Safeguarding the Identity and Integrity of XML Messages - The DigestMethod Element (Page 16 of 21 )
The DigestMethod element represents an identifier for the algorithm used to calculate the digest of the Reference URI plus all the Transforms.
Note - The digest algorithm itself must receive its information in the form of octets. Typically, if a conversion is needed from an XML nodeset to octets, it is handled automatically. However, in some cases, an additional Transform may be required to ensure that the input to the digest algorithm is in the form of octets or to ensure that the XML nodeset is canonicalized so that it has the highest likelihood of valid comparison on any platform.
The only required digest algorithm is SHA1, and it is designated by
Algorithm=http://www.w3.org/2000/09/xmldsig#sha1
The DigestValue Element
The DigestValue element contains the Base-64 encoded value of the digest. The following DigestValue element is taken from the example we gave near the beginning of the chapter:
<DigestValue>60NvZvtdTB+7UnlLp/H24p7h4bs=</DigestValue>
The SignatureValue Element
At this point, the SignatureValue element must seem anti-climactic even though it represents the signature itself. The SignatureValue element is the Base-64 encoded resulting value of encrypting a digest of the SignedInfo element. The particular signature method used is defined within the SignatureMethod element itself (for example, RSA-SHA1). Here is an example of a SignatureValue:
<SignatureValue> hTHQJyd3C6ww/OJz07P4bMOgjqBdznSUOsCh6P+0MpF69w2tln/PFLdx/EP4/VKX </SignatureValue>
So far, we have reviewed the core aspects of the XML Signature, SignedInfo, and SignatureValue elements. If you understand them well, you have a good basis for understanding and using XML Signatures. The next two elements, Object and KeyInfo, are optional, but in many circumstances, necessary and important elements.
 This chapter is from Securing Web Services Security with WS-Security, by Jothy Rosenberg and David Remy (Sams, 2004, ISBN: 0672326515). Check it out at your favorite bookstore today.
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