The exclusive C14N API provided by LibXML2 will generate extra default 
namespace (xmlns="")

For example:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope 
	xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" 
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
	xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
	xmlns:wsa="http://schemas.xmlsoap.org/ws/2004/03/addressing" 
	xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-
wssecurity-secext-1.0.xsd" 
	xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-
wssecurity-utility-1.0.xsd">
<soap:Header>
	<Correlation xmlns="urn:liberty:sb:2004-12" messageID="msg:273627d5wd4" 
refToMessageID="ref:dfhdf73" timestamp="2005-09-23T14:09:15.7722111+08:00" 
id="cid:tjerthur"/>
	<Provider xmlns="urn:liberty:sb:2004-12" providerID="pid:ww8e9w" 
affiliationID="sff:2348ufe" id="dsjd37yerw"/>
	<wsa:Action>urn:liberty:id-wsf-tma:2005-06:Modify</wsa:Action>
	<wsa:MessageID>uuid:44f0def9-5589-482e-b74c-e74c4cb5a50e</wsa:MessageID>
	<wsa:ReplyTo>
	
	<wsa:Address>http://schemas.xmlsoap.org/ws/2004/03/addressing/role/anony
mous</wsa:Address>
	</wsa:ReplyTo>
	<wsa:To>http://172.16.221.14:1234/MID/TMAgent</wsa:To>
	<wsse:Security soap:mustUnderstand="1">
		<wsu:Timestamp wsu:Id="Timestamp-9ccd6975-1526-4f9d-af11-
6289b8b42c1b">
			<wsu:Created>2005-09-23T06:09:16Z</wsu:Created>
			<wsu:Expires>2005-09-23T06:14:16Z</wsu:Expires>
		</wsu:Timestamp>
		<wsse:BinarySecurityToken 
			xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-
200401-wss-wssecurity-utility-1.0.xsd" 
			ValueType="http://docs.oasis-open.org/wss/2004/01/oasis-
200401-wss-x509-token-profile-1.0#X509v3" 
			EncodingType="http://docs.oasis-
open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary" 
			wsu:Id="SecurityToken-dd8ff9d1-0a69-451b-8807-
f33279623e92">MIIDRjCCAq+gAwIBAgIBBDANBgkqhkiG9w0BAQQFADBuMQswCQYDVQQGEwJDaDELMA
kGA1UECBMCU2gxCzAJBgNVBAcTAlNoMQwwCgYDVQQKEwNmZXMxDTALBgNVBAsTBGljZnMxCzAJBgNVBA
MTAmNhMRswGQYJKoZIhvcNAQkBFgxjYS5pbnRlbC5jb20wHhcNMDUwNDI1MDY1NzU2WhcNMTUwNDIzMD
Y1NzU2WjBrMQswCQYDVQQGEwJDaDELMAkGA1UECBMCU2gxDDAKBgNVBAoTA2ZlczENMAsGA1UECxMEaW
NmczEQMA4GA1UEAxMHY2xpZW50NDEgMB4GCSqGSIb3DQEJARYRY2xpZW50NC5pbnRlbC5jb20wgZ8wDQ
YJKoZIhvcNAQEBBQADgY0AMIGJAoGBANrdvkCPUuaj3BAVF/8QvjR3Fh6DGrg4epE9Wr8T7AWliqEMsJ
BaVcfIYKd3okLWgWcDxeOtcxWYKVVJlXofxcP8DktpZsBqUonnxcuI6YgnIQGkF8MLqxq2lQSOIWq0CF
Qzyltkk/ZE1VaRVIMY3HP8VHcNpNF5xrT04qyCf8fZAgMBAAGjgfYwgfMwCQYDVR0TBAIwADAsBglghk
gBhvhCAQ0EHxYdT3BlblNTTCBHZW5lcmF0ZWQgQ2VydGlmaWNhdGUwHQYDVR0OBBYEFKcH1w38RkdsVB
GAo/wNu4rBPfaKMIGYBgNVHSMEgZAwgY2AFKStdC2kEgxum61YIaEu2zqO7awRoXKkcDBuMQswCQYDVQ
QGEwJDaDELMAkGA1UECBMCU2gxCzAJBgNVBAcTAlNoMQwwCgYDVQQKEwNmZXMxDTALBgNVBAsTBGljZn
MxCzAJBgNVBAMTAmNhMRswGQYJKoZIhvcNAQkBFgxjYS5pbnRlbC5jb22CAQAwDQYJKoZIhvcNAQEEBQ
ADgYEAGQMYin9C/xKFtFcY83T99iehtLpz/AGRwioP4lFKcZGJCFIybI8ZqWeBqYcTIcD5B/BLOQeLsG
cGUfnhs+O0N+ZU3WQ4sedc6TK4ynjsCDB93N6sC3duEFCo7meDDuQI+bTrQUWTdX/9FSpY3uLHr62p8A
ezLPcPb94Y8S6Ax3A=
		</wsse:BinarySecurityToken>
		<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
			<SignedInfo>
				<ds:CanonicalizationMethod 
xmlns:ds="http://www.w3.org/2000/09/xmldsig#" 
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
				<SignatureMethod 
Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
				<Reference URI="#ndid:1234-5678-0987">
					<Transforms>
						<Transform 
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
					</Transforms>
					<DigestMethod 
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
				
	<DigestValue>sj1VXDB2VMCU5qmva2OtDw3kKOs=</DigestValue>
				</Reference>
			</SignedInfo>
		
	<SignatureValue>2Ov9rDid1Qw9yKGsPQLMvSOS1QI3dpDu5hMZiVZqJJXpEuuvKTnn1Z1P
3MJHhv/n
WJ/1JWZ5Qjjn1MwyUgek+GmbXHcGe/+LrMUvFadpPrblG3oOS6O1M8YqCzO65siL
37Yo/k3hIeS/2IOQfhmqt94i/GU1AF8td0BaAsC0nnA=
			</SignatureValue>
			<KeyInfo>
				<X509Data>
					<X509Certificate>...</X509Certificate>
</X509Data></KeyInfo></Signature></wsse:Security></soap:Header><soap:Body></soap
:Body></soap:Envelope>


The result of C14N will be like:

<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
	<ds:CanonicalizationMethod xmlns="" 
xmlns:ds="http://www.w3.org/2000/09/xmldsig#" 
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"></ds:CanonicalizationMethod>
	<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-
sha1"></SignatureMethod>
	<Reference URI="#ndid:1234-5678-0987">
		<Transforms>
			<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-
c14n#"></Transform>
		</Transforms>
		<DigestMethod 
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></DigestMethod>
		<DigestValue>sj1VXDB2VMCU5qmva2OtDw3kKOs=</DigestValue>
	</Reference>
</SignedInfo>


It seems the 'xmlns="" ' should not be there according to exclusive C14N spec: 
	<ds:CanonicalizationMethod xmlns="" 
xmlns:ds="http://www.w3.org/2000/09/xmldsig#" 
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#">