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Digital Signatures Overview

Digital signatures are widely used as security tokens, not just in XML. A digital signature provides a mechanism for assuring integrity of data, the authentication of its signer, and the non-repudiation of the entire signature to an external party:

  • A digital signature must provide a way to verify that the data has not been modified or replaced to ensure integrity.
  • The signature must provide a way to establish the identity of the data's signer for authentication.
  • The signature must provide the ability for the data's integrity and authentication to be provable to a third party for non-repudiation.

A public key system is used to create the digital signature and it is also used for verification. The signature binds the signer to the document because digitally signing a document requires the originator to create a hash of the message and then encrypt that hash value with their own private key. Only the originator has that private key and that person is the only one who can encrypt the hash so that it can be unencrypted using their public key. The recipient, upon receiving both the message and the encrypted hash value, can decrypt the hash value, knowing the originator's public key. The recipient must also try to generate the hash value of the message and compare the newly generated hash value with the unencrypted hash value received from the originator. If the hash values are identical, it proves that the originator created the message, because only the actual originator could encrypt the hash value correctly.

XML Signatures can be applied to any digital content (data object), including XML (see W3C Recommendation, XML-Signature Syntax and Processing). An XML Signature may be applied to the content of one or more resources:

  • Enveloped or enveloping signatures are applied over data within the same XML document as the signature
  • Detached signatures are applied over data external to the signature element; 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.

The XML Signature is a method of associating a key with referenced data. It does not normatively specify how keys are associated with persons or institutions, nor the meaning of the data being referenced and signed.

The original data is not actually signed. Instead, the signature is applied to the output of a chain of canonicalization and transformation algorithms, which are applied to the data in a designated sequence. This system provides the flexibility to accommodate whatever "normalization" or desired preprocessing of the data that might be required or desired before subjecting it to being signed.

Since the signature is dependent on the content it is signing, a signature produced from a non-canonicalized document could possibly be different from one produced from a canonicalized document. The canonical form of an XML document is physical representation of the document produced by the method described in this specification. The XML canonicalization method is the algorithm defined by this specification that generates the canonical form of a given XML document or document subset. XML canonicalization is designed to be useful for applications that require the ability to test whether or not the information content of a document or document subset has been changed. This is done by comparing the canonical form of the original document before application processing with the canonical form of the document result of the application processing.

A digital signature over the canonical form of an XML document or document subset allows the signature digest calculations to be oblivious to changes in the original document's physical representation. During signature generation, the digest is computed over the canonical form of the document. The document is then transferred to the relying party, which validates the signature by reading the document and computing a digest of the canonical form of the received document. The equivalence of the digests computed by the signing and relying parties (hence, the equivalence of the canonical forms that they were computed for) ensures that the information content of the document has not been altered since it was signed.

The following canonicalization algorithms are used in Oxygen XML Editor Eclipse plugin:

  • Canonical XML (or Inclusive XML Canonicalization) (XMLC14N) - Used for XML where the context doesn't change.

    Inclusive Canonicalization copies all the declarations, even if they are defined outside of the scope of the signature, and all the declarations you might use will be unambiguously specified. Inclusive Canonicalization is useful when it is less likely that the signed data will be inserted in other XML document and it is the safer method from the security standpoint because it requires no knowledge of the data that are to be secured to safely sign them. A problem may occur if the signed document is moved into another XML document that has other declarations because the Inclusive Canonicalization will copy them and the signature will be invalid.

  • Exclusive XML Canonicalization (EXCC14N) - Designed for canonicalization where the context might change.

    Exclusive Canonicalization just copies the namespaces you are actually using (the ones that are a part of the XML syntax). It does not look into attribute values or element content, so the namespace declarations required to process these are not copied. This is useful if you have a signed XML document that you want to insert into other XML documents (or you need self-signed structures that support placement within various XML contexts), as it will ensure the signature is verified correctly each time.

The canonicalization method can specify whether or not comments should be included in the canonical form output by the XML canonicalization method. If a canonical form contains comments corresponding to the comment nodes in the input node-set, the result is called canonical XML with comments. In an uncommented canonical form, comments are removed, including the delimiter for comments outside the document element.

The three operations. Canonicalize, Sign, and Verify Signature, are available from the Source submenu when invoking the contextual menu in Text mode or from the XML Tools menu.