6.6 Transform Algorithms
A Transform algorithm has a single implicit parameters: 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 base 64 decoding transforms is
[MIME]. The base64 Transform element has no content. The input is
decoded by the algorithms. This transform is useful if an
application needs to sign the raw data associated with the encoded
content of an element.
This transform requires an octet stream for input. If an XPath
node-set (or sufficiently functional alternative) is given as input,
then it is converted to an octet stream by performing operations
logically equivalent to 1) applying an XPath transform with
expression self::text(), then 2) taking the string-value of the
node-set. Thus, if an XML element is identified by a barename
XPointer in the Reference URI, and its content consists solely of
base64 encoded character data, then this transform automatically
strips away the start and end tags of the identified element and any
of its descendant elements as well as any descendant comments and
processing instructions. The output of this transform is an octet
6.6.3 XPath Filtering
The normative specification for XPath expression evaluation is
[XPath]. The XPath expression to be evaluated appears as the
character content of a transform parameter child element named XPath.
The input required by this transform is an XPath node-set. Note that
if the actual input is an XPath node-set resulting from a null URI or
barename XPointer dereference, then comment nodes will have been
omitted. If the actual input is an octet stream, then the
application MUST convert the octet stream to an XPath node-set
suitable for use by Canonical XML with Comments (a subsequent
application of the REQUIRED Canonical XML algorithm would strip away
these comments). In other words, the input node-set should be
equivalent to the one that would be created by the following process:
1. Initialize an XPath evaluation context by setting the initial node
equal to the input XML document's root node, and set the context
position and size to 1.
2. Evaluate the XPath expression (//. | //@* | //namespace::*)
The evaluation of this expression includes all of the document's
nodes (including comments) in the node-set representing the octet
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
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
The primary purpose of this transform is to ensure that only
specifically defined changes to the input XML document are permitted
after the signature is affixed. This is done by omitting precisely
those nodes that are allowed to change once the signature is affixed,
and including all other input nodes in the output. It is the
responsibility of the XPath expression author to include all nodes
whose change could affect the interpretation of the transform output
in the application context.
An important scenario would be a document requiring two enveloped
signatures. Each signature must omit itself from its own digest
calculations, but it is also necessary to exclude the second
signature element from the digest calculations of the first signature
so that adding the second signature does not break the first
The XPath transform establishes the following evaluation context for
each node of the input node-set:
* A context node equal to a node of the input node-set.
* A context position, initialized to 1.
* A context size, initialized to 1.
* A library of functions equal to the function set defined in
XPath plus a function named here.
* A set of variable bindings. No means for initializing these is
defined. Thus, the set of variable bindings used when
evaluating the XPath expression is empty, and use of a variable
reference in the XPath expression results in an error.
* The set of namespace declarations in scope for the XPath
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:
Function: node-set here()
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
Note: The function definition for here() is intended to be consistent
with its definition in XPointer. However, some minor differences are
presently being discussed between the Working Groups.
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.
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
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
6.6.4 Enveloped Signature Transform
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:
The input and output requirements of this transform are identical to
those of the XPath transform. Note that it is not necessary to use
an XPath expression evaluator to create this transform. However,
this transform MUST produce output in exactly the same manner as the
XPath transform parameterized by the XPath expression above.
6.6.5 XSLT Transform
The normative specification for XSL Transformations is [XSLT]. The
XSL style sheet or transform to be evaluated appears as the character
content of a transform parameter child element named XSLT. The root
element of a XSLT style sheet SHOULD be <xsl:stylesheet>.
This transform requires an octet stream as input. If the actual
input is an XPath node-set, then the signature application should
attempt to covert it to octets (apply Canonical XML]) as described in
the Reference Processing Model (section 188.8.131.52).
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 transformauthors
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 transformafter the XSLT
transformto perform 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
7.0 XML Canonicalization and Syntax Constraint Considerations
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
The kinds of changes in XML that may need to be canonicalized can be
divided into three 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. And, 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.
Any canonicalization algorithm should yield output in a specific
fixed coded character set. For both the minimal canonicalization
defined in this specification and Canonical XML [XML-C14N] that coded
character set is UTF-8 (without a byte order mark (BOM)).Neither the
minimal canonicalization nor the Canonical XML [XML-C14N] algorithms
provide character normalization. We RECOMMEND that signature
applications create XML content (Signature elements and their
descendents/content) in Normalization Form C [NFC] 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.
7.1 XML 1.0, Syntax Constraints, and Canonicalization
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,
1. line endings are normalized to the single character #xA by
dropping #xD characters if they are immediately followed by a #xA
and replacing them with #xA in all other cases,
2. missing attributes declared to have default values are provided to
the application as if present with the default value,
3. character references are replaced with the corresponding
4. entity references are replaced with the corresponding declared
5. attribute values are normalized by
A. replacing character and entity references as above,
B. replacing occurrences of #x9, #xA, and #xD with #x20 (space)
except that the sequence #xD#xA is replaced by a single space,
C. if the attribute is not declared to be CDATA, stripping all
leading and trailing spaces and replacing all interior runs of
spaces with a single space.
Note that items (2), (4), and (5C) depend on the presence of a
schema, DTD or similar declarations. The Signature element type is
laxly schema valid [XML-schema], consequently external XML or even
XML within the same document as the signature may be (only) well
formed or from another namespace (where permitted by the signature
schema); the noted items may not be present. Thus, a signature with
such content will only be verifiable by other signature applications
if the following syntax constraints are observed when generating any
signed material including the SignedInfo element:
1. attributes having default values be explicitly present,
2. all entity references (except "amp", "lt", "gt", "apos", "quot",
and other character entities not representable in the encoding
chosen) be expanded,
3. attribute value white space be normalized
7.2 DOM/SAX Processing and Canonicalization
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
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 XML1.0 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 octect stream that was signed.
8.0 Security Considerations
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
Note, core validation behavior does not confirm that the signed data
was obtained by applying each step of the indicated transforms.
(Though it does check that the digest of the resulting content
matches that specified in the signature.) For example, some
application may be satisfied with verifying an XML signature over a
cached copy of already transformed data. Other applications might
require that content be freshly dereferenced and transformed.
8.1.1 Only What is Signed is Secure
First, obviously, signatures over a transformed document do not
secure any information discarded by transforms: only what is signed
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.
8.1.2 Only What is "Seen" Should be 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.
8.1.3 "See" What is Signed
Just as a user should only sign what it "sees," persons and automated
mechanisms 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 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
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. Consequently, while we RECOMMEND all documents
operated upon and generated by signature applications be in [NFC]
(otherwise intermediate processors might unintentionally break the
signature) encoding normalizations SHOULD NOT be done as part of a
signature transform, or (to state it another way) if normalization
does occur, the application SHOULD always "see" (operate over) the
8.2 Check the Security Model
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
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.
8.3 Algorithms, Key Lengths, Certificates, Etc.
The strength of a particular signature depends on all links in the
security chain. This includes the signature and digest algorithms
used, the strength of the key generation [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
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.
9.0 Schema, DTD, Data Model, and Valid Examples
XML Signature Schema Instance
core-schema.xsd Valid XML schema instance based on the
20000922 Schema/DTD [XML-Schema].
XML Signature DTD
RDF Data Model
XML Signature Object Example
example.xml A cryptographical invalid XML example that
includes foreign content and validates under the schema. (It
validates under the DTD when the foreign content is removed or
the DTD is modified accordingly).
RSA XML Signature Example
An XML Signature example with generated cryptographic values by
Merlin Hughes and validated by Gregor Karlinger.
DSA XML Signature Example
example-dsa.xml Similar to above but uses DSA.
A value generated from the application of a shared key to a
message via a cryptographic algorithm such that it has the
properties of message authentication (integrity) but not signer
"A signature should identify what is signed, making it
impracticable to falsify or alter either the signed matter or
the signature without detection." [Digital Signature
"A signature should indicate who signed a document, message or
record, and should be difficult for another person to produce
without authorization." [Digital Signature Guidelines, ABA]
The syntax and processing defined by this specification,
including core validation. We use this term to distinguish
other markup, processing, and applications semantics from our
Data Object (Content/Document)
The actual binary/octet data being operated on (transformed,
digested, or signed) by an application -- frequently an HTTP
entity [HTTP]. Note that the proper noun 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].
The inability to change a message without also changing the
signature value. See message authentication.
An XML Signature element wherein arbitrary (non-core) data may
be placed. An Object element is merely one type of digital
data (or document) that can be signed via a Reference.
"A resource can be anything that has identity. Familiar
examples include an electronic document, an image, a service
(e.g., 'today's weather report for Los Angeles'), and a
collection of other resources.... The resource is the
conceptual mapping to an entity or set of entities, not
necessarily the entity which corresponds to that mapping at any
particular instance in time. Thus, a resource can remain
constant even when its content---the entities to which it
currently corresponds---changes over time, provided that the
conceptual mapping is not changed in the process." [URI] In
order to avoid a collision of the term entity within the URI
and XML specifications, we use the term data object, content or
document to refer to the actual bits being operated upon.
Formally speaking, a value generated from the application of a
private key to a message via a cryptographic algorithm such
that it has the properties of signer authentication and message
authentication (integrity). (However, we sometimes use the
term signature generically such that it encompasses
Authentication Code values as well, but we are careful to make
the distinction when the property of signer authentication is
relevant to the exposition.) A signature may be (non-
exclusively) described as detached, enveloping, or enveloped.
An application that implements the MANDATORY (REQUIRED/MUST)
portions of this specification; these conformance requirements
are over the structure of the Signature element type and its
children (including SignatureValue) and mandatory to support
The signature is over content external to the 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
The signature is over content found within an Object element of
the signature itself. The Object(or its content) is identified
via a Reference (via a URI fragment identifier or transform).
The signature is over the XML content that contains the
signature as an element. The content provides the root XML
document element. Obviously, enveloped signatures must take
care not to include their own value in the calculation of the
The processing of a octet stream from source content to derived
content. Typical transforms include XML Canonicalization,
XPath, and XSLT.
The core processing requirements of this specification
requiring signature validation and SignedInfo reference
The hash value of the identified and transformed content,
specified by Reference, matches its specified DigestValue.
The SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified
in Core Validation (section 3.2).
The application determines that the semantics associated with a
signature are valid. For example, an application may validate
the time stamps or the integrity of the signer key -- though
this behavior is external to this core specification.
ABA Digital Signature Guidelines.
Bourret Declaring Elements and Attributes in an XML DTD.
Ron Bourret. http://www.informatik.tu-
DOM Document Object Model (DOM) Level 1 Specification.
W3C Recommendation. V. Apparao, S. Byrne, M.
Champion, S. Isaacs, I. Jacobs, A. Le Hors, G.
Nicol, J. Robie, R. Sutor, C. Wilson, L. Wood.
October 1998. http://www.w3.org/TR/1998/REC-DOM-
DSS FIPS PUB 186-1. Digital Signature Standard (DSS).
U.S. Department of Commerce/National Institute of
Standards and Technology.
HMAC Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC
2104, February 1997.
HTTP Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P. and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC
2616, June 1999.
KEYWORDS Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
LDAP-DN Wahl, M., Kille, S. and T. Howes, "Lightweight
Directory Access Protocol (v3): UTF-8 String
Representation of Distinguished Names", RFC 2253,
December 1997. http://www.ietf.org/rfc/rfc2253.txt
MD5 Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
MIME Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
NFC TR15. Unicode Normalization Forms. M. Davis, M.
Drst. Revision 18: November 1999.
PGP Callas, J., Donnerhacke, L., Finney, H. and R.
Thayer, "OpenPGP Message Format", November 1998.
RANDOM Eastlake, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
RDF RDF Schema W3C Candidate Recommendation. D.
Brickley, R.V. Guha. March 2000.
RDF Model and Syntax W3C Recommendation. O.
Lassila, R. Swick. February 1999.
1363 IEEE 1363: Standard Specifications for Public Key
Cryptography. August 2000.
PKCS1 Kaliski, B. and J. Staddon, "PKCS #1: RSA
Cryptography Specifications Version 2.0", RFC 2437,
October 1998. http://www.ietf.org/rfc/rfc2437.txt
SAX SAX: The Simple API for XML David Megginson et. al.
May 1998. http://www.megginson.com/SAX/index.html
SHA-1 FIPS PUB 180-1. Secure Hash Standard. U.S.
Department of Commerce/National Institute of
Standards and Technology.
Unicode The Unicode Consortium. The Unicode Standard.
UTF-16 Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
UTF-8 Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
URI Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
URI-Literal Hinden, R., Carpenter, B. and L. Masinter, "Format
for Literal IPv6 Addresses in URL's", RFC 2732,
December 1999. http://www.ietf.org/rfc/rfc2732.txt
URL Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)", RFC 1738,
December 1994. http://www.ietf.org/rfc/rfc1738.txt
URN Moats, R., "URN Syntax" RFC 2141, May 1997.
Daigle, L., van Gulik, D., Iannella, R. and P.
Faltstrom, "URN Namespace Definition Mechanisms",
RFC 2611, June 1999.
X509v3 ITU-T Recommendation X.509 version 3 (1997).
"Information Technology - Open Systems
Interconnection - The Directory Authentication
Framework" ISO/IEC 9594-8:1997.
XHTML 1.0 XHTML(tm) 1.0: The Extensible Hypertext Markup
Language Recommendation. S. Pemberton, D. Raggett,
et. al. January 2000.
XLink XML Linking Language. Working Draft. S. DeRose, D.
Orchard, B. Trafford. July 1999.
XML Extensible Markup Language (XML) 1.0
Recommendation. T. Bray, J. Paoli, C. M. Sperberg-
McQueen. February 1998.
XML-C14N J. Boyer, "Canonical XML Version 1.0", RFC 3076,
September 2000. http://www.w3.org/TR/2000/CR-xml-
XML-Japanese XML Japanese Profile. W3C NOTE. M. MURATA April
XML-MT Whitehead, E. and M. Murata, "XML Media Types",
July 1998. http://www.ietf.org/rfc/rfc2376.txt
XML-ns Namespaces in XML Recommendation. T. Bray, D.
Hollander, A. Layman. Janury 1999.
XML-schema XML Schema Part 1: Structures Working Draft. D.
Beech, M. Maloney, N. Mendelshohn. September 2000.
XML Schema Part 2: Datatypes Working Draft. P.
Biron, A. Malhotra. September 2000.
XML-Signature-RD Reagle, J., "XML Signature Requirements", RFC 2907,
April 2000. http://www.w3.org/TR/1999/WD-xmldsig-
XPath XML Path Language (XPath)Version 1.0.
Recommendation. J. Clark, S. DeRose. October 1999.
XPointer XML Pointer Language (XPointer). Candidate
Recommendation. S. DeRose, R. Daniel, E. Maler.
XSL Extensible Stylesheet Language (XSL) Working Draft.
S. Adler, A. Berglund, J. Caruso, S. Deach, P.
Grosso, E. Gutentag, A. Milowski, S. Parnell, J.
Richman, S. Zilles. March 2000.
XSLT XSL Transforms (XSLT) Version 1.0. Recommendation.
J. Clark. November 1999.
12. Authors' Addresses
Donald E. Eastlake 3rd
Motorola, Mail Stop: M2-450
20 Forbes Boulevard
Mansfield, MA 02048 USA
Joseph M. Reagle Jr., W3C
Massachusetts Institute of Technology
Laboratory for Computer Science
NE43-350, 545 Technology Square
Cambridge, MA 02139
909 Third Ave, 16th Floor
NY, NY 10043 USA
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