Copyright © 1999 The Internet Society & W3C (MIT, INRIA, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
This document specifies the syntax and processing rules for the encoding of digital signatures using XML. Such signatures can provide integrity, message authentication, and/or signer authentication services for data of any type, whether located within the XML that includes the signature or locatable elsewhere.
This is the second (and rough) public draft of this specification. This draft covers most of the topics the final specification will cover, however parts of the text and syntax within this specification are subject to change (and may be incorrect or inconsistent.)
Please send comments to the editors and cc: the list <w3c-ietf-xmldsig@w3.org>. Publication as a Working Draft does not imply endorsement by the W3C membership or IESG. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite W3C Drafts as other than "work in progress." A list of current W3C working drafts can be found at http://www.w3.org/TR
Patent disclosures relevant to this specification may be found on the WG's patent disclosure page.
This document describes the proposed syntax and processing rules for the XML Digital Signature specification. This specification provides a mechanism for applying digital signatures to XML documents and other Internet resources and encoding those signatures as XML.
The structure allows for both embedded and detached signatures. An embedded signature can include the signature within the signed object or embed the signed object within the signature. A detached signature allows the signature to be independent of the object. The processing structure allows for switching between embedded and detached signatures without necessailry invalidating the signature.
In addition to the basic signature type, this document also defines other useful types including methods of referencing multiple resources and key management and algorithm definitions.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The XML namespace [XML-namespace] URI that MUST be used by experimental implementations of this dated specification is:
xmlns="http://www.w3.org/1999/10/signature-core"
While applications MUST support XML-namespaces, the use of our "dsig" XML namespace prefix and defaulting/scoping conventions are OPTIONAL -- we use these facilities so as to provide compact and readable examples.
The URI in the namespace declaration above is also used as a prefix for URIs which
identify resources, algorithms, or semantics under control of this specification. We use
MIME types to identify algorithms, resources, or their characteristics under the control
of IANA. Otherwise we define a URN Namespace Identifiers [RFC2141]
for other organizations, for example: urn:ietf-org:hmac-sha1
This document includes the following abbreviations for long words. (The acronyms are generated by wrapping the word_length-2 in the first and last letter):
Finally, this document includes a list of open issues which are still being addressed by the working group.
Readers unfamiliar with DTD syntax may wish to refer to Ron Bourret's "Declaring Elements and Attributes in an XML DTD."
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 is expected to use a different Namespace.
This section provides a general top down overview of XML digital signature syntax and processing. The formal specification is provided in later sections. General familiarity with digital signature concepts and XML syntax is assumed.
Signature ElementXML digital signatures are very flexible and may be used to apply signatures to any type of resource. The object(s) being signed may be included within the signature, outside the signature in the same document, or completely outside of the document.
XML digital signatures are represented by the Signature element which has
the following structure:
<Signature>
(SignedInfo)
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The required SignedInfo element is the information which is actually
signed. SignedInfo includes a digest calculated over each of the data objects
being signed. The core signature verification includes the verification of these digests.
The algorithms used in calculating the SignatureValue are also included in
the signed information. The signature can not cover itself so the SignatureValue
element is outside SignedInfo.
KeyInfo indicates what key was used to create the signature. It is
optional because in some applications the key is implied by the circumstances. A wide
variety of KeyInfo forms are available including certificates, key names, key
agreement algorithms and information, etc. The keying information is outside of the signed
information so that it need not be signed. KeyInfo might contain auxiliary
information it is not desired to reveal to all signature verifiers. If KeyInfo
were signed, it would be necessary to pass all of it to all verifiers. On the other hand,
if it is desired to bind the keying information in to the signature, its digest and a
pointer to it can easily be included in the signed information.
Object is an optional element for carrying the signed data. A signature
can be applied to a mix of external and embedded objects. The data can be optionally
typed and/or encoded. While Object elements can appear inside a signature as
shown above, they can also appear outside of the Signature element in the
same document or in other documents.
There is no explicit provision for additional signature properties such as time of
signing. (These are traditional called "attributes" although that term in that
context has no relationship to the XML term "attribute".) Signature properties
can be included as a type of Object and thus can easily be signed or not as
appropriate.
SignedInfo ElementThe SignedInfo element has the structure indicated below.
<Signature>
<SignedInfo>
(CanonicalizationMethod)
(SignatureMethod)
(ObjectReference)+
</SignedInfo>
(SignatureValue)
(KeyInfo)?
(Object)*
</Signature>
The CanonicalizationMethod is the algorithm which is used to canonicalize
the SignedInfo element before it is digested as part of the signature
operation. [The working group is considering specifying a fixed CanonicalizationMethod
which would eliminate the need for this element.]
The SignatureMethod is the algorithm used to convert the canonicalized SignedInfo
into the SignatureValue. It is a combination of a digest algorithm and a key
dependent algorithm such as RSA-SHA1 or HMAC-SHA1. The algorithm names are signed to
resist attacks based on substituting a weaker algorithm.
To promote interoperability, there are mandatory to implement canonicalization and signature algorithms. Additional standard algorithms are specified as Recommended or Optional and user defined algorithms are permitted.
The ObjectReference elements specify the things secured by the signature.
As discussed in more detail below, they point to the thing, specify any transformations,
specify the digest algorithm, and include the digest value itself. It is the signing of
this digest value and its verification as part of the signature verification that secures
the thing pointed to.
The indirect reference to secured things via the ObjectReference means
that it is possible to change a Signature from one where the data in enclosed
as an Object within the Signature to one where the Object
appears elsewhere or to move a secure item between locations outside a Signature
without invalidating the signature provided the secured data can still be located from the
same ObjectReference.
ObjectReference ElementThe ObjectReference element has the structure indicated below.
...
<SignedInfo>
(CanonicalizationMethod)?
(SignatureMethod)
<ObjectReference Location=? Type=? >
(Transforms)?
(DigestMethod)
(DigestValue)
</ObjectReference>+
</SignedInfo>
...
The optional Location attribute says where the secured thing is.
The optional Type attribute provides information about the content of the
thing at Location. In particular, it can indicate that the thing consists of
signature Properties or is a Manifest or Package
(see below).
Transforms is an optional ordered list of processing steps that are
applied to the thing at Location before it is digested. These transforms can
include any number of canonicalizations, encoding and decoding including compression and
inflation, and XPath based and other transforms. XPath based transforms permit parts of an
XML thing to be omitted. For example, if a thing being secured encloses the signature
itself, such a transform must be used to exclude the signature from the data covered. If
no Transforms element is present, the data pointed at by Location
is digested directly.
To promote interoperability, there are mandatory to implement canonicalization and decoding algorithms. Additional standard canonicalization, coding, and XPath based transform algorithms are specified as Recommended or Optional and user defined transform algorithms are permitted.
DigestMethod is the algorithm which, when applied to the thing at Location
after Transforms is applied, results in DigestValue. The signing
of the DigestValue is what secures the thing pointed to.
Manifest and Package ElementsThere are cases where it is efficient to have one signature covering many items. One
approach is to include multiple object references within SignedInfo. Since the core
verification behavior of this specification includes verifying the digests of objects
referenced within SignedInfo, some applications may need an alternative
approach which allows pushing the validation decision to the application. This allows more
complex processing to be defined on an application specific basis; for example, it may be
sufficient if the signature's validity for n out of m of the items can be verified or
there may be a large number of items that it is desired to sign with multiple signature
algorithms and / or keys where listing all of the items within the SignedInfo
element of each Signature is too bulky.
To answer these requirements, additional object types have been defined which may be
referenced by SignedInfo. The Manifest element is provided which
similarly contains a collection of references and objects (like SignedInfo),
but leaves it entirely up to the application which digest or digests it will verify.
Multiple signatures over the possibly large number of items in a Manifest
need only point to the Manifest from one ObjectReference in each
signature's SignedInfo.
The structure of Manifest, which reuses the ObjectReference
and Object elements described above, is as follows:
<Manifest>
(ObjectReference)+
(Object)*
</Manifest>
A Package is syntactically identical to a Manifest but
asserts the identity of each of its ObjectReference elements after Transforms
application.
Manifest and Package appear as the content of Objects.
The general structure of an XML signature is described in section 2 above. This section provides detailed syntax of the core signature features and actual exampes (TBD). Syntax in this section is mandatory to implement unless othewise indicated.
Signature element<!ELEMENT Signature (SignedInfo, SignatureValue, KeyInfo?,
Object*)>
<!ATTLIST SignedInfo
Id
ID #IMPLIED>
A simple example follows:
<Signature
xmlns="http://www.w3.org/1999/10/signature-core">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/.../xml-c14n"/>
<SignatureMethod Algorithm="dsig:dsaWithSHA-1"/>
<ObjectReference Location="http://www.ietf.org">
<DigestMethod
Algorithm="urn:nist-gov:sha1"/>
<DigestValue
encoding="urn:ietf-org:base64">a23bcd43</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue
encoding="urn:ietf-org:base64">dd2323dd</SignatureValue>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
Note: this example will be revised to ensure hash/signature validate.
SignatureValue ElementThe SignatureValue element contains the actual value of the digital
signature. The ability to define a SignatureMethod and SignatureValue
pair which includes multiple distinct signatures is explicitly permitted (e.g.
"rsawithsha-1 and ecdsawithsha-1").
<!ELEMENT SignatureValue CDATA >
<!-- base64 encoded signature value -->
<!ATTLIST SignatureValue
encoding
CDATA "urn:ietf-org:base64" >
SignedInfo ElementThe structure of SignedInfo includes a canonicalization algorithm, a
signature algorithm, and one or more references to objects. The SignedInfo
element may contain an optional ID attribute that will allow it to be referenced by other
signatures and objects.
<!ELEMENT SignedInfo(CanonicalizationMethod, SignatureMethod,
ObjectReference+ ) >
<!ATTLIST SignedInfo
Id
ID #IMPLIED >
SignedInfo does not include explicit signature properties. If an
application needs to associate properties (such as signing time, signing device, etc.)
with the signature, it may add an additional Object that includes that data
and reference that Object via an ObjectReference. See the Properties
element below.
CanonicalizationMethodElement CanonicalizationMethod is a mandatory element which specifies the
canonicalization algorithm applied to the SignedInfo element prior to
performing signature calculations. This element uses the general structure here for
algorithms in which a URI is included as an attribute naming the algorithm and optional Parameter
contents of the element contain any parameter, value, or other information for the
algorithm. Possible options may include a minimal algorithm (CRLF and charset
normalization), or more extensive operations such as [XML-C14N].
An expected default for this value will be defined once the specification of XML aware
canonicalization algorithms are finalized.
<!ELEMENT CanonicalizationMethod Parameter* >
<!ATTLIST CanonicalizationMethod
Algorithm
CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI]
-->
[The working group is considering specifying a fixed CanonicalizationMethod
which would eliminate the need for this element.]
SignatureMethod ElementSignatureMethod is a required element which specifies the algorithm used
for signature generation and validation. This algorithm identifies all cryptographic
functions involved in the signature operation (e.g. hashing, public key algorithms, MACs,
etc.). This element uses the general structure here for algorithms in which a URI is
included as an attribute naming the algorithm and optional Parameter contents
of the element contain any parameter, value, or other information for the algorithm. While
there is a single identifier, that identifier may specify a format containing multiple
distinct signature values.
<!ELEMENT SignatureMethod Parameter* >
<!ATTLIST SignatureMethod
Algorithm
CDATA #REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI]
-->
ObjectReference ElementObjectReference is an element that may occur one or more times. It
includes a pointer to the object being signed, the type of the object, an optional list of
transforms to be applied prior to digesting, a digest algorithm and digest value.
<!ELEMENT ObjectReference ( Transforms?, DigestMethod,
DigestValue ) >
<!ATTLIST ObjectReference
Id ID
#IMPLIED
Location
CDATA #IMPLIED>
Type
CDATA #IMPLIED>
<!-- The values of Location and Type conform
to the productions specified by [URI] -->
There is an optional ID attribute so that it can be referenced from elsewhere.
The optional Location attribute identifies where to find the Object using
a URI. As the terms are defined in RFC2396 [URI], some URIs are used in
conjunction with a fragment identifier by use of a separating hash (#), but the URI does
not include the fragment identifier. Location only permits a URI, and
fragment identification is covered under Transforms. Note that a
null URI is proper an indicates the current document (Location="").
If this attribute is omitted, then the receiving application is expected to be able to
determine the object to which the signature applies. For example, this attribute might be
omitted for a signature in a lightweight protocol data unit. The location may be omitted
only if there is a single object reference. If there are multiple object references, they
each must contain an explicit location. Note, it is the content yielded after the URI is
dereferenced, decoded, and transformed that the digest algorithm is applied to. If the URI
indicates an XML document, the document is assumed to be unparsed prior to the application
of Transforms. If there are no Transforms, then the indicated
resource is passed to the digest algorithm unmodified.
The optional Type attribute contains information about the type of object
being signed (e.g. manifest, package, document, SignedInfo. This is
represented as a URI.
Examples follow:
Type="http://www.w3.org/1999/10/signature-core/manifest"
Type="urn:ietf-org:hmac-sha1"
Transforms Element Transforms is an optional element that contains one or more operations to
be performed on an indicated resource prior to digest calculation. (These operations are
different from those specified in the Signature; those are are applied over SignedInfo.)
If the Transforms element is omitted, the exact data referenced is digested
byte for byte.
The Transforms element contains an ordered list of Transform
elements. The output of each Transform serves as input to the next Transform.
The input to the first Transform is the raw data result of obtaining the
resource given by the Location attrbiute or as determined by the application
if no Location is meaningful for that application. The output from the last Transform
is the input for the digest algorithm.
<!ELEMENT Transforms (Transformation+) >
Each Transform consists of an Algorithm attribute, optional Encoding,
Type, and Charset attributes, and content appropriate for the
given algorithm. The Algorithm attribute value specifies the name of the
algorithm to be performed, and the Transform content is provided as
additional data to govern the algorithm's processing of the input resource. If the
Encoding attribute indicates that the Transform content is base 64 encoded,
then the Transform content will be base-64 decoded before it is presented to
the transformation algorithm.
The optional Type and Charset attributes are made available
to algorithms which need and are otherwise unable to deduce that inforamtion about the
data they are processing.
<!ELEMENT Transform ANY>
<!ATTLIST Transform
Algorithm
CDATA #REQUIRED
Encoding
CDATA #IMPLIED
Type CDATA #IMPLIED
Charset
CDATA #IMPLIED >
<!-- The CDATA for Algoirhtm and Encoding conforms
to the productions specified by [URI] -->
Examples of resource transforms include but are not limited to base-64 decoding,
canonicalization, XPath filtering, and XSLT. The generic definition of the Transform
element also allows application-specific transform algorithms. For example, the transform
could be a decompression routine given by a base-64 encoded Java class appearing in the Transform
content. However, applications should refrain from using application-specific transforms
whenever possible since the resulting signature will not necessarily be verifiable outside
of the application domain. The section Transform Algorithms
defines the list of standard transformations.
Implementation Comment: When transformations are applied the signer is not signing the native (original) document but the resulting (transformed) document that is not captured explicitly in the signature syntax.
DigestMethod ElementDigestMethod is a required element which identifies the digest algorithm
to be applied to the signed object. This element uses the general structure here for
algorithms in which a URI is included as an attribute naming the algorithm and optional Parameter
contents of the element contain any parameter, value, or other information for the
algorithm.
<!ELEMENT DigestMethod Parameter* >
<!ATTLIST DigestMethod
Algorithm CDATA
#REQUIRED >
<!-- Where CDATA conforms to the
productions specified by [URI]
-->
DigestValue ElementDigestValue is an element which contains the base64 encoded value of the
digest.
<!ELEMENT DigestValue CDATA>
<!-- base64 encoded signature value -->
<!ATTLIST DigestValue
Encoding
CDATA "urn:ietf-org:base64">
KeyInfo ElementKeyInfo may contain keys, names, certificates and other public key
management information (such as inband key distribution or agreement data or data
supporting any other method.) This specification defines a few simple types but
applications may place (embed) their own key identification and exchange semantics within
this element through the XML-namespace facility. [XML-namespace]
<!ELEMENT KeyInfo (#PCDATA | (KeyName | KeyValue |
SubjectName | RetrievalMethod |
x509Data |
PGPData | MgmtData)* )>
KeyInfo is an optional element which enables the recipient(s) to obtain
the key(s) needed to validate the signature. If omitted, the recipient is expected to be
able to identify the key based on application context information. This element contains
one KeyInfo data element providing information for the recipient(s).
Applications may define and use any mechanism they choose through inclusion of elements
from a different namespace.
Compliant versions implementing KeyInfo MUST implement KeyValue,
and SHOULD implement RetrievalMethod.
KeyName contains an identifier for the key which may be useful to the
recipient. This may be a name, index, etc.KeyValue contains the actual key(s) used to validate the signature. If the
key is sent in protected form, the MgmtData element should be used. Specific
types must be defined for each algorithm type (see algorithms).SubjectName contains one or more names for the sender. Forms to be
supported include a simple name string, encoded DN, email address, etc.RetrievalMethod is a URI which may be used to obtain key and/or certificate
information. The URI should contain the complete string for retrieving the key needed for
this message (rather than a generic URI).X509Data contains an identifier of the key/cert used for validation (either
an IssuerSerial value, a subject name, or a subjectkeyID) and an optional collection of
certificates and revocation/status information which may be used by the recipient.
IssuerSerial contains the encoded issuer name (RFC 2253) along with the serial number.PGPData data associated with a PGP key.MgmtData contains in-band key distribution or agreement data. Examples may
include DH key exchange, RSA key encryption etc.<!ELEMENT KeyValue ANY>
<!ELEMENT KeyName (#PCDATA) >
<!ELEMENT SubjectName (#PCDATA) >
<!ELEMENT RetrievalMethod (#PCDATA) >
<!ELEMENT X509Data ((X509IssuerSerial | X509SKI | X509Name),
(X509Certificate | X509CRL | ANY)* ) >
<!ELEMENT MgmtData (#PCDATA)>
<!ELEMENT PGPData (PGPKeyID, PGPKeyPacket?)>
<!ELEMENT X509IssuerSerial (X509Name, X509SerialNumber)>
<!ELEMENT X509Name (#PCDATA)>
<!-- Where the name is encoded accroding to RFC 2253 -->
<!ELEMENT X509SerialNumber (#PCDATA)>
<!-- Where the data is the serial number encoded as a decimal integer -->
<!ELEMENT X509SKI (#PCDATA)>
<!-- Where the data consists of the SKI base64 encoded -->
<!ELEMENT X509Certificate (#PCDATA)>
<!-- Where the data conists of the base64 encoded certificate -->
<!ELEMENT X509CRL (#PCDATA)>
<!-- Where the data consists of the base64 encoded CRL -->
<!ELEMENT PGPKeyID (#PCDATA)>
<!-- Where the data conists of the hex encoding of the key ID. -->
<!ELEMENT PGPKeyPacket (#PCDATA)>
<!-- Where the data consists of the base64 encoded key packet --->
Note: This section is preliminary. A more detailed version will be included in a subsequent version of this specification.
Object ElementObject is an optional element which may occur one or more times. When
present this element may contain any item and specifies the encoding. The digest is
calculated over the entire Object element including start and end tags. If
the application wishes to exclude the <Object> tags from the digest
calculation, then a transform must be used. Exclusion of the object tags may be desired
for cases where the signature is intended to survive a change between embedded and
detached objects or where the content of the Object is an encoding of an
original binary document and it is desired to extract and decode so as to sign the
original as closely as possible.
<!ELEMENT Object ANY>
<!ATTLIST Object
Id ID
#IMPLIED
Type
CDATA #IMPLIED
Encoding
CDATA #IMPLIED >
<!-- Where type and encoding CDATA conforms to the
productions specified by [URI]
-->
The Object's ID is referenced from the ObjectReference in SignedInfo.
This element is used for embedded signatures where the object being signed is to be
included in the signature document. The Object element may include optional
type, ID, and encoding attributes.
Parameter ElementAlgorithms are provided with parameters and input data, when necessary, by having Parameter
elements in the content of the algorithm element. Algorithms also have an implicit input
such as the canonicalized SignedInfo for SignatureMethod and the
tranformed data pointed to for DigestMethod.
Where more than one Parameter appears, they are passed to the algorithm as an ordered vector corresponding to the order they appear in the algorithm element content.
<!ELEMENT Parameter #PCDATA>
<!ATTLIST Parameter
Encoding
CDATA #IMPLIED >
<!-- Encoding CDATA conforms to the productions
specified by [URI] -->
This section describes the optional to implement Manifest and Package
elements and describes the handling of XML Processing Instructions and Comments.
Manifest and Package ElementsThe Manifest element provides a list of ObjectReferences. The
difference from the list in SignedInfo is that it is application defined
which, if any, of the digests are actually checked against the objects referenced and what
to do if the object is inaccessible or the digest compare fails. If a Manifest
is pointed to from SignedInfo, the digest over the Manifest
itself will be checked by the core signture verifciation behavior. The digests within such
a Manifest are checked at application discretion. If a Manifest
is referenced from another Manifest, even the overall digest of this two
level deep Manifest might not be checked.
A Package has the same syntax as a Manifest but also asserts
the equality of each of its referenced objects, after any transforms. The testing of this
equality and action if it fails is also entirely at the discretion of the applicaiton.
<!ELEMENT Manifest ( ObjectReference+, Object* ) >
<!ATTLIST Manifest
Id
ID #IMPLIED >
<!ELEMENT Package ( ObjectReference+, Object* ) >
<!ATTLIST Package
Id
ID #IMPLIED >
Properties ElementAdditional information concerning the signature overall or as it applied to particular ObjectReferences
can be placed in a Properties element inside an Object. This
should be such information as signing time or the serial number of crypto hardware used.
Additional information concerning data being signed should be with that data.
<!ELEMENT Property ANY >
<!ATTLIST Property
Target
IDREF #REQUIRED >
TDB - will specify the use, if any, of XML processing instructions by this specification and the handling of PIs appearing within elements specified in this document.
TDB - will specify the use, if any, and handling of XML comments appearing within elements specified in this document.
This section identifies algorithms used with the XML digital signature standard. Entries contain the identifier to be used in signature documents, a reference to the formal specification, and definitions, where applicable, for the representation of keys and the results of cryptographic operations.
The specification defines a set of algorithms, their URIs, and requirements for implementation. Requirements are specified over implementation, not over requirements for signature use. Furthermore, the mechanism is extensible, alternative algorithms may be used by signature applications.
| Algorithm Type | Algorithm | Requirements | Algorithm URI | URN Derivation |
| Digest | ||||
| SHA1 | REQUIRED | urn:nist-gov:sha1 | IOTP | |
| Encoding | ||||
| Base64 | REQUIRED | urn:ietf-org:base64 | suggested | |
| MAC | ||||
| HMAC-SHA1 | REQUIRED | urn:ietf-org:hmac-sha1 | extrapolated from IOTP | |
| Signature | ||||
| DSAwithSHA1 (DSS) | REQUIRED | urn:nist-gov:dsa | IOTP | |
| RSAwithSHA1 | RECOMMENDED | urn:rsasdi-com:rsa-sha1 | extrapolated from IOTP | |
| ECDSA | OPTIONAL | urn:nist-gov:ecdsa | extrapolated from IOTP | |
| Canonicalization | ||||
| minimal | REQUIRED | http://www.w3.org/1999/10/signature-core/minimal | suggested W3C | |
| XML-Canonicalization | RECOMMENDED | http://www.w3.org/1999/07/WD-xml-c14n-19990729 | W3C | |
| Transform | ||||
| XSLT | RECOMMENDED | http://www.w3.org/TR/1999/PR-xslt-19991008 | W3C | |
| XPath | RECOMMENDED | http://www.w3.org/TR/1999/PR-xpath-19991008 | W3C | |
| XPointer | RECOMMENDED | http://www.w3.org/1999/07/WD-xptr-19990709 | W3C |
The SHA-1 algorithm identifier is urn:nist-gov:sha1.
The SHA-1 algorithm takes no parameters. An example of an SHA-1 DigestAlg element is
<DigestMethod Algorithm="urn:nist-gov:sha1"/>
An SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. Example: the DigestValue element for the message digest
A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D
from Appendix A of the SHA-1 standard would be
<DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>
The HMAC algorithm identifiers are
urn:ietf-org:hmac-sha1 and urn:ietf-org:hmac-md5. The HMAC
algorithm takes the truncation length in bits as a parameter (parameter identifier
urn:ietf-org:hmac-outputlength). An example of an HMAC SignatureMethod
element:
<SignatureMethod
Algorithm="urn:ietf-org:hmac-sha1">
<Parameter type="urn:ietf-org:hmac-outputlength">
128
</Parameter>
</SignatureMethod>
The output of the HMAC algorithm is ultimately the output (possibly truncated) of the chosen digest algorithm. This value shall be base64 encoded in the same straightforward fashion as the output of the digest algorithms. Example: the SignatureValue element for the HMAC-MD5 digest
9294727A 3638BB1C 13F48EF8 158BFC9D
from the test vectors in [RFC 2104] would be
<SignatureValue>kpRyejY4uxwT9I74FYv8nQ==</SignatureValue>
The DSA algorithm identifier is urn:nist-gov:dsa.
The DSA algorithm takes no parameters. An example of a DSA SignatureMethod
element is
<SignatureMethod Algorithm="urn:nist-gov:dsa"/>
The output of the DSA algorithm consists of a pair of integers usually referred by the pair (r, s). The signature value shall consist of the base64 encoding of the concatenation of two octet-streams that respectively result from the octet-encoding of the values r and s. Integer to octet-stream conversion shall be done according to the I2OSP operation defined in the PKCS #1 specification with a k parameter equal to 20. Example: the SignatureValue element for a DSA signature (r, s) with values specified in hexadecimal
r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8
from the example in Appendix 5 of the DSS standard would be
<SignatureValue>i6watmQQQ1y3GB+VsWq5fJKzQcBB4jRfH1bfJFj0JtFVtLotttzYyA==</SignatureValue>
DSA key values have the following set of fields: P, Q, G and Y are mandatory when appearing as a key value, J, seed and pgenCounter are optional but SHOULD be present. (The seed and pgenCounter fields MUST both either appear or be absent). All parameters are encoded as base64 values.
<!ELEMENT DssKeyValue (P, Q, G, Y, J?, (seed, pgenCounter)?)
>
<!-- Each of these fields consists a CDATA where the data is base64 encoded -->
The expression "RSA algorithm" as used in this document refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437.
The RSA algorithm identifiers are urn:rsasdi-com:rsa-sha1
and urn:rsasdi-com:rsa-md5. The RSA algorithm takes no parameters. An example
of an RSA SignatureMethod element is
<SignatureMethod name="urn:rsasdi-com:rsa-sha1"/>
The output of the RSA algorithm is an octet string. The SignatureValue content for an RSA signature shall be the base64 encoding of this octet string. Example: TBD
RSA key values have two fields: Modulus and Exponent.
<!ELEMENT RSAKeyValue ( Modulus, Exponent ) > <!-- Each
field contains a CDATA which is the value for that item base64 encoded -->
The expression ECDSA as used in this document refers to the signature algorithms specified in ANSI X9.62. Additional details are to be provided.
Null canonicalization, i.e., no modification whatsoever, can be achieved for signed data by simply not putting any canonicalization in the Transforms element (omitting it entirely if no other tranforms are needed).
The algorithm identifier for the minimal canonicalization is http://www.w3.org/1999/10/signature-core/minimal.
An example of a minimal canonicalization CanonicalizationAlg element is
<CanonicalizationMethod name="http://www.w3.org/1999/10/signature-core/minimal"/>
The minimal canonicalization algorithm:
This algorithm is only applicable to XML resources.
The algorithm identifier for XML canonicalization is http://www.w3.org/1999/07/WD-xml-c14n-19990729.
An example of an XML canonicalization CanonicalizationAlg element is
<CanonicalizationMethod name="http://www.w3.org/1999/07/WD-xml-c14n-19990729"/>
See the Canonical XML specification.
Application developers are strongly encouraged to support all transforms listed in this section as RECOMMENDED unless the application environment has severe resource constraints that would make such support impractical. The working group goal is to maximize application interoperability on XML signatures, and the working group expects ubiquitous availability of software to support these transforms that can be incorporated into applications without extensive development.
The Algorithm value for canonicalization is defined above.
The Transform element content MUST include a Canonicalization
element, which specifies the canonicalization algorithm that will be applied to the input
of the Transform element.
The Algorithm value for the base 64 decoding transform is urn:ietf-org:base64.
The base-64 decoding algorithm identifier is urn:ietf-org:base64.
The base-64 Transform element has no content. The input (from the Location
or from the previous Transform) is base-64 decoded by this algorithm. This
transform is useful if an application needs to sign the raw data associated with base-64
encoded content of an element.
The Algorithm value for the XPath filtering transform is "http://www.w3.org/TR/1999/PR-xpath-19991008"
The Transform element content MUST conform to the XML Path Language (XPath) syntax.
XPath assumes that an XML processor has processed the input resource. So, for example, entity reference expansion, normalization of linefeeds and attribute values are normalized, and CDATA section replacement are expected. As well, XPath joins all consecutive characters into a single text node.
The input resource MUST be a well-formed XML document. The result of applying the XPath to the input resource MUST be a node-set (as defined in XPath). The output of this transform is a new XML document with the following characteristics:
& and <, respectively.It is RECOMMENDED that the XPath be constructed such that the result of this operation is a well-formed XML document. This should be the case if root element of the input resource is included by the XPath (even if a number of its descendant elements and attributes are omitted by the XPath).
The Algorithm value for the XPointer filtering transform is "http://www.w3.org/1999/07/WD-xptr-19990709".
The Transform element content MUST conform to the XML Pointer Language (XPointer) syntax.
The processing rules for XPointer filtering are identical to those for XPath filtering (stated above), except that the additional functionality offered by XPointer can be utilized in constructing the output node-set.
The XPointer filter is particularly important if the input resource is processed by a validating XML processor since the XPointer barename shortcut could then be used to implement the well-known fragment identification by ID attribute.
NOTE: In application environments with severe resource limitations, applications MAY constrain XPointer support to barename processing and also to determination of the ID attribute by means other than a validating XML processor. In fact, the use of an XML processor for barename resolution is OPTIONAL. However, the output expectations of this transform MUST be supported by the application.
The Algorithm value for the XSLT transform is "http://www.w3.org/TR/1999/PR-xslt-19991008"
The Transform element content MUST conform to the XSL Transforms (XSLT) language syntax.
The processing rules for the XSLT transform are stated in the XSLT specification.
The Algorithm value for the Java transform is urn:ECMA-org:java.
Details to be determined.
Although the Algorithm attribute of a Transform can take
application-specific values, having a Java transform seems to be the most reasonable way
to allow application-specific transforms that can be processed outside of the application
domain.
These sections describe the operations to be performed as part of signature generation and validation. The description is of a logical behavior and does not specify an order of execution, nor specify discrete steps.
Transforms determined by application to each object being signed.ObjectReference element(s) including location of object, digest,
digest algorithm, and transform elements, if required.SignedInfo element with SignatureMethod, CanonicalizationMethod,
and ObjectReference(s).SignedInfo based on algorithms in
step 4.SignedInfo, Object (s) (if
desired, encoding may be different than that used for signing), KeyInfo (if
required), and SignatureValue.Transforms to the specified resource based on
each ObjectReference(s) in the SignedInfo element. Each
transform is applied in order from left to right to the object with the output of each
transform being the input to the next.ObjectReference(s).SignedInfo for each reference (if any
mismatch, validation fails).SignedInfo element based on the CanonicalizationMethod in SignedInfo.KeyInfo or externally.SignatureValue based on the SignatureMethod in
the SignedInfo element, the key obtained in step 5, and the results of step
4. - Digest calculation is performed over the SignedInfo element including
start and end tags.Any processing beyond cryptographic validation (e.g. certificate validation, applicability decisions, time related processing) is outside the scope of this specification.
The XML digital signature standard provides a very flexible mechanism. In designing a system to make use of it, due consideration should be given to the threat model being defended against and to the factors covered in the subsections below.
The flexible Transforms mechanism, including canonicalization and explicit filtering and extraction, permit securing only a subset of data in an object. This is good for many applications where a limited portion of an object must change after the signature or different signatures secure different parts or the application modifies aspects of the object that are not significant and can be omitted from signature coverage or the like. Keep in mind that whenever this is done, those aspects that are not signed can be arbitrarily modified and the signature will still validate.
If signing is intended to convey the judgment or consent of an automated mechanism or person concerning some information, then it is normally necessary to secure as exactly as possible the information that was presented to that mechanism or person. Note that this can be accomplished by literally signing what was presented, for example the screen images shown a user. However, this may result in data which it is difficult for subsequent software to manipulate. It can be effective instead to secure the full data along with whatever filters, style sheets, or the like were used to control the part of the information that was presented.
This standard specifies public key signatures and secret key keyed hash authentication codes. These have substantially different security models. Furthermore, it permits user specified additions which may have other models.
With public key signatures, any number of parties can hold the public key and verify signatures while only the parties with the secret key can create signatures. The number of holders of the secret key should be minimized and preferably be one. Confidence by verifiers in the public key they are using and its binding to the entity or capabilities represented by the corresponding secret key is an important issue, usually addressed by certificate or on line authority systems.
Keyed hash authentication codes, based on secret keys, are typically much more efficient in terms of the computational effort required but have the characteristic that all verifiers need to have possession of the same key as the signer. Thus any verifier can forge signatures.
This standard permits user provided signature algorithms and keying information designators. Such user provided algorithms may have further different security models. For example, methods involving biometrics usually depend on a "key" which is a physical characteristic of the user and thus can not be changed the way public or secret keys can be and may have other security model differences.
The strength of a particular signature depends on all links in the security chain. This includes the signature and digest algorithms used, the strength of the key generation [RFC 1750] and the size of the key, the security of key and certificate authentication and distribution mechanisms, protection of all cryptographic processing from hostile observation and tampering, etc. The security of an overall system would also depend on the security and integrity of its operating procedures, its personnel, and on the administrative enforcement of those procedures. The factors listed in this paragraph, while critical to the overall security of a system, are mostly beyond the scope of this document.
<Signature xmlns="http://www.w3.org/1999/10/signature-core">
<SignedInfo Id="5">
<CanonicalizationMethod
Algorithm="http://www.w3.org/.../xml-c14n"/>
<SignatureMethod Algorithm="urn:nist-gov:dsa"/>
<ObjectReference Location="...">
<!-- pointer to external
signedobject -->
<Transforms>
<Transform
Algorithm="http://www.w3.org/1999/10/signature-core/null">
<Encoding
Algorithm="urn:ietf-org:base64"/>
</Transforms>
<DigestMethod
Algorithm="urn:nist-gov:sha1"/>
<DigestValue>a23bcd43"</DigestValue>
</ObjectReference>
<ObjectReference Locations="#timestamp"
Type="http://www.w3.org/1999/10/signature-core/signatureattributes"> <!--
points to Object below -->
<Transforms>
<CanonicalizationMethod
name="http://..."/>
</Transforms>
<DigestMethod
Algorithm="urn:nist-gov:sha1"/>
<DigestValue>a53uud43"</DigestValue>
</ObjectReference>
</SignedInfo>
<SignatureValue
encoding="urn:ietf-org:base64">dd2323dd</SignatureValue>
<Object ID="timestamp"
type="http://www.w3.org/1999/10/signature-core/signatureattributes " >
<timestamp about="#5"
xmlns="http://www.ietf.org/rfc/1234">
<date>19990908</date>
<time>14:34:34:34</time>
</timestamp>
</Object>
<KeyInfo>
<keyname>Solo</keyname>
</KeyInfo>
</Signature>
[TBD: Combined DTD]
SignedInfo and
for objects. Other defaults. Mandatory to implement cryptographic algorithms.KeyInfo types.We define the following types for use in identifying XML resources that include Signture semantics.
Other references can be found in section 5.7.