XML Signature

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XML Signature (also called XMLDsig, XML-DSig, XML-Sig) is a W3C recommendation that defines an XML syntax for digital signatures. Functionally, it has much in common with PKCS#7 but is more extensible and geared towards signing XML documents. It is used by various Web technologies such as SOAP, SAML, and others.

XML signatures can be used to sign data–a resource–of any type, typically XML documents, but anything that is accessible via a URL can be signed. An XML signature used to sign a resource outside its containing XML document is called a detached signature; if it is used to sign some part of its containing document, it is called an enveloped signature; if it contains the signed data within itself it is called an enveloping signature.

[edit] Structure

An XML Signature consists of a Signature element in the http://www.w3.org/2000/09/xmldsig# namespace. The basic structure is as follows:

<Signature>
  <SignedInfo>
    <SignatureMethod />
    <CanonicalizationMethod />
    <Reference>
       <Transforms>
       <DigestMethod>
       <DigestValue>
    </Reference>
    <Reference /> etc.
  </SignedInfo>
  <SignatureValue />
  <KeyInfo />
  <Object />
</Signature>

• The SignedInfo element contains or references the signed data and specifies what algorithms are used.
  The SignatureMethod and CanonicalizationMethod elements are used by the SignatureValue element and are included in SignedInfo to protect them from tampering.
  One or more Reference elements specify the resource being signed by URI reference; and any transforms to be applied to the resource prior to signing. DigestMethod specifies the hash algorithm before applying the hash.
  DigestValue contains the result of applying the hash algorithm to the transformed resource(s).

• The SignatureValue element contains the Base64 encoded signature result - the signature generated with the parameters specified in the SignatureMethod element - of the SignedInfo element after applying the algorithm specified by the CanonicalizationMethod.

• KeyInfo element optionally allows the signer to provide recipients with the key that validates the signature, usually in the form of one or more X.509 digital certificates. The relying party must identify the key from context if KeyInfo is not present.

• The Object element (optional) contains the signed data if this is an enveloping signature.

[edit] Validation and Security Considerations

When validating an XML Signature, a procedure called Core Validation is followed.

1. Reference Validation: Each Reference's digest is verified by retrieving the corresponding resource and applying any transforms and then the specified digest method to it. The result is compared to the recorded DigestValue; if they do not match, validation fails.
2. Signature Validation: The SignedInfo element is serialized using the canonicalization method specified in CanonicalizationMethod, the key data is retrieved using KeyInfo or by other means, and the signature is verified using the method specified in SignatureMethod.

This procedure establishes whether the resources were really signed by the alleged party. However, because of the extensibility of the canonicalization and transform methods, the verifying party must also make sure that what was actually signed or digested is really what was present in the original data, in other words, that the algorithms used there can be trusted not to change the meaning of the signed data.

[edit] XML Canonicalization

The creation of XML Signatures is a bit more complex than the creation of an ordinary digital signature because a given XML Document (an "Infoset," in common usage among XML developers) may have more than one legal serialized representation. For example, whitespace inside an XML Element is not syntactically significant, so that <Elem > is syntactically identical to <Elem>.

Since the digital signature is created by using an asymmetric key algorithm (typically RSA) to encrypt the results of running the serialized XML document through a Cryptographic hash function (typically SHA1), a single-byte difference would cause the digital signature to vary.

To avoid this problem and guarantee that logically-identical XML documents give identical digital signatures, an XML canonicalization transform (frequently abbreviated C14n) is nearly always employed when signing XML documents (for signing the SignedInfo, a canonicalization is mandatory). These algorithms guarantee that logically-identical documents produce exactly identical serialized representations.

Another complication arises because of the way that the default canonicalization algorithm handles namespace declarations; frequently a signed XML document needs to be embedded in another document; in this case the original canonicalization algorithm will not yield the same result as if the document is treated alone. For this reason, the so-called Exclusive Canonicalization, which serializes XML namespace declarations independently of the surrounding XML, was created.

[edit] Criticisms

There are criticisms directed at the suitability of XML canonicalization as a front end to signing and encrypting XML data due to its complexity, inherent processing requirement, and poor performance characteristics. The argument is that performing XML canonicalization causes excessive latency that is simply too much to overcome for transactional, performance sensitive SOA applications.

[edit] See also

• Canonical XML
• XML Encryption
• XAdES, extensions to XML-DSig for use with advanced electronic signature

[edit] External links

• XML-Signature Syntax and Processing (W3C)
• Performance of Web Services Security
• W3C workshop presentation on XML security
• Performance Comparison of Security Mechanisms for Grid Services
• Why XML canonicalization is bad for Web Services Security
• Canonical XML
• Additional XML Security Uniform Resource Identifiers (URIs)
• Exclusive XML Canonicalization
• Why XML Security is Broken
• XMLSignatures Java binding for XMLBeans and JAXB.