<Signature> -> <dsig:Signature> & Validation

Hi everyone,

I really need some help. I have two problems :

1 - how can I change Signature structure generation from 

	<Signature>
		<SignedInfo>
		....

To 
	<dsig:Signature>
		<dsig:SignedInfo>
		...

2 - I insert my Signature structure in a document (enveloped signature),
but the validation fails.
Shoul I change my canonicalization method for signing. I'm currently
using EXCLUSIVE_WITH_COMMENTS.

A lot of thanks,

Othman