XML syntax for digital signatures
XML Signature
(also called
XMLDSig
,
XML-DSig
,
XML-Sig
) defines an
XML
syntax for
digital signatures
and is defined in the
W3C recommendation
XML Signature Syntax and Processing
. 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;
[1]
if it contains the signed data within itself it is called an
enveloping
signature.
[2]
Structure
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]
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>
<CanonicalizationMethod
/>
<SignatureMethod
/>
<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 transformations to be applied to the resource prior to signing.
Transforms
contains the transformations applied to the resource prior to signing. A transformation can be a XPath-expression that selects a defined subset of the document tree.
[3]
DigestMethod
specifies the hash algorithm before applying the hash.
DigestValue
contains the
Base64
encoded result of applying the hash algorithm to the transformed resource(s) defined in the
Reference
element attributes.
- 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
.
Validation and security considerations
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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.
Because the signed document's structure can be tampered with leading to "signature wrapping" attacks, the validation process should also cover XML document structure. Signed element and signature element should be selected using absolute
XPath
expression, not
getElementByName
methods.
[4]
XML canonicalization
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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 ensures data integrity, a single-byte difference would cause the signature to vary. Moreover, if an XML document is transferred from computer to computer, the
line terminator
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
transform (frequently abbreviated
C14n
) is employed when signing XML documents (for signing the
SignedInfo
, a canonicalization is mandatory). These algorithms guarantee that semantically-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.
Benefits
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XML Signature is more flexible than other forms of digital signatures such as
Pretty Good Privacy
and
Cryptographic Message Syntax
, because it does not operate on
binary data
, but on the
XML Infoset
, allowing to work on subsets of the data (this is also possible with binary data in non-standard ways, for example encoding blocks of binary data in base64 ASCII), 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.
Issues
[
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There are criticisms directed at the architecture of XML security in general,
[5]
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.
[6]
[7]
[8]
The argument is that performing XML canonicalization causes excessive latency that is simply too much to overcome for transactional, performance sensitive
SOA
applications.
These issues are being addressed in the
XML Security Working Group
.
[9]
[10]
Without proper policy and implementation
[4]
the use of XML Dsig in SOAP and WS-Security can lead to vulnerabilities,
[11]
such as XML signature wrapping.
[12]
Applications
[
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An example of applications of XML Signatures:
See also
[
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References
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External links
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