ACTION-142: New identifiers for new variants of DSA in FIPS 186-3

Folks,

I've committed revision 1.73 of xmldsig-core-11\Overview.htm, which includes text to resolve ACTION-142.  Specifically, now that FIPS 186-3 is out defining DSAwithSHA224 and DSAwithSHA256, I've made the following updates:

In Section 6, define the identifier DSAwithSHA256 (http://www.w3.org/2009/xmldsig11#dsa-sha256) as an OPTIONAL signature algorithm.

In Section 6.4.1, added DSAwithSHA256, updated the language in paragraph 1 to describe the four variants of DSA, and updated the Security Considerations section (there was a duplicate paragraph there, among other problems).  (Cynthia, I made these changes before seeing your comments - see if you're OK with the new version or if I need to update/revise.)

In keeping with the way we did RSA, where we didn't put the key size in the algorithm URI, I chose to do the same thing with DSA.  So the intent is that the DSAwithSHA256 AlgID should be used for both 2048-bit DSA and 3072-bit DSA with SHA-256.  Similarly, since we don't use SHA-224 anywhere else in the XMLDSIG spec, I did not define a corresponding DSAwithSHA224 (which would be 2048-bit keys & SHA-224).  We can add that if people think it's necessary, but I didn't see a compelling reason.

                                                                                --bal