The Xml Digital Signatures provides
security services of data integrity, authentication, and nonrepudiation.In a simple shorthand notation structures of DSIG signatures with its four elements. Elements appear zero or more times if followed by "*", zero or once if followed by"?", and once or more if followed by "+". When not followed by a symbol, elements appear only once.
Removing attributes and contents in notation it becomes an example of signature object using three of its four elements.
The signature object contains cryptographic hash of any signed information, and a reference to information itself. The signed information may be an arbitrary document. However, often, it will be an XML object. The ability to sign only specific elements of XML documents is one of most important features of DSIG. It lets unsigned parts of XML document is enhanced, modified, or removed for privacy or efficiency, keeping signature valid.
DSIG signatures may contain either signed XML object contained in XML object, or detached from signed object or document. When signed XML object envelops signature, enveloped signature value itself is not included in signature calculation and validation computation. For this you use enveloped-signature transform, removing whole signature element in which it is contained from digest calculation.
Public key digital signatures that provide nonrepudiation, such as RSA, are computationally intensive operations; therefore, DSIG also allows shared-key authentication that provides authentication but nonrepudiation. Collision resistant hashing of signed content is also used to save computational requirements.
Generating DSIG signatures:
1. Identity resources to be signed. 2. Calculating digest value and composing reference elements for each resource. 3. Composing signed info element from all references. 4. Computing value of signature method over identity resources to be signed element by applying algorithms like DSA, RSA-SHA1, etc 5. Composing signature elements with signedInfo, signature value, identity key used to sign, and other optional objects like signature properties.
