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RFC 3275

(Extensible Markup Language) XML-Signature Syntax and Processing

Pages: 73
Draft Standard
Obsoletes:  3075
Part 3 of 3 – Pages 44 to 73
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6.0 Algorithms

This section identifies algorithms used with the XML digital signature specification. Entries contain the identifier to be used in Signature elements, a reference to the formal specification, and definitions, where applicable, for the representation of keys and the results of cryptographic operations.

6.1 Algorithm Identifiers and Implementation Requirements

Algorithms are identified by URIs that appear as an attribute to the element that identifies the algorithms' role (DigestMethod, Transform, SignatureMethod, or CanonicalizationMethod). All
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   algorithms used herein take parameters but in many cases the
   parameters are implicit.  For example, a SignatureMethod is
   implicitly given two parameters: the keying info and the output of
   CanonicalizationMethod.  Explicit additional parameters to an
   algorithm appear as content elements within the algorithm role
   element.  Such parameter elements have a descriptive element name,
   which is frequently algorithm specific, and MUST be in the XML
   Signature namespace or an algorithm specific namespace.

   This specification defines a set of algorithms, their URIs, and
   requirements for implementation.  Requirements are specified over
   implementation, not over requirements for signature use.
   Furthermore, the mechanism is extensible; alternative algorithms may
   be used by signature applications.

      1. Required SHA1
      1. Required base64
      1. Required HMAC-SHA1
      1. Required DSAwithSHA1 (DSS)
      2. Recommended RSAwithSHA1
      1. Required Canonical XML (omits comments)
      2. Recommended Canonical XML with Comments
      1. Optional XSLT
      2. Recommended XPath
      3. Required Enveloped Signature*

   *  The Enveloped Signature transform removes the Signature element
   from the calculation of the signature when the signature is within
   the content that it is being signed.  This MAY be implemented via the
   RECOMMENDED XPath specification specified in 6.6.4: Enveloped
   Signature Transform; it MUST have the same effect as that specified
   by the XPath Transform.
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6.2 Message Digests

Only one digest algorithm is defined herein. However, it is expected that one or more additional strong digest algorithms will be developed in connection with the US Advanced Encryption Standard effort. Use of MD5 [MD5] is NOT RECOMMENDED because recent advances in cryptanalysis have cast doubt on its strength.

6.2.1 SHA-1

Identifier: The SHA-1 algorithm [SHA-1] takes no explicit parameters. An example of an SHA-1 DigestAlg element is: <DigestMethod Algorithm=""/> A SHA-1 digest is a 160-bit string. The content of the DigestValue element shall be the base64 encoding of this bit string viewed as a 20-octet octet stream. For example, the DigestValue element for the message digest: A9993E36 4706816A BA3E2571 7850C26C 9CD0D89D from Appendix A of the SHA-1 standard would be: <DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>

6.3 Message Authentication Codes

MAC algorithms take two implicit parameters, their keying material determined from KeyInfo and the octet stream output by CanonicalizationMethod. MACs and signature algorithms are syntactically identical but a MAC implies a shared secret key.

6.3.1 HMAC

Identifier: The HMAC algorithm (RFC2104 [HMAC]) takes the truncation length in bits as a parameter; if the parameter is not specified then all the bits of the hash are output. An example of an HMAC SignatureMethod element:
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   The output of the HMAC algorithm is ultimately the output (possibly
   truncated) of the chosen digest algorithm.  This value shall be
   base64 encoded in the same straightforward fashion as the output of
   the digest algorithms.  Example: the SignatureValue element for the
   HMAC-SHA1 digest

      9294727A 3638BB1C 13F48EF8 158BFC9D

   from the test vectors in [HMAC] would be


      Schema Definition:

      <simpleType name="HMACOutputLengthType">
        <restriction base="integer"/>


      <!ELEMENT HMACOutputLength (#PCDATA)>

6.4 Signature Algorithms

Signature algorithms take two implicit parameters, their keying material determined from KeyInfo and the octet stream output by CanonicalizationMethod. Signature and MAC algorithms are syntactically identical but a signature implies public key cryptography.

6.4.1 DSA

Identifier: The DSA algorithm [DSS] takes no explicit parameters. An example of a DSA SignatureMethod element is: <SignatureMethod Algorithm=""/>
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   The output of the DSA algorithm consists of a pair of integers
   usually referred by the pair (r, s).  The signature value consists of
   the base64 encoding of the concatenation of two octet-streams that
   respectively result from the octet-encoding of the values r and s in
   that order.  Integer to octet-stream conversion must be done
   according to the I2OSP operation defined in the RFC 2437 [PKCS1]
   specification with a l parameter equal to 20.  For example, the
   SignatureValue element for a DSA signature (r, s) with values
   specified in hexadecimal:

      r = 8BAC1AB6 6410435C B7181F95 B16AB97C 92B341C0
      s = 41E2345F 1F56DF24 58F426D1 55B4BA2D B6DCD8C8

   from the example in Appendix 5 of the DSS standard would be


6.4.2 PKCS1 (RSA-SHA1)

Identifier: The expression "RSA algorithm" as used in this document refers to the RSASSA-PKCS1-v1_5 algorithm described in RFC 2437 [PKCS1]. The RSA algorithm takes no explicit parameters. An example of an RSA SignatureMethod element is: <SignatureMethod Algorithm=""/> The SignatureValue content for an RSA signature is the base64 [MIME] encoding of the octet string computed as per RFC 2437 [PKCS1, section 8.1.1: Signature generation for the RSASSA-PKCS1-v1_5 signature scheme]. As specified in the EMSA-PKCS1-V1_5-ENCODE function RFC 2437 [PKCS1, section 9.2.1], the value input to the signature function MUST contain a pre-pended algorithm object identifier for the hash function, but the availability of an ASN.1 parser and recognition of OIDs are not required of a signature verifier. The PKCS#1 v1.5 representation appears as: CRYPT (PAD (ASN.1 (OID, DIGEST (data)))) Note that the padded ASN.1 will be of the following form: 01 | FF* | 00 | prefix | hash
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   where "|" is concatenation, "01", "FF", and "00" are fixed octets of
   the corresponding hexadecimal value, "hash" is the SHA1 digest of the
   data, and "prefix" is the ASN.1 BER SHA1 algorithm designator prefix
   required in PKCS1 [RFC 2437], that is,

      hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14

   This prefix is included to make it easier to use standard
   cryptographic libraries.  The FF octet MUST be repeated the maximum
   number of times such that the value of the quantity being CRYPTed is
   one octet shorter than the RSA modulus.

   The resulting base64 [MIME] string is the value of the child text
   node of the SignatureValue element, e.g.,


6.5 Canonicalization Algorithms

If canonicalization is performed over octets, the canonicalization algorithms take two implicit parameters: the content and its charset. The charset is derived according to the rules of the transport protocols and media types (e.g., RFC2376 [XML-MT] defines the media types for XML). This information is necessary to correctly sign and verify documents and often requires careful server side configuration. Various canonicalization algorithms require conversion to [UTF-8]. The two algorithms below understand at least [UTF-8] and [UTF-16] as input encodings. We RECOMMEND that externally specified algorithms do the same. Knowledge of other encodings is OPTIONAL. Various canonicalization algorithms transcode from a non-Unicode encoding to Unicode. The two algorithms below perform text normalization during transcoding [NFC, NFC-Corrigendum]. We RECOMMEND that externally specified canonicalization algorithms do the same. (Note, there can be ambiguities in converting existing charsets to Unicode, for an example see the XML Japanese Profile [XML-Japanese] Note.)

6.5.1 Canonical XML

Identifier for REQUIRED Canonical XML (omits comments):
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   Identifier for Canonical XML with Comments:

   An example of an XML canonicalization element is:

   The normative specification of Canonical XML is [XML-C14N].  The
   algorithm is capable of taking as input either an octet stream or an
   XPath node-set (or sufficiently functional alternative).  The
   algorithm produces an octet stream as output.  Canonical XML is
   easily parameterized (via an additional URI) to omit or retain

6.6 Transform Algorithms

A Transform algorithm has a single implicit parameter: an octet stream from the Reference or the output of an earlier Transform. Application developers are strongly encouraged to support all transforms listed in this section as RECOMMENDED unless the application environment has resource constraints that would make such support impractical. Compliance with this recommendation will maximize application interoperability and libraries should be available to enable support of these transforms in applications without extensive development.

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: The normative specification for base64 decoding transforms is [MIME]. The base64 Transform element has no content. The input is decoded by the algorithms. This transform is useful if an application needs to sign the raw data associated with the encoded content of an element. This transform requires an octet stream for input. If an XPath node-set (or sufficiently functional alternative) is given as input, then it is converted to an octet stream by performing operations logically equivalent to 1) applying an XPath transform with expression self::text(), then 2) taking the string-value of the
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   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

Identifier: 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
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   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

   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
   [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()
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   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

   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,

      <Signature xmlns="">
          <Reference URI="">
                <XPath xmlns:dsig="&dsig;">

   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;.
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   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]) >

   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

Identifier: An enveloped signature transform T removes the whole Signature element containing T from the digest calculation of the Reference element containing T. The entire string of characters used by an XML processor to match the Signature with the XML production element is removed. The output of the transform is equivalent to the output that would result from replacing T with an XPath transform containing the following XPath parameter element: <XPath xmlns:dsig="&dsig;"> count(ancestor-or-self::dsig:Signature | here()/ancestor::dsig:Signature[1]) > count(ancestor-or-self::dsig:Signature)</XPath> The input and output requirements of this transform are identical to those of the XPath transform, but may only be applied to a node-set from its parent XML document. Note that it is not necessary to use an XPath expression evaluator to create this transform. However, this transform MUST produce output in exactly the same manner as the XPath transform parameterized by the XPath expression above.

6.6.5 XSLT Transform

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   The normative specification for XSL Transformations is [XSLT].
   Specification of a namespace-qualified stylesheet element, which MUST
   be the sole child of the Transform element, indicates that the
   specified style sheet should be used.  Whether this instantiates in-
   line processing of local XSLT declaration within the resource is
   determined by the XSLT processing model; the ordered application of
   multiple stylesheet may require multiple Transforms.  No special
   provision is made for the identification of a remote stylesheet at a
   given URI because it can be communicated via an xsl:include or
   xsl:import within the stylesheet child of the Transform.

   This transform requires an octet stream as input.  If the actual
   input is an XPath node-set, then the signature application should
   attempt to convert it to octets (apply Canonical XML]) as described
   in the Reference Processing Model (section

   The output of this transform is an octet stream.  The processing
   rules for the XSL style sheet or transform element are stated in the
   XSLT specification [XSLT].  We RECOMMEND that XSLT transform authors
   use an output method of xml for XML and HTML.  As XSLT
   implementations do not produce consistent serializations of their
   output, we further RECOMMEND inserting a transform after the XSLT
   transform to canonicalize the output.  These steps will help to
   ensure interoperability of the resulting signatures among
   applications that support the XSLT transform.  Note that if the
   output is actually HTML, then the result of these steps is logically
   equivalent [XHTML].

7. 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.
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   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 four categories.  There are those related to the basic
   [XML], as described in 7.1 below.  There are those related to [DOM],
   [SAX], or similar processing as described in 7.2 below.  Third, there
   is the possibility of coded character set conversion, such as between
   UTF-8 and UTF-16, both of which all  [XML] compliant processors are
   required to support, which is described in the paragraph immediately
   below.  And, fourth, there are changes that related to namespace
   declaration and XML namespace attribute context as described in 7.3

   Any canonicalization algorithm should yield output in a specific
   fixed coded character set.  All canonicalization algorithms
   identified in this document use UTF-8 (without a byte order mark
   (BOM)) and do not provide character normalization.  We RECOMMEND that
   signature applications create XML content (Signature elements and
   their descendents/content) in Normalization Form C [NFC, NFC-
   Corrigendum] and check that any XML being consumed is in that form as
   well; (if not, signatures may consequently fail to validate).
   Additionally, none of these algorithms provide data type
   normalization.  Applications that normalize data types in varying
   formats (e.g., (true, false) or (1,0)) may not be able to validate
   each other's signatures.

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,
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   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
      5.1 replacing character and entity references as above,
      5.2 replacing occurrences of #x9, #xA, and #xD with #x20 (space)
          except that the sequence #xD#xA is replaced by a single space,
      5.3 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 (5.3) depend on the presence of a
   schema, DTD or similar declarations.  The Signature element type is
   laxly schema valid [XML-schema], consequently external XML or even
   XML within the same document as the signature may be (only) well-
   formed or from another namespace (where permitted by the signature
   schema); the noted items may not be present.  Thus, a signature with
   such content will only be verifiable by other signature applications
   if the following syntax constraints are observed when generating any
   signed material including the SignedInfo element:

   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.
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   If an XML Signature is to be produced or verified on a system using
   DOM or SAX processing, a canonical method is needed to serialize the
   relevant part of a DOM tree or sequence of SAX events.  XML
   canonicalization specifications, such as [XML-C14N], are based only
   on information which is preserved by DOM and SAX.  For an XML
   Signature to be verifiable by an implementation using DOM or SAX, not
   only must the XML 1.0 syntax constraints given in the previous
   section be followed, but an appropriate XML canonicalization MUST be
   specified so that the verifier can re-serialize DOM/SAX mediated
   input into the same octet stream that was signed.

7.3 Namespace Context and Portable Signatures

In [XPath] and consequently the Canonical XML data model an element has namespace nodes that correspond to those declarations within the element and its ancestors: "Note: An element E has namespace nodes that represent its namespace declarations as well as any namespace declarations made by its ancestors that have not been overridden in E's declarations, the default namespace if it is non-empty, and the declaration of the prefix xml." [XML-C14N] When serializing a Signature element or signed XML data that's the child of other elements using these data models, that Signature element and its children, may contain namespace declarations from its ancestor context. In addition, the Canonical XML and Canonical XML with Comments algorithms import all xml namespace attributes (such as xml:lang) from the nearest ancestor in which they are declared to the apex node of canonicalized XML unless they are already declared at that node. This may frustrate the intent of the signer to create a signature in one context which remains valid in another. For example, given a signature which is a child of B and a grandchild of A: <A xmlns:n1="&foo;"> <B xmlns:n2="&bar;"> <Signature xmlns="&dsig;"> ... <Reference URI="#signme"/> ... </Signature> <C ID="signme" xmlns="&baz;"/> </B> </A> when either the element B or the signed element C is moved into a [SOAP] envelope for transport:
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          <B xmlns:n2="&bar;">
            <Signature xmlns="&dsig;">
            <C ID="signme" xmlns="&baz;"/>

   The canonical form of the signature in this context will contain new
   namespace declarations from the SOAP:Envelope context, invalidating
   the signature.  Also, the canonical form will lack namespace
   declarations it may have originally had from element A's context,
   also invalidating the signature.  To avoid these problems, the
   application may:

   1. Rely upon the enveloping application to properly divorce its body
      (the signature payload) from the context (the envelope) before the
      signature is validated.  Or,
   2. Use a canonicalization method that "repels/excludes" instead of
      "attracts" ancestor context.  [XML-C14N] purposefully attracts
      such context.

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 a 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.
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   Note, core validation behavior does not confirm that the signed data
   was obtained by applying each step of the indicated transforms.
   (Though it does check that the digest of the resulting content
   matches that specified in the signature.)  For example, some
   applications may be satisfied with verifying an XML signature over a
   cached copy of already transformed data.  Other applications might
   require that content be freshly dereferenced and transformed.

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 cannot rely upon canonicalization to do this for them. Also, users concerned with the integrity of the element type definitions associated with the XML instance being signed may wish to sign those definitions as well (i.e., the schema, DTD, or natural language description associated with the namespace/identifier). Second, an envelope containing signed information is not secured by the signature. For instance, when an encrypted envelope contains a signature, the signature does not protect the authenticity or integrity of unsigned envelope headers nor its ciphertext form, it only secures the plaintext actually signed.

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.
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8.1.3 'See' What is Signed

Just as a user should only sign what he or she "sees," persons and automated mechanism that trust the validity of a transformed document on the basis of a valid signature should operate over the data that was transformed (including canonicalization) and signed, not the original pre-transformed data. This recommendation applies to transforms specified within the signature as well as those included as part of the document itself. For instance, if an XML document includes an embedded style sheet [XSLT] it is the transformed document that should be represented to the user and signed. To meet this recommendation where a document references an external style sheet, the content of that external resource should also be signed via a signature Reference, otherwise the content of that external content might change which alters the resulting document without invalidating the signature. Some applications might operate over the original or intermediary data but should be extremely careful about potential weaknesses introduced between the original and transformed data. This is a trust decision about the character and meaning of the transforms that an application needs to make with caution. Consider a canonicalization algorithm that normalizes character case (lower to upper) or character composition ('e and accent' to 'accented-e'). An adversary could introduce changes that are normalized and consequently inconsequential to signature validity but material to a DOM processor. For instance, by changing the case of a character one might influence the result of an XPath selection. A serious risk is introduced if that change is normalized for signature validation but the processor operates over the original data and returns a different result than intended. As a result: * All documents operated upon and generated by signature applications MUST be in [NFC, NFC-Corrigendum] (otherwise intermediate processors might unintentionally break the signature) * Encoding normalizations SHOULD NOT be done as part of a signature transform, or (to state it another way) if normalization does occur, the application SHOULD always "see" (operate over) the normalized form.
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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.
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   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. Schema, DTD, Data Model, and Valid Examples

XML Signature Schema Instance schema.xsd Valid XML schema instance based on the 20001024 Schema/DTD [XML-Schema]. XML Signature DTD schema.dtd RDF Data Model 20000112.gif XML Signature Object Example A cryptographical fabricated XML example that includes foreign content and validates under the schema, it also uses schemaLocation to aid automated schema fetching and validation. RSA XML Signature Example rsa.xml An XML Signature example with generated cryptographic values by Merlin Hughes and validated by Gregor Karlinger. DSA XML Signature Example dsa.xml Similar to above but uses DSA.

10. Definitions

Authentication Code (Protected Checksum) 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 (and integrity) but not signer authentication. Equivalent to protected checksum, "A
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      checksum that is computed for a data object by means that protect
      against active attacks that would attempt to change the checksum
      to make it match changes made to the data object."  [SEC]

   Authentication, Message
      The property, given an authentication code/protected checksum,
      that tampering with both the data and checksum, so as to introduce
      changes while seemingly preserving integrity, are still detected.
      "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
      The property of the identity of the signer is as claimed.  "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] Note,
      signer authentication is an application decision (e.g., does the
      signing key actually correspond to a specific identity) that is
      supported by, but out of the scope of, this specification.
      "A value that (a) is computed by a function that is dependent on
      the contents of a data object and (b) is stored or transmitted
      together with the object, for the purpose of detecting changes in
      the data." [SEC]
      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].
      "The property that data has not been changed, destroyed, or lost
      in an unauthorized or accidental manner." [SEC] A simple checksum
      can provide integrity from incidental changes in the data; message
      authentication is similar but also protects against an active
      attack to alter the data whereby a change in the checksum is
      introduced so as to match the change in the data.
      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.
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      "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/octets
      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 integrity, message authentication and/or
      signer authentication.  (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 application behavior, the structure of the Signature element
      type and its children (including SignatureValue) and the specified
   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.
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      The processing of a data from its source to its derived form.
      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.
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Appendix: Changes from RFC 3075

Numerous minor editorial changes were made. In addition, the following substantive changes have occurred based on interoperation experience or other considerations: 1. Minor but incompatible changes in the representation of DSA keys. In particular, the optionality of several fields was changed and two fields were re-ordered. 2. Minor change in the X509Data KeyInfo structure to allow multiple CRLs to be grouped with certificates and other X509 information. Previously CRLs had to occur singly and each in a separate X509Data structure. 3. Incompatible change in the type of PGPKeyID, which had previously been string, to the more correct base64Binary since it is actually a binary quantity. 4. Several warnings have been added. Of particular note, because it reflects a problem actually encountered in use and is the only warning added that has its own little section, is the warning of canonicalization problems when the namespace context of signed material changes.


[ABA] Digital Signature Guidelines. [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. 19981001/ [DSS] FIPS PUB 186-2 . Digital Signature Standard (DSS). U.S. Department of Commerce/National Institute of Standards and Technology. 2/fips186-2.pdf [HMAC] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
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   [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.

   [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

   [NFC]              TR15, Unicode Normalization Forms. M. Davis, M.
                      Drst. Revision 18: November 1999.
                      18.html.  NFC-Corrigendum Normalization
                      Corrigendum. The Unicode Consortium.

   [PGP]              Callas, J., Donnerhacke, L., Finney, H. and R.
                      Thayer, "OpenPGP Message Format", RFC 2440,
                      November 1998.

   [RANDOM]           Eastlake, 3rd, D., Crocker, S. and J. Schiller,
                      "Randomness Recommendations for Security", RFC
                      1750, December 1994.

   [RDF]              Resource Description Framework (RDF) Schema
                      Specification 1.0. W3C Candidate Recommendation.
                      D. Brickley, R.V. Guha. March 2000.
                      Resource Description Framework (RDF) Model and
                      Syntax Specification.  W3C Recommendation. O.
                      Lassila, R. Swick. February 1999.
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   [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.

   [SAX]              SAX: The Simple API for XML. D. Megginson, et al.
                      May 1998.
                      (THIS PAGE OUT OF DATE; GO TO

   [SEC]              Shirey, R., "Internet Security Glossary", FYI 36,
                      RFC 2828, May 2000.

   [SHA-1]            FIPS PUB 180-1. Secure Hash Standard. U.S.
                      Department of Commerce/National Institute of
                      Standards and Technology.

   [SOAP]             Simple Object Access Protocol (SOAP) Version 1.1.
                      W3C Note. D. Box, D. Ehnebuske, G. Kakivaya, A.
                      Layman, N. Mendelsohn, H. Frystyk Nielsen, S.
                      Thatte, D. Winer. May 2001.

   [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, R., "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.
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   [URL]              Berners-Lee, T., Masinter, L. and M. McCahill,
                      "Uniform Resource Locators (URL)", RFC 1738,
                      December 1994.

   [URN]              Moats, R., "URN Syntax", RFC 2141, May 1997.

   [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. W3C Recommendation. S. Pemberton, D.
                      Raggett, et al. January 2000.

   [XLink]            XML Linking Language. W3C Recommendation. S.
                      DeRose, E. Maler, D. Orchard. June 2001.

   [XML]              Extensible Markup Language (XML) 1.0 (Second
                      Edition). W3C Recommendation. T. Bray, E. Maler,
                      J. Paoli, C. M. Sperberg-McQueen.  October 2000.

   [XML-C14N]         Boyer, J., "Canonical XML Version 1.0", RFC 3076,
                      March 2001.

   [XML-Japanese]     XML Japanese Profile. W3C Note. M. Murata April

   [XML-MT]           Whitehead, E. and M. Murata, "XML Media Types",
                      RFC 2376, July 1998.

   [XML-ns]           Namespaces in XML. W3C Recommendation. T. Bray, D.
                      Hollander, A. Layman. January 1999.
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   [XML-schema]       XML Schema Part 1: Structures. W3C Recommendation.
                      D. Beech, M. Maloney, N. Mendelsohn, H. Thompson.
                      May 2001.
                      xmlschema-1-20010502/ XML Schema Part 2: Datatypes
                      W3C Recommendation. P. Biron, A. Malhotra.  May

   [XML-Signature-RD] Reagle, J., "XML Signature Requirements", RFC
                      2807, July 2000.

   [XPath]            XML Path Language (XPath) Version 1.0. W3C
                      Recommendation. J. Clark, S. DeRose. October 1999.

   [XPointer]         XML Pointer Language (XPointer). W3C Working
                      Draft. S. DeRose, R. Daniel, E. Maler. January

   [XSL]              Extensible Stylesheet Language (XSL). W3C Proposed
                      Recommendation. S.  Adler, A. Berglund, J. Caruso,
                      S. Deach, P. Grosso, E. Gutentag, A. Milowski, S.
                      Parnell, J. Richman, S. Zilles. August 2001.

   [XSLT]             XSL Transforms (XSLT) Version 1.0. W3C
                      Recommendation. J. Clark. November 1999.
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Authors' Addresses

Donald E. Eastlake 3rd Motorola, 20 Forbes Boulevard Mansfield, MA 02048 USA Phone: 1-508-851-8280 EMail: 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: David Solo Citigroup 909 Third Ave, 16th Floor NY, NY 10043 USA Phone +1-212-559-2900 EMail:
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