Please refer to the errata for this document, which may include some normative corrections.
This document is also available in these non-normative formats: XHTML with color-coded revision indicators against the previous recommendation version.
See also translations.
Copyright © 2008 The Internet Society & W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use 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 section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
The original version of this specification was produced by the IETF/W3C XML Signature Working Group which believes the specification is sufficient for the creation of independent interoperable implementations; the Interoperability Report shows at least 10 implementations with at least two interoperable implementations over every feature.
This Second Edition was produced by the W3C XML Security Specifications Maintenance Working Group, part of the W3C Security Activity (Activity Statement).
This Second Edition of XML Signature Syntax and
Processing adds Canonical XML 1.1 as a required
canonicalization algorithm and recommends its use for inclusive
canonicalization. This version of Canonical XML enables use of
xml:id
and xml:base
Recommendations
with XML Signature and also enables other possible future
attributes in the XML namespace. Additional minor changes,
including the incorporation of known errata, are documented in
Changes in XML Signature Syntax and Processing
(Second Edition).
The Working Group conducted an interoperability test as part of its activity. The Test Cases for C14N 1.1 and XMLDSig Interoperability [TESTCASES] are available as a companion Working Group Note. The Implementation Report for XML Signature, Second Edition is also publicly available.
Please send comments about this document to<EMAIL_ADDRESS>(with public archive).
This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.
This document is governed by the 24 January 2002 CPP as amended by the W3C Patent Policy Transition Procedure. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy. Patent disclosures relevant to this specification may be found on the IETF Page of Intellectual Property Rights Notices, in conformance with IETF policy.
The English version of this specification is the only normative version.
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 an 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 key words 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 Namespaces in XML 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:
The following members of the XML Security Specification Maintenance Working Group contributed to the second edition:
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 ID?> <SignedInfo> <CanonicalizationMethod/> <SignatureMethod/> (<Reference URI? > (<Transforms>)? <DigestMethod> <DigestValue> </Reference>)+ </SignedInfo> <SignatureValue> (<KeyInfo>)? (<Object ID?>)* </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
Reference
)sThe 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/2006/12/xml-c14n11"/> [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/2006/12/xml-c14n11"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK.../DigestValue> [s11] </Reference> [s12] </SignedInfo> [s13] <SignatureValue>...</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/2006/12/xml-c14n11"/> [s08] </Transforms> [s09] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [s10] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</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] <Transforms> [p06] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [p07] </Transforms> [p08] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [p09] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...</DigestValue> [p10] </Reference> [p11] </SignedInfo> [p12] ... [p13] <Object> [p14] <SignatureProperties> [p15] <SignatureProperty Id="AMadeUpTimeStamp" Target="#MySecondSignature"> [p16] <timestamp xmlns="http://www.ietf.org/rfcXXXX.txt"> [p17] <date>19990914</date> [p18] <time>14:34:34:34</time> [p19] </timestamp> [p20] </SignatureProperty> [p21] </SignatureProperties> [p22] </Object> [p23]</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.
[p13]
Object
is an optional element
for including data objects within the signature element or
elsewhere. The Object
can be optionally typed and/or
encoded.
[p14-21]
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] <Transforms> [m04] <Transform Algorithm="http://www.w3.org/2006/12/xml-c14n11"/> [m05] </Transforms> [m06] <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/> [m07] <DigestValue>dGhpcyBpcyBub3QgYSBzaWduYXR1cmUK...=</DigestValue> [m08] </Reference> [ ] ... [m09] <Object> [m10] <Manifest Id="MyFirstManifest"> [m11] <Reference> [m12] ... [m13] </Reference> [m14] <Reference> [m15] ... [m16] </Reference> [m17] </Manifest> [m18] </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 .)Transform
elements is
a node-set. We RECOMMEND that, when generating signatures,
signature applications do not rely on this default behavior, but
explicitly identify the transformation that is applied to perform
this mapping. In cases in which inclusive canonicalization is
desired, we RECOMMEND that Canonical XML 1.1 [XML-C14N11] be used.
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
.
Note, if the Signature
includes same-document
references, [XML] or
[XML-schema]
validation of the document might introduce changes that break
the signature. Consequently, applications should be careful
to consistently process the document or refrain from using
external contributions (e.g., defaults and entities).
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
.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 applications that implement a text-based instead of XML-based canonicalization -- such as resource constrained apps -- 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 Reference
s 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 [URI].
The mapping from this attribute's value to a URI reference MUST be performed as specified in section 3.2.17 of [XMLSCHEMA Datatypes, 2nd Edition]. Additionally: Some existing implementations are known to verify the value of the URI attribute against the grammar in [URI]. It is therefore safest to perform any necessary escaping while generating the URI attribute.
We RECOMMEND XML signature applications 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 after all ds:Reference
transforms have been applied. 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 results in the
digesting of 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
.
Note: The Reference Generation Model (section 3.1.1) includes further restrictions on the reliance upon defined default transformations when applications generate signatures.
In this specification, a 'same-document' reference is defined as a URI-Reference that consists of a hash sign ('#') followed by a fragment or alternatively consists of an empty URI [URI].
Unless the URI-Reference is such a 'same-document' reference , 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 <IP_ADDRESS>).)
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 in this case 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
shortname XPointer [XPointer-Framework]. 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 due to the processing
defined in Same-Document URI-References (section <IP_ADDRESS>).) All
other support for XPointers is OPTIONAL, especially all support
for shortname 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).
'#xpointer(/)
' MUST be interpreted to identify
the root node [XPath] of
the document that contains the URI
attribute.
'#xpointer(id('ID'))
' MUST be
interpreted to identify the element node identified by
'#element(ID)
' [XPointer-Element] when
evaluated with respect to the document that contains the
URI
attribute.
The original edition of this specification [XMLDSIG-2002] referenced the
XPointer Candidate Recommendation [XPTR-2001] and some implementations support it
optionally. That Candidate Recommendation has been superseded by
the [XPointer-Framework], [XPointer-xmlns] and [XPointer-Element]
Recommendations, and -- at the time of this edition -- the
[XPointer-xpointer] Working Draft. Therefore, the use
of the xpointer()
scheme [XPointer-xpointer]
beyond the usage discussed in this section is discouraged.
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 [XML-C14N]. 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 [XPointer-Framework]. When processing an XPointer, the
application MUST behave as if the XPointer was evaluated with
respect to the XML document containing the URI
attribute . The application MUST behave as if the result of
XPointer processing [XPointer-Framework] were a node-set derived from the
resultant subresource 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 and shortname
XPointers . It is necessary because when [XML-C14N] or [XML-C14N11] is passed a node-set, it processes the
node-set as is: with or without comments. Only when it is called
with an octet stream does it invoke its 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 scheme-based
XPointer. To retain comments while selecting an element by an
identifier ID, use the following scheme-based XPointer:
URI='#xpointer(id('ID'))'
. To retain
comments while selecting the entire document, use the following
scheme-based XPointer: URI='#xpointer(/)'
.
The interpretation of these XPointers is defined in The Reference Processing Model (section <IP_ADDRESS>).
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. (See Algorithm
Identifiers and Implementation Requirements (section 6).)
As described in The Reference Processing Model (section <IP_ADDRESS>), 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 <IP_ADDRESS>). 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
Y
J
seed
pgenCounter
Parameter J is available 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 Definition:
<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="G" type="ds:CryptoBinary" minOccurs="0"/>
<element name="Y" type="ds:CryptoBinary"/>
<element name="J" type="ds:CryptoBinary" minOccurs="0"/>
<sequence minOccurs="0">
<element name="Seed" type="ds:CryptoBinary"/>
<element name="PgenCounter" type="ds:CryptoBinary"/>
</sequence>
</sequence>
</complexType>
DTD Definition:
<!ELEMENT DSAKeyValue ((P, Q)?, G?, Y, J?, (Seed, PgenCounter)?) >
<!ELEMENT P (#PCDATA) >
<!ELEMENT Q (#PCDATA) >
<!ELEMENT G (#PCDATA) >
<!ELEMENT Y (#PCDATA) >
<!ELEMENT J (#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 Definition:
<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 Definition:
<!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 <IP_ADDRESS>) and
The
Reference Processing Model (section <IP_ADDRESS>) 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 retrieved after all transforms have been applied. 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 ref="ds:Transforms" 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 >
Note: The schema for the URI
attribute of RetrievalMethod erroneously omitted the attribute:
use="required"
The DTD is correct. However, this error only results in a more lax schema which permits all valid RetrievalMethod elements. Because the existing schema is embedded in many applications, which may include the schema in their signatures, the schema has not been corrected to be more restrictive.
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. The distinguished name SHOULD be represented as a
string that complies with section 3 of RFC4514 [LDAP-DN], to be generated
according to the Distinguished Name Encoding Rules section
below,X509SubjectName
element, which
contains an X.509 subject distinguished name that SHOULD be
represented as a string that complies with section 3 of
RFC4514 [LDAP-DN],
to be generated according to the Distinguished Name Encoding Rules section
below,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.
To encode a distinguished name
(X509IssuerSerial
,X509SubjectName
, and
KeyName
if appropriate), the encoding rules in
section 2 of RFC 4514 [LDAP-DN] SHOULD be applied, except that the character
escaping rules in section 2.4 of RFC 4514 [LDAP-DN] MAY be augmented as follows:
Since a XML document logically consists of characters, not octets, the resulting Unicode string is finally encoded according to the character encoding used for producing the physical representation of the XML document.
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 permitX509Data
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 element 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
'http://www.w3.org/2000/09/xmldsig#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
normative 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 specification 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 concatenation, "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 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 output of these algorithms will be in NFC [NFC, NFC-Corrigendum]. This is because the XML processor used to prepare the XPath data model input is required (by the Data Model) to use Normalization Form C when converting an XML document to the UCS character domain from any encoding that is not UCS-based.
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.)
This specification REQUIRES implementation of both Canonical XML 1.0 [XML-C14N] and Canonical XML 1.1 [XML-C14N11]. We RECOMMEND that applications that generate signatures choose Canonical XML 1.1 [XML-C14N11] when inclusive canonicalization is desired.
Note: Canonical XML 1.0 [XML-C14N] and Canonical XML 1.1 [XML-C14N11] specify a standard serialization of XML that, when applied to a subdocument, includes the subdocument's ancestor context including all of the namespace declarations and some attributes in the 'xml:' namespace. However, some applications require a method which, to the extent practical, excludes unused ancestor context from a canonicalized subdocument. The Exclusive XML Canonicalization Recommendation [XML-exc-C14N] may be used to address requirements resulting from scenarios where a subdocument is moved between contexts.
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 XML1.0 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.
The normative specification of Canonical XML 1.1 is [XML-C14N11]. 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 1.1 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 shortname 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 shortname 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.
Note that the XML-Signature XPath Filter 2.0 Recommendation [XPath-Filter-2] may be used for this purpose. This recommendation defines an XPath transform that permits the easy specification of subtree selection and omission that can be efficiently implemented.
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, but may only be applied to a node-set from its parent XML document. 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 <IP_ADDRESS>).
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].
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 (5.3) 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 transforms,
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 applications 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:
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 Laboratories
111 Locke Drive
Marlborough, MA 01752 USA
Phone: +1-508-786-7554
Email<EMAIL_ADDRESS>Joseph M. Reagle Jr.
Department of Media, Culture, and Communication
New York University
Email<EMAIL_ADDRESS>David Solo
Citigroup
909 Third Ave, 16th Floor
NY, NY 10043 USA
Phone +1-212-559-2900
Email: dsolo@alum.mit.edu