Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
A W3C Recommendation is the final stage of a ratification process of the World Wide Web Consortium working group concerning a technical standard. This designation signifies that a document has been subjected to a public and W3C-member organization's review. It aims to standardise the Web technology...
In cryptography, PKCS refers to a group of public-key cryptography standards devised and published by RSA Security.RSA Data Security Inc was assigned the licensing rights for the patent on the RSA asymmetric key algorithm and acquired the licensing rights to several other key patents as well...
#7 but is more extensible and geared towards signing XML documents. It is used by various Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
Security Assertion Markup Language is an XML-based open standard for exchanging authentication and authorization data between security domains, that is, between an identity provider and a service provider...
, 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
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
can be signed. An XML signature used to sign a resource outside its containing XML document is called a detached signature
Detached signature
A detached signature is a type of digital signature that is kept separate from its signed data, as opposed to bundled together into a single file....
; 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.
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:
etc.
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. A transformation can be a XPath-expression that selects a defined subset of the document tree.
DigestMethod specifies the hash algorithm before applying the hash. DigestValue contains the result of applying the hash algorithm to the transformed resource(s).
Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation...
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
X.509
In cryptography, X.509 is an ITU-T standard for a public key infrastructure and Privilege Management Infrastructure . X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation...
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.
Validation and Security Considerations
When validating an XML Signature, a procedure called Core Validation is followed.
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.
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.
XML Canonicalization
The creation of XML Signatures is substantially 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
Cryptographic hash function
A cryptographic hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the hash value, such that an accidental or intentional change to the data will change the hash value...
(typically SHA1), a single-byte difference would cause the digital signature to vary.
Moreover, if an XML document is transferred from computer to computer, the line terminator
Newline
In computing, a newline, also known as a line break or end-of-line marker, is a special character or sequence of characters signifying the end of a line of text. The name comes from the fact that the next character after the newline will appear on a new line—that is, on the next line below the...
may be changed from CR to LF to CR LF, etc. A program that digests and validates an XML document may later render the XML document in a different way, e.g. adding excess space between attribute definitions with an element definition, or using relative (vs. absolute) URLs, or by reordering namespace definitions. Canonical XML is especially important when an XML Signature refers to a remote document, which may be rendered in time-varying ways by an errant remote server.
To avoid these problems and guarantee that logically-identical XML documents give identical digital signatures, an XML canonicalization
Canonicalization
In computer science, canonicalization , is a process for converting data that has more than one possible representation into a "standard", "normal", or canonical form...
transform (frequently abbreviated C14n) is 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
XML Namespace
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
declarations independently of the surrounding XML, was created.
Benefits
XML DSig is more flexible than other forms of digital signatures such as Pretty Good Privacy
Pretty Good Privacy
Pretty Good Privacy is a data encryption and decryption computer program that provides cryptographic privacy and authentication for data communication. PGP is often used for signing, encrypting and decrypting texts, E-mails, files, directories and whole disk partitions to increase the security...
The Cryptographic Message Syntax is the IETF's standard for cryptographically protected messages. It can be used to digitally sign, digest, authenticate or encrypt any form of digital data....
, because it does not operate on binary data, but on the XML Infoset, allowing to work on subsets of the data, having various ways to bind the signature and signed information, and perform transformations. Another core concept is canonicalization, that is to sign only the "essence", eliminating meaningless differences like whitespace and line endings.
Criticisms
There are criticisms directed at the architecture of XML security in general, and at the suitability of XML canonicalization in particular 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
Service-oriented architecture
In software engineering, a Service-Oriented Architecture is a set of principles and methodologies for designing and developing software in the form of interoperable services. These services are well-defined business functionalities that are built as software components that can be reused for...
Canonical XML is a profile or subset of XML. Any XML document can be converted to Canonical XML, thus normalizing away specific kinds of minor differences while remaining an XML document...
XML Encryption, also known as XML-Enc, is a specification, governed by a W3C recommendation, that defines how to encrypt the contents of an XML element....
