyValue
element may include
externally defined public keys values represented as PCDATA or
element types from an external namespace.
Schema Definition: <element name="KeyValue" type="ds:KeyValueType"/> <complexType name="KeyValueType" mixed="true"> <choice> <element ref="ds:DSAKeyValue"/> <element ref="ds:RSAKeyValue"/> <any namespace="##other" processContents="lax"/> </choice> </complexType>
DTD: <!ELEMENT KeyValue (#PCDATA|DSAKeyValue|RSAKeyValue %KeyValue.ANY;)* >
DSAKeyValue
ElementType="http//www.w3.org/2000/09/xmldsig#DSAKeyValue"
(this can be used within a RetrievalMethod
or
Reference
element to identify the referent's
type)DSA keys and the DSA signature algorithm are specified in [DSS]. DSA public key values can have the following fields:
P
Q
G
J
Y
seed
pgenCounter
Parameter J is avilable for inclusion solely for efficiency as
it is calculatable from P and Q. Parameters seed and pgenCounter
are used in the DSA prime number generation algorithm specified in
[DSS]. As such, they are optional but must either both be present
or both be absent. This prime generation algorithm is designed to
provide assurance that a weak prime is not being used and it yields
a P and Q value. Parameters P, Q, and G can be public and common to
a group of users. They might be known from application context. As
such, they are optional but P and Q must either both appear or both
be absent. If all of P
, Q
,
seed
, and pgenCounter
are present,
implementations are not required to check if they are consistent
and are free to use either P
and Q
or
seed
and pgenCounter
. All parameters are
encoded as base64 [MIME] values.
Arbitrary-length integers (e.g. "bignums" such as RSA moduli)
are represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema:
<element name="DSAKeyValue" type="ds:DSAKeyValueType"/>
<complexType name="DSAKeyValueType">
<sequence>
<sequence minOccurs="0">
<element name="P" type="ds:CryptoBinary"/>
<element name="Q" type="ds:CryptoBinary"/>
</sequence>
<element name="J" type="ds:CryptoBinary" minOccurs="0"/>
<element name="G" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Y" type="ds:CryptoBinary"/>
<sequence minOccurs="0">
<element name="Seed" type="ds:CryptoBinary"/>
<element name="PgenCounter" type="ds:CryptoBinary"/>
</sequence>
</sequence>
</complexType>
DTD:
<!ELEMENT DSAKeyValue (P, Q)?, J?, G?, Y, (Seed, PgenCounter)?) >
<!ELEMENT P (#PCDATA) >
<!ELEMENT Q (#PCDATA) >
<!ELEMENT J (#PCDATA) >
<!ELEMENT G (#PCDATA) >
<!ELEMENT Y (#PCDATA) >
<!ELEMENT Seed (#PCDATA) >
<!ELEMENT PgenCounter (#PCDATA) >
RSAKeyValue
ElementType="http//www.w3.org/2000/09/xmldsig#RSAKeyValue"
(this can be used within a RetrievalMethod
or
Reference
element to identify the referent's
type)RSA key values have two fields: Modulus and Exponent.
<RSAKeyValue> <Modulus>xA7SEU+e0yQH5rm9kbCDN9o3aPIo7HbP7tX6WOocLZAtNfyxSZDU16ksL6W jubafOqNEpcwR3RdFsT7bCqnXPBe5ELh5u4VEy19MzxkXRgrMvavzyBpVRgBUwUlV 5foK5hhmbktQhyNdy/6LpQRhDUDsTvK+g9Ucj47es9AQJ3U= </Modulus> <Exponent>AQAB</Exponent> </RSAKeyValue>
Arbitrary-length integers (e.g. "bignums" such as RSA moduli)
are represented in XML as octet strings as defined by the ds:CryptoBinary
type.
Schema:
<element name="RSAKeyValue" type="ds:RSAKeyValueType"/>
<complexType name="RSAKeyValueType">
<sequence>
<element name="Modulus" type="ds:CryptoBinary"/>
<element name="Exponent" type="ds:CryptoBinary"/>
</sequence>
</complexType>
DTD:
<!ELEMENT RSAKeyValue (Modulus, Exponent) >
<!ELEMENT Modulus (#PCDATA) >
<!ELEMENT Exponent (#PCDATA) >
RetrievalMethod
ElementA RetrievalMethod
element within
KeyInfo
is used to convey a reference to
KeyInfo
information that is stored at another
location. For example, several signatures in a document might use a
key verified by an X.509v3 certificate chain appearing once in the
document or remotely outside the document; each signature's
KeyInfo
can reference this chain using a single
RetrievalMethod
element instead of including the
entire chain with a sequence of X509Certificate
elements.
RetrievalMethod
uses the same syntax and
dereferencing behavior as Reference
's URI (section 4.3.3.1)
and The Reference
Processing Model (section 4.3.3.2) except that there is no
DigestMethod
or DigestValue
child
elements and presence of the URI is mandatory.
Type
is an optional identifier for the type of data
to be retrieved. The result of dereferencing a
RetrievalMethod
Reference
for all KeyInfo
types defined by this
specification (section 4.4) with a corresponding XML structure
is an XML element or document with that element as the root. The
rawX509Certificate
KeyInfo
(for which
there is no XML structure) returns a binary X509 certificate.
Schema Definition <element name="RetrievalMethod" type="ds:RetrievalMethodType"/> <complexType name="RetrievalMethodType"> <sequence> <element name="Transforms" type="ds:TransformsType" minOccurs="0"/> </sequence> <attribute name="URI" type="anyURI"/> <attribute name="Type" type="anyURI" use="optional"/> </complexType>
DTD <!ELEMENT RetrievalMethod (Transforms?) > <!ATTLIST RetrievalMethod URI CDATA #REQUIRED Type CDATA #IMPLIED >
X509Data
ElementType="http://www.w3.org/2000/09/xmldsig#X509Data
"RetrievalMethod
or
Reference
element to identify the referent's
type)An X509Data
element within KeyInfo
contains one or more identifiers of keys or X509 certificates (or
certificates' identifiers or a revocation list). The content of
X509Data
is:
X509IssuerSerial
element, which contains an
X.509 issuer distinguished name/serial number pair that SHOULD be
compliant with RFC2253 [LDAP-DN],X509SubjectName
element, which contains an
X.509 subject distinguished name that SHOULD be compliant with
RFC2253 [LDAP-DN],X509SKI
element, which contains the base64
encoded plain (i.e. non-DER-encoded) value of a X509 V.3
SubjectKeyIdentifier extension.X509Certificate
element, which contains a
base64-encoded [X509v3] certificate,
andX509CRL
element, which contains a
base64-encoded certificate revocation list (CRL) [X509v3].Any X509IssuerSerial
, X509SKI
, and
X509SubjectName
elements that appear MUST refer to the
certificate or certificates containing the validation key. All such
elements that refer to a particular individual certificate MUST be
grouped inside a single X509Data
element and if the
certificate to which they refer appears, it MUST also be in that
X509Data
element.
Any X509IssuerSerial
, X509SKI
, and
X509SubjectName
elements that relate to the same key
but different certificates MUST be grouped within a single
KeyInfo
but MAY occur in multiple
X509Data
elements.
All certificates appearing in an X509Data
element
MUST relate to the validation key by either containing it or being
part of a certification chain that terminates in a certificate
containing the validation key.
No ordering is implied by the above constraints. The comments in the following instance demonstrate these constraints:
<KeyInfo>
<X509Data> <!-- two pointers to certificate-A -->
<X509IssuerSerial>
<X509IssuerName>CN=TAMURA Kent, OU=TRL, O=IBM,
L=Yamato-shi, ST=Kanagawa, C=JP</X509IssuerName>
<X509SerialNumber>12345678</X509SerialNumber>
</X509IssuerSerial>
<X509SKI>31d97bd7</X509SKI>
</X509Data>
<X509Data><!-- single pointer to certificate-B -->
<X509SubjectName>Subject of Certificate B</X509SubjectName>
</X509Data>
<X509Data> <!-- certificate chain -->
<!--Signer cert, issuer CN=arbolCA,OU=FVT,O=IBM,C=US, serial 4-->
<X509Certificate>MIICXTCCA..</X509Certificate>
<!-- Intermediate cert subject CN=arbolCA,OU=FVT,O=IBM,C=US
issuer CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICPzCCA...</X509Certificate>
<!-- Root cert subject CN=tootiseCA,OU=FVT,O=Bridgepoint,C=US -->
<X509Certificate>MIICSTCCA...</X509Certificate>
</X509Data>
</KeyInfo>
Note, there is no direct provision for a PKCS#7 encoded "bag" of
certificates or CRLs. However, a set of certificates and CRLs can
occur within an X509Data
element and multiple
X509Data
elements can occur in a KeyInfo
.
Whenever multiple certificates occur in an X509Data
element, at least one such certificate must contain the public key
which verifies the signature.
Also, strings in DNames
(X509IssuerSerial
,X509SubjectName
, and
KeyName
if approriate) should be encoded as
follows:
Schema Definition <element name="X509Data" type="ds:X509DataType"/> <complexType name="X509DataType"> <sequence maxOccurs="unbounded"> <choice> <element name="X509IssuerSerial" type="ds:X509IssuerSerialType"/> <element name="X509SKI" type="base64Binary"/> <element name="X509SubjectName" type="string"/> <element name="X509Certificate" type="base64Binary"/> <element name="X509CRL" type="base64Binary"/> <any namespace="##other" processContents="lax"/> </choice> </sequence> </complexType> <complexType name="X509IssuerSerialType"> <sequence> <element name="X509IssuerName" type="string"/> <element name="X509SerialNumber" type="integer"/> </sequence> </complexType>
DTD <!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509SubjectName | X509Certificate)+ | X509CRL %X509.ANY;)> <!ELEMENT X509IssuerSerial (X509IssuerName, X509SerialNumber) > <!ELEMENT X509IssuerName (#PCDATA) > <!ELEMENT X509SubjectName (#PCDATA) > <!ELEMENT X509SerialNumber (#PCDATA) > <!ELEMENT X509SKI (#PCDATA) > <!ELEMENT X509Certificate (#PCDATA) > <!ELEMENT X509CRL (#PCDATA) > <!-- Note, this DTD and schema permitsX509Data
to be empty; this is precluded by the text inKeyInfo
Element (section 4.4) which states that at least one element from the dsig namespace should be present in the PGP, SPKI, and X509 structures. This is easily expressed for the other key types, but not for X509Data because of its rich structure. -->
PGPData
ElementType="http://www.w3.org/2000/09/xmldsig#PGPData
"RetrievalMethod
or
Reference
element to identify the referent's
type)The PGPData
element within KeyInfo
is
used to convey information related to PGP public key pairs and
signatures on such keys. The PGPKeyID
's value is a
base64Binary sequence containing a standard PGP public key
identifier as defined in [PGP, section
11.2]. The PGPKeyPacket
contains a base64-encoded Key
Material Packet as defined in [PGP, section
5.5]. These children element types can be complemented/extended by
siblings from an external namespace within PGPData
, or
PGPData
can be replaced all-together with an
alternative PGP XML structure as a child of KeyInfo
.
PGPData
must contain one PGPKeyID
and/or
one PGPKeyPacket
and 0 or more elements from an
external namespace.
Schema Definition: <element name="PGPData" type="ds:PGPDataType"/> <complexType name="PGPDataType"> <choice> <sequence> <element name="PGPKeyID" type="base64Binary"/> <element name="PGPKeyPacket" type="base64Binary" minOccurs="0"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> <sequence> <element name="PGPKeyPacket" type="base64Binary"/> <any namespace="##other" processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </sequence> </choice> </complexType>
DTD: <!ELEMENT PGPData ((PGPKeyID, PGPKeyPacket?) | (PGPKeyPacket) %PGPData.ANY;) > <!ELEMENT PGPKeyPacket (#PCDATA) > <!ELEMENT PGPKeyID (#PCDATA) >
SPKIData
ElementType="http://www.w3.org/2000/09/xmldsig#SPKIData
"RetrievalMethod
or
Reference
element to identify the referent's
type)The SPKIData
element within KeyInfo
is
used to convey information related to SPKI public key pairs,
certificates and other SPKI data. SPKISexp
is the
base64 encoding of a SPKI canonical S-expression.
SPKIData
must have at least one SPKISexp
;
SPKISexp
can be complemented/extended by siblings from
an external namespace within SPKIData
, or
SPKIData
can be entirely replaced with an alternative
SPKI XML structure as a child of KeyInfo
.
Schema Definition: <element name="SPKIData" type="ds:SPKIDataType"/> <complexType name="SPKIDataType"> <sequence maxOccurs="unbounded"> <element name="SPKISexp" type="base64Binary"/> <any namespace="##other" processContents="lax" minOccurs="0"/> </sequence> </complexType>
DTD: <!ELEMENT SPKIData (SPKISexp %SPKIData.ANY;) > <!ELEMENT SPKISexp (#PCDATA) >
MgmtData
ElementType="http://www.w3.org/2000/09/xmldsig#MgmtData
"RetrievalMethod
or
Reference
element to identify the referent's
type)The MgmtData
element within KeyInfo
is
a string value used to convey in-band key distribution or agreement
data. For example, DH key exchange, RSA key encryption, etc. Use of
this elemet is NOT RECOMMENDED. It provides a syntactic hook where
in-band key distribution or agreement data can be placed. However,
superior interoperable child elements of KeyInfo
for
the transmission of encrypted keys and for key agreement are being
specified by the W3C XML Encryption Working Group and they should
be used instead of MgmtData
.
Schema Definition: <element name="MgmtData" type="string"/>
DTD: <!ELEMENT MgmtData (#PCDATA)>
Object
ElementType="http://www.w3.org/2000/09/xmldsig#Object"
(this can be used within a Reference
element
to identify the referent's type)Object
is an optional element that may occur one or
more times. When present, this element may contain any data. The
Object
element may include optional MIME type, ID, and
encoding attributes.
The Object
's Encoding
attributed may
be used to provide a URI that identifies the method by which the
object is encoded (e.g., a binary file).
The MimeType
attribute is an optional attribute
which describes the data within the Object
(independent of its encoding). This is a string with values defined
by [MIME]. For example, if the
Object
contains base64 encoded PNG, the
Encoding
may be specified as 'base64' and the
MimeType
as 'image/png'. This attribute is purely
advisory; no validation of the MimeType
information is
required by this specification. Applications which require
normatiave type and encoding information for signature validation
should specify Transforms
with well defined
resulting types and/or encodings.
The Object
's Id
is commonly referenced
from a Reference
in SignedInfo
, or
Manifest
. This element is typically used for enveloping
signatures where the object being signed is to be included in
the signature element. The digest is calculated over the entire
Object
element including start and end tags.
Note, if the application wishes to exclude the
<Object>
tags from the digest calculation the
Reference
must identify the actual data object (easy
for XML documents) or a transform must be used to remove the
Object
tags (likely where the data object is non-XML).
Exclusion of the object tags may be desired for cases where one
wants the signature to remain valid if the data object is moved
from inside a signature to outside the signature (or vice versa),
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 bitwise representation.
Schema Definition: <element name="Object" type="ds:ObjectType"/> <complexType name="ObjectType" mixed="true"> <sequence minOccurs="0" maxOccurs="unbounded"> <any namespace="##any" processContents="lax"/> </sequence> <attribute name="Id" type="ID" use="optional"/> <attribute name="MimeType" type="string" use="optional"/> <attribute name="Encoding" type="anyURI" use="optional"/> </complexType>
DTD: <!ELEMENT Object (#PCDATA|Signature|SignatureProperties|Manifest %Object.ANY;)* > <!ATTLIST Object Id ID #IMPLIED MimeType CDATA #IMPLIED Encoding CDATA #IMPLIED >
This section describes the optional to implement
Manifest
and SignatureProperties
elements
and describes the handling of XML processing instructions and
comments. With respect to the elements Manifest
and
SignatureProperties
this section specifies syntax and
little behavior -- it is left to the application. These elements
can appear anywhere the parent's content model permits; the
Signature
content model only permits them within
Object
.
Manifest
ElementType="http://www.w3.org/2000/09/xmldsig#Manifest"
(this can be used within a Reference
element
to identify the referent's type)The Manifest
element provides a list of
Reference
s. 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 signature validation behavior.
The digests within such a Manifest
are checked at the
application's discretion. If a Manifest
is referenced
from another Manifest
, even the overall digest of this
two level deep Manifest
might not be checked.
Schema Definition: <element name="Manifest" type="ds:ManifestType"/> <complexType name="ManifestType"> <sequence> <element ref="ds:Reference" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType>
DTD: <!ELEMENT Manifest (Reference+) > <!ATTLIST Manifest Id ID #IMPLIED >
SignatureProperties
ElementType="http://www.w3.org/2000/09/xmldsig#SignatureProperties"
(this can be used within a Reference
element
to identify the referent's type)Additional information items concerning the generation of the
signature(s) can be placed in a SignatureProperty
element (i.e., date/time stamp or the serial number of
cryptographic hardware used in signature generation).
Schema Definition: <element name="SignatureProperties" type="ds:SignaturePropertiesType"/> <complexType name="SignaturePropertiesType"> <sequence> <element ref="ds:SignatureProperty" maxOccurs="unbounded"/> </sequence> <attribute name="Id" type="ID" use="optional"/> </complexType> <element name="SignatureProperty" type="ds:SignaturePropertyType"/> <complexType name="SignaturePropertyType" mixed="true"> <choice maxOccurs="unbounded"> <any namespace="##other" processContents="lax"/> <!-- (1,1) elements from (1,unbounded) namespaces --> </choice> <attribute name="Target" type="anyURI" use="required"/> <attribute name="Id" type="ID" use="optional"/> </complexType>
DTD: <!ELEMENT SignatureProperties (SignatureProperty+) > <!ATTLIST SignatureProperties Id ID #IMPLIED > <!ELEMENT SignatureProperty (#PCDATA %SignatureProperty.ANY;)* > <!ATTLIST SignatureProperty Target CDATA #REQUIRED Id ID #IMPLIED >
No XML processing instructions (PIs) are used by this specification.
Note that PIs placed inside SignedInfo
by an
application will be signed unless the
CanonicalizationMethod
algorithm discards them. (This
is true for any signed XML content.) All of the
CanonicalizationMethod
s identified within this
specification retain PIs. When a PI is part of content that is
signed (e.g., within SignedInfo
or referenced XML
documents) any change to the PI will obviously result in a
signature failure.
XML comments are not used by this specification.
Note that unless CanonicalizationMethod
removes
comments within SignedInfo
or any other referenced XML
(which [XML-C14N] does), they will be
signed. Consequently, if they are retained, a change to the comment
will cause a signature failure. Similarly, the XML signature over
any XML data will be sensitive to comment changes unless a
comment-ignoring canonicalization/transform method, such as the
Canonical XML [XML-C14N], is
specified.
This section identifies algorithms used with the XML digital
signature specification. Entries contain the identifier to be used
in Signature
elements, a reference to the formal
specification, and definitions, where applicable, for the
representation of keys and the results of cryptographic
operations.
Algorithms are identified by URIs that appear as an attribute to
the element that identifies the algorithms' role
(DigestMethod
, Transform
,
SignatureMethod
, or
CanonicalizationMethod
). All algorithms used herein
take parameters but in many cases the parameters are implicit. For
example, a SignatureMethod
is implicitly given two
parameters: the keying info and the output of
CanonicalizationMethod
. Explicit additional parameters
to an algorithm appear as content elements within the algorithm
role element. Such parameter elements have a descriptive element
name, which is frequently algorithm specific, and MUST be in the
XML Signature namespace or an algorithm specific namespace.
This 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.
* The Enveloped Signature transform removes the
Signature
element from the calculation of the
signature when the signature is within the content that it is being
signed. This MAY be implemented via the RECOMMENDED XPath
specification specified in 6.6.4: Enveloped Signature Transform;
it MUST have the same effect as that specified by the XPath Transform.
Only one digest algorithm is defined herein. However, it is expected that one or more additional strong digest algorithms will be developed in connection with the US Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptanalysis have cast doubt on its strength.
The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1 DigestAlg element is:
<DigestMethod Algorithm="
http://www.w3.org/2000/09/xmldsig#sha1"/>
A 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. For 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>
MAC algorithms take two implicit parameters, their keying
material determined from KeyInfo
and the octet stream
output by CanonicalizationMethod
. MACs and signature
algorithms are syntactically identical but a MAC implies a shared
secret key.
The HMAC
algorithm (RFC2104 [HMAC]) takes the
truncation length in bits as a parameter; if the parameter is not
specified then all the bits of the hash are output. An example of
an HMAC SignatureMethod
element:
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#hmac-sha1"> <HMACOutputLength>128</HMACOutputLength> </SignatureMethod>
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-SHA1 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [HMAC] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
Schema Definition: <simpleType name="HMACOutputLengthType"> <restriction base="integer"/> </simpleType>
DTD: <!ELEMENT HMACOutputLength (#PCDATA)>
Signature algorithms take two implicit parameters, their keying
material determined from KeyInfo
and the octet stream
output by CanonicalizationMethod
. Signature and MAC
algorithms are syntactically identical but a signature implies
public key cryptography.
The DSA algorithm [DSS] takes no explicit
parameters. An example of a DSA SignatureMethod
element is:
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
The output of the DSA algorithm consists of a pair of integers
usually referred by the pair (r, s). The signature value consists
of the base64 encoding of the concatenation of two octet-streams
that respectively result from the octet-encoding of the values r
and s in that order. Integer to octet-stream conversion must be
done according to the I2OSP operation defined in the RFC 2437 [PKCS1] specification with a l
parameter equal to 20. For 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>
The expression "RSA algorithm" as used in this draft refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is:
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
The SignatureValue
content for an RSA signature is
the base64 [MIME] encoding of the octet
string computed as per RFC 2437 [PKCS1, section 8.1.1: Signature generation
for the RSASSA-PKCS1-v1_5 signature scheme]. As specified in the
EMSA-PKCS1-V1_5-ENCODE function RFC 2437 [PKCS1, section 9.2.1], the value input to the
signature function MUST contain a pre-pended algorithm object
identifier for the hash function, but the availability of an ASN.1
parser and recognition of OIDs is not required of a signature
verifier. The PKCS#1 v1.5 representation appears as:
CRYPT (PAD (ASN.1 (OID, DIGEST (data))))
Note that the padded ASN.1 will be of the following form:
01 | FF* | 00 | prefix | hash
where "|" is concatentation, "01", "FF", and "00" are fixed octets of the corresponding hexadecimal value, "hash" is the SHA1 digest of the data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1 [RFC 2437], that is,
hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14
This prefix is included to make it easier to use standard cryptographic libraries. The FF octet MUST be repeated the maximum number of times such that the value of the quantity being CRYPTed is one octet shorter than the RSA modulus.
The resulting base64 [MIME] string is the value of the child text node of the SignatureValue element, e.g.
<SignatureValue> IWijxQjUrcXBYoCei4QxjWo9Kg8D3p9tlWoT4t0/gyTE96639In0FZFY2/rvP+/bMJ01EArmKZsR5VW3rwoPxw= </SignatureValue>
If canonicalization is performed over octets, the canonicalization algorithms take two implicit parameters: the content and its charset. The charset is derived according to the rules of the transport protocols and media types (e.g, RFC2376 [XML-MT] defines the media types for XML). This information is necessary to correctly sign and verify documents and often requires careful server side configuration.
Various canonicalization algorithms require conversion to [UTF-8].The two algorithms below understand at least [UTF-8] and [UTF-16] as input encodings. We RECOMMEND that externally specified algorithms do the same. Knowledge of other encodings is OPTIONAL.
Various canonicalization algorithms transcode from a non-Unicode encoding to Unicode. The two algorithms below perform text normalization during transcoding [NFC, NFC-Corrigendum]. We RECOMMEND that externally specified canonicalization algorithms do the same. (Note, there can be ambiguities in converting existing charsets to Unicode, for an example see the XML Japanese Profile [XML-Japanese] NOTE.)
An example of an XML canonicalization element is:
<CanonicalizationMethod Algorithm="
http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
The normative specification of Canonical XML is [XML-C14N]. The algorithm is capable of taking as input either an octet stream or an XPath node-set (or sufficiently functional alternative). The algorithm produces an octet stream as output. Canonical XML is easily parameterized (via an additional URI) to omit or retain comments.
Transform
AlgorithmsA Transform
algorithm has a single implicit
parameter: an octet stream from the Reference
or the
output of an earlier Transform
.
Application developers are strongly encouraged to support all transforms listed in this section as RECOMMENDED unless the application environment has resource constraints that would make such support impractical. Compliance with this recommendation will maximize application interoperability and libraries should be available to enable support of these transforms in applications without extensive development.
Any canonicalization algorithm that can be used for
CanonicalizationMethod
(such as those in Canonicalization Algorithms (section 6.5))
can be used as a Transform
.
The normative specification for base64 decoding transforms is
[MIME]. The base64 Transform
element has no content. The input is decoded by the algorithms.
This transform is useful if an application needs to sign the raw
data associated with the encoded content of an element.
This transform requires an octet stream for input. If an XPath
node-set (or sufficiently functional alternative) is given as
input, then it is converted to an octet stream by performing
operations logically equivalent to 1) applying an XPath transform
with expression self::text()
, then 2) taking the
string-value of the node-set. Thus, if an XML element is identified
by a barename XPointer in the Reference
URI, and its
content consists solely of base64 encoded character data, then this
transform automatically strips away the start and end tags of the
identified element and any of its descendant elements as well as
any descendant comments and processing instructions. The output of
this transform is an octet stream.
The normative specification for XPath expression evaluation is
[XPath].
The XPath expression to be evaluated appears as the character
content of a transform parameter child element named
XPath
.
The input required by this transform is an XPath node-set. Note that if the actual input is an XPath node-set resulting from a null URI or barename XPointer dereference, then comment nodes will have been omitted. If the actual input is an octet stream, then the application MUST convert the octet stream to an XPath node-set suitable for use by Canonical XML with Comments. (A subsequent application of the REQUIRED Canonical XML algorithm would strip away these comments.) In other words, the input node-set should be equivalent to the one that would be created by the following process:
(//. | //@* |
//namespace::*)
The evaluation of this expression includes all of the document's nodes (including comments) in the node-set representing the octet stream.
The transform output is also an XPath node-set. The XPath
expression appearing in the XPath
parameter is
evaluated once for each node in the input node-set. The result is
converted to a boolean. If the boolean is true, then the node is
included in the output node-set. If the boolean is false, then the
node is omitted from the output node-set.
Note: Even if the input node-set has had comments
removed, the comment nodes still exist in the underlying parse tree
and can separate text nodes. For example, the markup
<e>Hello, <!-- comment -->world!</e>
contains two text nodes. Therefore, the expression
self::text()[string()="Hello, world!"]
would fail.
Should this problem arise in the application, it can be solved by
either canonicalizing the document before the XPath transform to
physically remove the comments or by matching the node based on the
parent element's string value (e.g. by using the expression
self::text()[string(parent::e)="Hello, world!"]
).
The primary purpose of this transform is to ensure that only specifically defined changes to the input XML document are permitted after the signature is affixed. This is done by omitting precisely those nodes that are allowed to change once the signature is affixed, and including all other input nodes in the output. It is the responsibility of the XPath expression author to include all nodes whose change could affect the interpretation of the transform output in the application context.
An important scenario would be a document requiring two enveloped signatures. Each signature must omit itself from its own digest calculations, but it is also necessary to exclude the second signature element from the digest calculations of the first signature so that adding the second signature does not break the first signature.
The XPath transform establishes the following evaluation context for each node of the input node-set:
As a result of the context node setting, the XPath expressions
appearing in this transform will be quite similar to those used in
used in [XSLT], except that the size and
position are always 1 to reflect the fact that the transform is
automatically visiting every node (in XSLT, one recursively calls
the command apply-templates
to visit the nodes of the
input tree).
The function here()
is defined as
follows:
The here function returns a node-set containing the attribute or processing instruction node or the parent element of the text node that directly bears the XPath expression. This expression results in an error if the containing XPath expression does not appear in the same XML document against which the XPath expression is being evaluated.
As an example, consider creating an enveloped signature (a
Signature
element that is a descendant of an element
being signed). Although the signed content should not be changed
after signing, the elements within the Signature
element are changing (e.g. the digest value must be put inside the
DigestValue
and the SignatureValue
must
be subsequently calculated). One way to prevent these changes from
invalidating the digest value in DigestValue
is to add
an XPath Transform
that omits all
Signature
elements and their descendants. For
example,
<Document> ... <Signature xmlns="http://www.w3.org/2000/09/xmldsig#"> <SignedInfo> ... <Reference URI=""> <Transforms> <Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116"> <XPath xmlns:dsig="&dsig;"> not(ancestor-or-self::dsig:Signature) </XPath> </Transform> </Transforms> <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> <DigestValue></DigestValue> </Reference> </SignedInfo> <SignatureValue></SignatureValue> </Signature> ... </Document>
Due to the null Reference
URI in this example, the
XPath transform input node-set contains all nodes in the entire
parse tree starting at the root node (except the comment nodes).
For each node in this node-set, the node is included in the output
node-set except if the node or one of its ancestors has a tag of
Signature
that is in the namespace given by the
replacement text for the entity &dsig;
.
A more elegant solution uses the here function to omit only the
Signature
containing the XPath Transform, thus
allowing enveloped signatures to sign other signatures. In the
example above, use the XPath
element:
<XPath xmlns:dsig="&dsig;"> count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) > count(ancestor-or-self::dsig:Signature)</XPath>
Since the XPath equality operator converts node sets to string
values before comparison, we must instead use the XPath union
operator (|). For each node of the document, the predicate
expression is true if and only if the node-set containing the node
and its Signature
element ancestors does not include
the enveloped Signature
element containing the XPath
expression (the union does not produce a larger set if the
enveloped Signature
element is in the node-set given
by ancestor-or-self::Signature
).
An enveloped signature transform T removes the
whole Signature
element containing T
from the digest calculation of the Reference
element
containing T. The entire string of characters used by
an XML processor to match the Signature
with the XML
production element
is removed. The output of the
transform is equivalent to the output that would result from
replacing T with an XPath transform containing the
following XPath
parameter element:
<XPath xmlns:dsig="&dsig;"> count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) > count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to those of the XPath transform. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform MUST produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.
The normative specification for XSL Transformations is [XSLT].
Specification of a namespace-qualified stylesheet element, which
MUST be the sole child of the Transform
element,
indicates that the specified style sheet should be used. Whether
this instantiates in-line processing of local XSLT declarations
within the resource is determined by the XSLT processing model; the
ordered application of multiple stylesheet may require multiple
Transforms
. No special provision is made for the
identification of a remote stylesheet at a given URI because it can
be communicated via an
xsl:include
or
xsl:import
within the stylesheet
child of the Transform
.
This transform requires an octet stream as input. If the actual input is an XPath node-set, then the signature application should attempt to convert it to octets (apply Canonical XML]) as described in the Reference Processing Model (section 4.3.3.2).
The output of this transform is an octet stream. The processing
rules for the XSL style sheet or transform element are stated in
the XSLT specification [XSLT]. We
RECOMMEND that XSLT transform authors use an output method of
xml
for XML and HTML. As XSLT implementations do not
produce consistent serializations of their output, we further
RECOMMEND inserting a transform after the XSLT transform to
canonicalize the output. These steps will help to ensure
interoperability of the resulting signatures among applications
that support the XSLT transform. Note that if the output is
actually HTML, then the result of these steps is logically
equivalent [XHTML].
The normative specification for XML Schema is [XML-Schema]. Use of the schema
validation transform without any parameters indicates that the
document should be processed according to information within the
resource being transformed. Use of a name-space qualified
schema
element, which must be the sole child of the
Transform
, indicates the specified schema should be
used for validation; whether this instantiates other validation
using other schema is determined by the XML Schema processing
model; the ordered application of multiple schema validations may
require multiple Transforms
. No special provision is
made for the identification of a remote stylesheet at a given URI
because it can be communicated via an
xsd:include
or
xsd:import
within the schema
child of
the Transform
.
This transform requires a specified set of "Required Information Set Items and Properties" [XML-schema, Appendix D]. If the input is octets, the octets must be parsed. If the input is an XPath node-set, this node-set may be able to serve as the necessary information set. Note, while the changes made to an information set by schema validation are largely augmentations, and consequently not contained in the XPath data model, schema validation can affect default attribute and element content values. Consequently, the presense and order of schema validation may affect the canonical form.
Digital signatures only work if the verification calculations are performed on exactly the same bits as the signing calculations. If the surface representation of the signed data can change between signing and verification, then some way to standardize the changeable aspect must be used before signing and verification. For example, even for simple ASCII text there are at least three widely used line ending sequences. If it is possible for signed text to be modified from one line ending convention to another between the time of signing and signature verification, then the line endings need to be canonicalized to a standard form before signing and verification or the signatures will break.
XML is subject to surface representation changes and to processing which discards some surface information. For this reason, XML digital signatures have a provision for indicating canonicalization methods in the signature so that a verifier can use the same canonicalization as the signer.
Throughout this specification we distinguish between the
canonicalization of a Signature
element and other
signed XML data objects. It is possible for an isolated XML
document to be treated as if it were binary data so that no changes
can occur. In that case, the digest of the document will not change
and it need not be canonicalized if it is signed and verified as
such. However, XML that is read and processed using standard XML
parsing and processing techniques is frequently changed such that
some of its surface representation information is lost or modified.
In particular, this will occur in many cases for the
Signature
and enclosed SignedInfo
elements since they, and possibly an encompassing XML document,
will be processed as XML.
Similarly, these considerations apply to Manifest
,
Object
, and SignatureProperties
elements
if those elements have been digested, their
DigestValue
is to be checked, and they are being
processed as XML.
The kinds of changes in XML that may need to be canonicalized can be divided into four categories. There are those related to the basic [XML], as described in 7.1 below. There are those related to [DOM], [SAX], or similar processing as described in 7.2 below. Third, there is the possibility of coded character set conversion, such as between UTF-8 and UTF-16, both of which all [XML] compliant processors are required to support, which is described in the paragraph immediately below. And, fourth, there are changes that related to namespace declaration and xml namespace attribute context as described in 7.3 below.
Any canonicalization algorithm should yield output in a specific
fixed coded character set. All canonicalization algorithms identified in this document use
UTF-8 (without a byte order mark (BOM)) and do not provide
character normalization. We RECOMMEND that signature applications
create XML content (Signature
elements and their
descendents/content) in Normalization Form C [NFC, NFC-Corrigendum] and check that any
XML being consumed is in that form as well; (if not, signatures may
consequently fail to validate). Additionally, none of these
algorithms provide data type normalization. Applications that
normalize data types in varying formats (e.g., (true, false) or
(1,0)) may not be able to validate each other's signatures.
XML 1.0 [XML] defines an interface where a conformant application reading XML is given certain information from that XML and not other information. In particular,
Note that items (2), (4), and (5C) depend on the presence of a
schema, DTD or similar declarations. The Signature
element type is
laxly schema valid [XML-schema],
consequently external XML or even XML within the same document as
the signature may be (only) well-formed or from another namespace
(where permitted by the signature schema); the noted items may not
be present. Thus, a signature with such content will only be
verifiable by other signature applications if the following syntax
constraints are observed when generating any signed material
including the SignedInfo
element:
In addition to the canonicalization and syntax constraints discussed above, many XML applications use the Document Object Model [DOM] or The Simple API for XML [SAX]. DOM maps XML into a tree structure of nodes and typically assumes it will be used on an entire document with subsequent processing being done on this tree. SAX converts XML into a series of events such as a start tag, content, etc. In either case, many surface characteristics such as the ordering of attributes and insignificant white space within start/end tags is lost. In addition, namespace declarations are mapped over the nodes to which they apply, losing the namespace prefixes in the source text and, in most cases, losing where namespace declarations appeared in the original instance.
If an XML Signature is to be produced or verified on a system using the DOM or SAX processing, a canonical method is needed to serialize the relevant part of a DOM tree or sequence of SAX events. XML canonicalization specifications, such as [XML-C14N], are based only on information which is preserved by DOM and SAX. For an XML Signature to be verifiable by an implementation using DOM or SAX, not only must the XML 1.0 syntax constraints given in the previous section be followed but an appropriate XML canonicalization MUST be specified so that the verifier can re-serialize DOM/SAX mediated input into the same octet stream that was signed.
In [XPath] and consequently the Canonical XML data model an element has namespace nodes that correspond to those declarations within the element and its ancestors:
"Note: An element E has namespace nodes that represent its namespace declarations as well as any namespace declarations made by its ancestors that have not been overridden in E's declarations, the default namespace if it is non-empty, and the declaration of the prefix
xml
." [XML-C14N]
When serializing a Signature
element or signed XML
data that's the child of other elements using these data models,
that Signature
element and its children, may contain
namespace declarations from its ancestor context. In addition, the
Canonical XML and Canonical XML with Comments algorithms import all
xml namespace attributes (such as xml:lang
) from the
nearest ancestor in which they are declared to the apex node of
canonicalized XML unless they are already declared at that node.
This may frustrate the intent of the signer to create a signature
in one context which remains valid in another. For example, given a
signature which is a child of B
and a grandchild of
A
:
<A xmlns:n1="&foo;"> <B xmlns:n2="&bar;"> <Signature xmlns="&dsig;"> ... <Reference URI="#signme"/> ... </Signature> <C ID="signme" xmlns="&baz;"/> </B> </A>
when either the element B
or the signed element
C
is moved into a [SOAP]
envelope for transport:
<SOAP:Envelope xmlns:SOAP="http://schemas.xmlsoap.org/soap/envelope/"> ... <SOAP:Body> <B xmlns:n2="&bar;"> <Signature xmlns="&dsig;"> ... </Signature> <C ID="signme" xmlns="&baz;"/> </B> </SOAP:Body> </SOAP:Envelope>
The canonical form of the signature in this context will contain
new namespace declarations from the SOAP:Envelope
context, invalidating the signature. Also, the canonical form will
lack namespace declarations it may have originally had from element
A
's context, also invalidating the signature. To avoid
these problems, the application may:
The XML Signature specification provides a very flexible digital signature mechanism. Implementors must give consideration to their application threat models and to the following factors.
A requirement of this specification is to permit signatures to
"apply to a part or totality of a XML document." (See [XML-Signature-RD, section 3.1.3].)
The Transforms
mechanism meets this requirement by
permitting one to sign data derived from processing the content of
the identified resource. For instance, applications that wish to
sign a form, but permit users to enter limited field data without
invalidating a previous signature on the form might use [XPath] to exclude those portions the user
needs to change. Transforms
may be arbitrarily
specified and may include encoding tranforms, canonicalization
instructions or even XSLT transformations. Three cautions are
raised with respect to this feature in the following sections.
Note, core validation behavior does not confirm that the signed data was obtained by applying each step of the indicated transforms. (Though it does check that the digest of the resulting content matches that specified in the signature.) For example, some application may be satisfied with verifying an XML signature over a cached copy of already transformed data. Other applications might require that content be freshly dereferenced and transformed.
First, obviously, signatures over a transformed document do not secure any information discarded by transforms: only what is signed is secure.
Note that the use of Canonical XML [XML-C14N] ensures that all internal
entities and XML namespaces are expanded within the content being
signed. All entities are replaced with their definitions and the
canonical form explicitly represents the namespace that an element
would otherwise inherit. Applications that do not canonicalize XML
content (especially the SignedInfo
element) SHOULD NOT
use internal entities and SHOULD represent the namespace explicitly
within the content being signed since they can not rely upon
canonicalization to do this for them. Also, users concerned with
the integrity of the element type definitions associated with the
XML instance being signed may wish to sign those definitions as
well (i.e,. the schema, DTD, or natural language description
associated with the namespace/identifier).
Second, an envelope containing signed information is not secured by the signature. For instance, when an encrypted envelope contains a signature, the signature does not protect the authenticity or integrity of unsigned envelope headers nor its ciphertext form, it only secures the plaintext actually signed.
Additionally, the signature secures any information introduced by the transform: only what is "seen" (that which is represented to the user via visual, auditory or other media) should be signed. If signing is intended to convey the judgment or consent of a user (an automated mechanism or person), then it is normally necessary to secure as exactly as practical the information that was presented to that user. Note that this can be accomplished by literally signing what was presented, such as the screen images shown a user. However, this may result in data which is difficult for subsequent software to manipulate. Instead, one can sign the data along with whatever filters, style sheets, client profile or other information that affects its presentation.
Just as a user should only sign what he or she "sees," persons
and automated mechanism that trust the validity of a transformed
document on the basis of a valid signature should operate over the
data that was transformed (including canonicalization) and signed,
not the original pre-transformed data. This recommendation applies
to transforms specified within the signature as well as those
included as part of the document itself. For instance, if an XML
document includes an
embedded style sheet [XSLT] it is the
transformed document that should be represented to the user and
signed. To meet this recommendation where a document references an
external style sheet, the content of that external resource should
also be signed as via a signature Reference
otherwise
the content of that external content might change which alters the
resulting document without invalidating the signature.
Some applications might operate over the original or intermediary data but should be extremely careful about potential weaknesses introduced between the original and transformed data. This is a trust decision about the character and meaning of the transforms that an application needs to make with caution. Consider a canonicalization algorithm that normalizes character case (lower to upper) or character composition ('e and accent' to 'accented-e'). An adversary could introduce changes that are normalized and consequently inconsequential to signature validity but material to a DOM processor. For instance, by changing the case of a character one might influence the result of an XPath selection. A serious risk is introduced if that change is normalized for signature validation but the processor operates over the original data and returns a different result than intended.
As a result of this, while we RECOMMEND all documents operated upon and generated by signature applications be in [NFC, NFC-Corrigendum] (otherwise intermediate processors might unintentionally break the signature) encoding normalizations SHOULD NOT be done as part of a signature transform, or (to state it another way) if normalization does occur, the application SHOULD always "see" (operate over) the normalized form.
This specification uses public key signatures and keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified algorithms 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 private key can create signatures. The number of holders of the private 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 private key is an important issue, usually addressed by certificate or online 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 specification permits user provided signature algorithms and keying information designators. Such user provided algorithms may have different security models. For example, methods involving biometrics usually depend on a physical characteristic of the authorized user that can not be changed the way public or secret keys can be and may have other security model differences.
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 [RANDOM] and the size of the key, the security of key and certificate authentication and distribution mechanisms, certificate chain validation policy, protection of cryptographic processing from hostile observation and tampering, etc.
Care must be exercised by applications in executing the various algorithms that may be specified in an XML signature and in the processing of any "executable content" that might be provided to such algorithms as parameters, such as XSLT transforms. The algorithms specified in this document will usually be implemented via a trusted library but even there perverse parameters might cause unacceptable processing or memory demand. Even more care may be warranted with application defined algorithms.
The security of an overall system will also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. All the factors listed in this section are important to the overall security of a system; however, most are beyond the scope of this specification.
schemaLocation
to aid automated schema fetching and
validation.Object
designates a specific XML element.
Occasionally we refer to a data object as a document or as
a resource's
content. The term element content is used to describe
the data between XML start and end tags [XML]. The term XML document is used to
describe data objects which conform to the XML specification [XML].Object
element is merely one type of digital data (or
document) that can be signed via a Reference
.Signature
element type and its children (including
SignatureValue
) and the specified algorithms.Signature
element, and can be identified via a
URI
or transform. Consequently, the signature is
"detached" from the content it signs. This definition typically
applies to separate data objects, but it also includes the instance
where the Signature
and data object reside within the
same XML document but are sibling elements.Object
element of the signature itself. The
Object
(or its content) is identified via a
Reference
(via a URI
fragment identifier
or transform).SignatureValue
.SignedInfo
reference validation.Reference
, matches its specified
DigestValue
.SignatureValue
matches the result of
processing SignedInfo
with
CanonicalizationMethod
and
SignatureMethod
as specified in Core Validation (section 3.2).Donald E. Eastlake 3rd
Motorola, 20 Forbes Boulevard
Mansfield, MA 02048 USA
Phone: 1-508-261-5434
Email: Donald.Eastlake@motorola.com
Joseph M. Reagle Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
Phone: + 1.617.258.7621
Email: reagle@w3.org
David Solo
Citigroup
909 Third Ave, 16th Floor
NY, NY 10043 USA
Phone +1-212-559-2900
Email: dsolo@alum.mit.edu