Is there a way to set the prefix on the Signature of a Signed XML Document (SignedXml class in .Net)?
So instead of:
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#>
...
</Signature>
I could have the following:
<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#>
...
</ds:Signature>
Answer
It can't be done. If you modify the XML after it has been signed it may not be able to be verified, which was the case in the example above. IMO this is a flaw in MSFT's digital signature implementation that you will have to live with.
A lot of people will say that there is no reason to do this, and they are technically correct. But when you are dealing with a huge vendor (i.e. a state government or bank), good luck getting them to change it on their end. Most reference implementations include it.
UPDATE: The signature signs everything in the SignedInfo element, so if you go updating that element after the fact, then the Signature is no longer valid. You have "tampered" with the message.