same message digest but different signature value

Question

I have 2 signature file that only different in namespace. Both these file provide the same message digest. I use the same key to sign with the same algo RSA-SHA1. But there are 2 different signature. I think that as long as the message digest, key and algo is the same, the signature should be the same, no matter how the origin file is.

I use python-xmlsec1 lib with openssl1.0.1.

The first file with namespace:

<?xml version="1.0" encoding="UTF-8"?>
<myns:MyElement xmlns:myns="urn:abc" ID="1">
    <myns:SubElement AssertionID="2" attr1="value1"/>
    <ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
        <ds:SignedInfo>
            <ds:CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"/>
        <ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
        <ds:Reference URI="">
            <ds:Transforms>
                <ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
            </ds:Transforms>
            <ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <ds:DigestValue>zzIulx6UyIN9BrYp0gpW9cCx05k=</ds:DigestValue>
        </ds:Reference>
    </ds:SignedInfo>
    <ds:SignatureValue>Gpcf/Yy1U9M6jvQL024pfMNHqB0+3kpUvFK9XF8wglgKIo9kPi2eyZ0qDEBm93ecXD1CfHLyHZc3gKR4cMsfobOkWDCoT5VA6FYwjyASnGlEyJE7dpvqU1BE7wDuCxDznI1NQsRTw4Ix8jFx20Cd91bbDibYG8Fn9ACVZdHRo=</ds:SignatureValue>
    </ds:Signature>
</myns:MyElement>

The second file with no namespace:

<?xml version="1.0" encoding="UTF-8"?>
<myns:MyElement xmlns:myns="urn:abc" ID="1">
    <myns:SubElement AssertionID="2" attr1="value1"/>
    <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
        <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
        <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
        <Reference>
            <Transforms>
                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
            </Transforms>
            <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
            <DigestValue>zzIulx6UyIN9BrYp0gpW9cCx05k=</DigestValue>
        </Reference>
    </SignedInfo>
    <SignatureValue>gs3U/ea2KmLbAy5Tle0196W+AcW7ftS2POgZHYaGLUX0QgPXRx5+a0ZhJ4d0NitzaTgXQtnPaMfqcsw8ZMBEQxYLdeulErtosX5bpiOt9SQpbRre7hB8VJPjb2ZT+Vu5V4SkAf1LI624JjeVUr0xrSjoCSP5ppxW87gQDPU=</SignatureValue>
    <KeyInfo>
        <KeyName/>
    </KeyInfo>
</Signature>
</myns:MyElement>

Both file can be verified by the xmlsec (given the public key).

Could anyone please explain how this can occur?

Many thanks,

Answer 1

I see that the signature value does not depend on digest value only. it is calculated based on the whole SignedInfo node. http://www.mismo.org/files/InformationSecurityGuidelines/XML_Signature_Section_for_General_I-Guide-1.pdf