RFC 3075
XML-Signature Syntax and Processing
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.6.6.1 Canonicalization
Any canonicalization algorithm that can be used for CanonicalizationMethod (such as those in Canonicalization Algorithms (section 6.5)) can be used as a Transform.6.6.2 Base64
Identifiers: http://www.w3.org/2000/09/xmldsig#base64 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 stream.6.6.3 XPath Filtering
Identifier: http://www.w3.org/TR/1999/REC-xpath-19991116 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 stream. The transform output is also an XPath node-set. The XPath expression appearing in the XPath parameter is evaluated once for each node in the input node-set. The result is converted to a boolean. If the boolean is true, then the node is included in the output node-set. If the boolean is false, then the node is omitted from the output node-set. Note: Even if the input node-set has had comments removed, the comment nodes still exist in the underlying parse tree and can separate text nodes. For example, the markup <e>Hello, <!-- comment --> world!</e> contains two text nodes. Therefore, the expression self::text()[string()="Hello, world!"] would fail. Should this problem arise in the application, it can be solved by either canonicalizing the document before the XPath transform to physically
remove the comments or by matching the node based on the parent
element's string value (e.g., by using the expression
self::text()[string(parent::e)="Hello, world!"]).
The primary purpose of this transform is to ensure that only
specifically defined changes to the input XML document are permitted
after the signature is affixed. This is done by omitting precisely
those nodes that are allowed to change once the signature is affixed,
and including all other input nodes in the output. It is the
responsibility of the XPath expression author to include all nodes
whose change could affect the interpretation of the transform output
in the application context.
An important scenario would be a document requiring two enveloped
signatures. Each signature must omit itself from its own digest
calculations, but it is also necessary to exclude the second
signature element from the digest calculations of the first signature
so that adding the second signature does not break the first
signature.
The XPath transform establishes the following evaluation context for
each node of the input node-set:
* 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
expression.
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
evaluated.
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.
For example,
<Document>
<Signature xmlns="&dsig;">
<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).
6.6.4 Enveloped Signature Transform
Identifier:
http://www.w3.org/2000/09/xmldsig#enveloped-signature
An enveloped signature transform T removes the whole Signature
element containing T from the digest calculation of the Reference
element containing T. The entire string of characters used by an XML
processor to match the Signature with the XML production element is
removed. The output of the transform is equivalent to the output
that would result from replacing T with an XPath transform containing
the following XPath parameter element:
<XPath xmlns:dsig="&dsig;">
count(ancestor-or-self::dsig:Signature |
here()/ancestor::dsig:Signature[1]) >
count(ancestor-or-self::dsig:Signature)</XPath>
The input and output requirements of this transform are identical to
those of the XPath transform. Note that it is not necessary to use
an XPath expression evaluator to create this transform. However,
this transform MUST produce output in exactly the same manner as the
XPath transform parameterized by the XPath expression above.
6.6.5 XSLT Transform
Identifier:
http://www.w3.org/TR/1999/REC-xslt-19991116
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 4.3.3.2). The output of this transform is an octet stream. The processing rules for the XSL style sheet or transform element are stated in the XSLT specification [XSLT]. We RECOMMEND that XSLT 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 equivalent [XHTML].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 XML. 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 character,
4. entity references are replaced with the corresponding declared
entity,
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,
and
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
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 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.8.1 Transforms
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 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 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.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 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. 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 normalized form.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 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.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 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.9.0 Schema, DTD, Data Model, and Valid Examples
XML Signature Schema Instance http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig- core-schema.xsd Valid XML schema instance based on the 20000922 Schema/DTD [XML-Schema]. XML Signature DTD http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig- core-schema.dtd RDF Data Model http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/xmldsig- datamodel-20000112.gif XML Signature Object Example http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature- 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 http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature- example-rsa.xml An XML Signature example with generated cryptographic values by Merlin Hughes and validated by Gregor Karlinger. DSA XML Signature Example http://www.w3.org/TR/2000/CR-xmldsig-core-20001031/signature- example-dsa.xml Similar to above but uses DSA.
10.0 Definitions
Authentication Code 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 authentication Authentication, Message "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 Guidelines, ABA] Authentication, Signer "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] Core 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 own. 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]. Integrity The inability to change a message without also changing the signature value. See message authentication. Object 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. Resource "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.
Signature
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.
Signature, Application
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
algorithms.
Signature, Detached
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
sibling elements.
Signature, Enveloping
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).
Signature, Enveloped
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
SignatureValue.
Transform
The processing of a octet stream from source content to derived
content. Typical transforms include XML Canonicalization,
XPath, and XSLT.
Validation, Core
The core processing requirements of this specification
requiring signature validation and SignedInfo reference
validation.
Validation, Reference
The hash value of the identified and transformed content,
specified by Reference, matches its specified DigestValue.
Validation, Signature
The SignatureValue matches the result of processing SignedInfo
with CanonicalizationMethod and SignatureMethod as specified
in Core Validation (section 3.2).
Validation, Trust/Application
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.
11.0 References
ABA Digital Signature Guidelines.
http://www.abanet.org/scitech/ec/isc/dsgfree.html
Bourret Declaring Elements and Attributes in an XML DTD.
Ron Bourret. http://www.informatik.tu-
darmstadt.de/DVS1/staff/bourret/xml/xmldtd.html
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-
Level-1-19981001/
DSS FIPS PUB 186-1. Digital Signature Standard (DSS).
U.S. Department of Commerce/National Institute of
Standards and Technology.
http://csrc.nist.gov/fips/fips1861.pdf
HMAC Krawczyk, H., Bellare, M. and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC
2104, February 1997.
http://www.ietf.org/rfc/rfc2104.txt
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.
http://www.ietf.org/rfc/rfc2616.txt
KEYWORDS Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
http://www.ietf.org/rfc/rfc2119.txt
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.
http://www.ietf.org/rfc/rfc1321.txt
MIME Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
http://www.ietf.org/rfc/rfc2045.txt
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.
http://www.ietf.org/rfc/rfc2440.txt
RANDOM Eastlake, D., Crocker, S. and J. Schiller,
"Randomness Recommendations for Security", RFC
1750, December 1994.
http://www.ietf.org/rfc/rfc1750.txt
RDF RDF Schema W3C Candidate Recommendation. D.
Brickley, R.V. Guha. March 2000.
http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
RDF Model and Syntax W3C Recommendation. O.
Lassila, R. Swick. February 1999.
http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/
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.
http://csrc.nist.gov/fips/fip180-1.pdf
Unicode The Unicode Consortium. The Unicode Standard.
http://www.unicode.org/unicode/standard/standard.html
UTF-16 Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
http://www.ietf.org/rfc/rfc2781.txt
UTF-8 Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.
http://www.ietf.org/rfc/rfc2279.txt
URI Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic
Syntax", RFC 2396, August 1998.
http://www.ietf.org/rfc/rfc2396.txt
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. http://www.ietf.org/rfc/rfc2141.txt Daigle, L., van Gulik, D., Iannella, R. and P. Faltstrom, "URN Namespace Definition Mechanisms", RFC 2611, June 1999. http://www.ietf.org/rfc/rfc2611.txt 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. http://www.w3.org/TR/2000/REC-xhtml1-20000126/ XLink XML Linking Language. Working Draft. S. DeRose, D. Orchard, B. Trafford. July 1999. http://www.w3.org/1999/07/WD-xlink-19990726 XML Extensible Markup Language (XML) 1.0 Recommendation. T. Bray, J. Paoli, C. M. Sperberg- McQueen. February 1998. http://www.w3.org/TR/1998/REC-xml-19980210 XML-C14N J. Boyer, "Canonical XML Version 1.0", RFC 3076, September 2000. http://www.w3.org/TR/2000/CR-xml- c14n-20001026 http://www.ietf.org/rfc/rfc3076.txt XML-Japanese XML Japanese Profile. W3C NOTE. M. MURATA April 2000 http://www.w3.org/TR/2000/NOTE-japanese-xml- 20000414/ 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. http://www.w3.org/TR/1999/REC-xml-names-19990114 XML-schema XML Schema Part 1: Structures Working Draft. D. Beech, M. Maloney, N. Mendelshohn. September 2000. http://www.w3.org/TR/2000/WD-xmlschema-1-20000922/
XML Schema Part 2: Datatypes Working Draft. P.
Biron, A. Malhotra. September 2000.
http://www.w3.org/TR/2000/WD-xmlschema-2-20000922/
XML-Signature-RD Reagle, J., "XML Signature Requirements", RFC 2907,
April 2000. http://www.w3.org/TR/1999/WD-xmldsig-
requirements-19991014
http://www.ietf.org/rfc/rfc2807.txt
XPath XML Path Language (XPath)Version 1.0.
Recommendation. J. Clark, S. DeRose. October 1999.
http://www.w3.org/TR/1999/REC-xpath-19991116
XPointer XML Pointer Language (XPointer). Candidate
Recommendation. S. DeRose, R. Daniel, E. Maler.
http://www.w3.org/TR/2000/CR-xptr-20000607
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.
http://www.w3.org/TR/2000/WD-xsl-
20000327/xslspec.html
XSLT XSL Transforms (XSLT) Version 1.0. Recommendation.
J. Clark. November 1999.
http://www.w3.org/TR/1999/REC-xslt-19991116.html
12. Authors' Addresses
Donald E. Eastlake 3rd Motorola, Mail Stop: M2-450 20 Forbes Boulevard Mansfield, MA 02048 USA Phone: 1-508-261-5434 EMail: Donald.Eastlake@motorola.com Joseph M. Reagle Jr., W3C Massachusetts Institute of Technology Laboratory for Computer Science NE43-350, 545 Technology Square Cambridge, MA 02139 Phone: 1.617.258.7621 EMail: reagle@w3.org David Solo Citigroup 909 Third Ave, 16th Floor NY, NY 10043 USA Phone: +1-212-559-2900 EMail: dsolo@alum.mit.edu
13. Full Copyright Statement
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