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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.
An XML Signature consists of a Signature element in the
When validating a XML Signature, a procedure called Core Validation is followed.
This procedure establishes whether the resources were really signed by the alleged party. However, because of the extensibiliy 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.
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
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 namespaceAn XML namespace is a W3C standard for providing uniquely named elements and attributes in an XML instance. An XML instance may contain element or attribute names from more than one XML vocabulary. If each vocabulary is given a namespace then the ambiguit declarations independently of the surrounding XML, was created.
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