XML Signature - Different signature value in PHP?

Question

I want to implement an XML digital signature in PHP. I'm testing the correctness of the signature at this verifier.

I'm getting the wrong signature value, so I'm going to explain step by step what I'm doing and please correct what I'm doing wrong.

The XML I want to sign (without new lines):

<root>
    <node1>test</node1>
    <node2 attr="value" />
</root>

First, I canonicalize the XML and then hash it using sha256 which yields the correct digest value.

Second, creation of SignedInfo XML element and canonicalizing it (without new lines):

<SignedInfo>
    <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"></CanonicalizationMethod>
    <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"></SignatureMethod>
    <Reference URI="">
        <Transforms>
            <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"></Transform>
        </Transforms>
        <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256"></DigestMethod>
        <DigestValue>EdtGJKqTvnmb0Z72M8ZxTO6Jv2jyXOohMgxVKe2Gkbo=</DigestValue>
    </Reference>
</SignedInfo>

Finally, signature of SignedInfo element using RSA-SHA256. This is the part that yields an incorrect result.

Signature is calculated using the following lines of code:

openssl_sign($c14nSignedInfo,$signature,$privateKey,"sha256WithRSAEncryption");
$signature = base64_encode($signature);

But it yields an incorrect signature value, as stated by the verifier. If I sign the same XML in C# using the same private key, I get the correct signature. So, what am I doing wrong? What am I missing? Clearly, I am signing the wrong thing, but what should I sign then? Should I hash the SignedInfo element before signing it?

This is my final result (which is incorrect):

<root><node1>test</node1><node2 attr="value" /><Signature xmlns="http://www.w3.org/2000/09/xmldsig#"><SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" /><SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" /><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" /></Transforms><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>EdtGJKqTvnmb0Z72M8ZxTO6Jv2jyXOohMgxVKe2Gkbo=</DigestValue></Reference></SignedInfo><SignatureValue>cqkvNxWf8Z03K01SQFk/J5Z7F/l8scSFpRECCknTH840rUm2L9lKHC8UF/SGKJ4LoVxOmYKJHyq0Dx8j6shTXTK1Abm0a3Ty3IP/V0Roj+3EApq4Hwr7VOpvZjcToQj1snuUtgPZFJ6pxPWdYJ5hZhxm0C+mDMlOCgcTuWP7UIDNQ3CSC1GMcKESEkxsfEAzIXNh9wHoIY2e2HnedceFzOsJLPaLnltd1qhewJvqYCq7M1gD8e+Hv5Lyo+wG7ipvxvhAQ4Ui+BAOD9mROzSQaiirrxg6nC/dMJyWketTjKwEprZDm5BOoMoJC+kb2PvShfXrdRgA/ezWZHIaT0mLGQ==</SignatureValue><KeyInfo><KeyValue><RSAKeyValue><Modulus>nENZ+u9FZ5DSDlFRZ+0f0tZ2Kvl6QY+cjUzSc89fhixKFuexGJQcEvCrs67x8QcgwxZCuoWHk/Cdh8qd3x0EZaC+ZoZ+AF7ofoxWPHioZ86CIuxhI4Zgk0bHWibSKx8Z7EIXVrQ1nM2OvX9CdM9iVjM0yfn1ohdd1o4EKmRBbJQf6kCZMTbCdOdr8UI0xGUMjjaZN6+vGj3VYoxlQXXi11NMHDlxsCyyyjBjyCvewnTerkXAomwf92xV77siOn1VZD2/YwWarv1Hk/0WtW1c6QGj0VNd4EbzUqvMtnkzY3301hz5MRqvPiPNez9tWMaawDzbMrGGkwbF2ivVAW3LHQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue></KeyValue></KeyInfo></Signature></root>

And this is the correct result (everything is the same, except signature value):

<root><node1>test</node1><node2 attr="value" /><Signature xmlns="http://www.w3.org/2000/09/xmldsig#"><SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" /><SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" /><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" /></Transforms><DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" /><DigestValue>EdtGJKqTvnmb0Z72M8ZxTO6Jv2jyXOohMgxVKe2Gkbo=</DigestValue></Reference></SignedInfo><SignatureValue>OzQtkphZ+eg8JnVHzf8BVZpUZ/xMFQJhkmw5j4h2+eu8PAJh8Z9mRZH5uN/hfPFgkByBTshS/VWDNyWyF6TclQRSc8m89L8sCBQhKqGxZKjCd7V8XUIXgRgh2+Zl1JZQ/hD36XESEPazFyQ26KWq+T+m1Tc541Rv3mklfOKv2qBOqZLd/n/nRnGhFJYMp6PtPMNk/BezosGaQFUu/IhSI5tiud5ia4qETk/1G1eXAXmE4RbnVMefkysarTjizJYkGRqW10f0cF0trGxbCPyohMMfb2msnYiQfZXZd0tW41mMpH0R0AHFeC7RPxK2GzxRMRJCkNeWe65brneUtUSHzA==</SignatureValue><KeyInfo><KeyValue><RSAKeyValue><Modulus>nENZ+u9FZ5DSDlFRZ+0f0tZ2Kvl6QY+cjUzSc89fhixKFuexGJQcEvCrs67x8QcgwxZCuoWHk/Cdh8qd3x0EZaC+ZoZ+AF7ofoxWPHioZ86CIuxhI4Zgk0bHWibSKx8Z7EIXVrQ1nM2OvX9CdM9iVjM0yfn1ohdd1o4EKmRBbJQf6kCZMTbCdOdr8UI0xGUMjjaZN6+vGj3VYoxlQXXi11NMHDlxsCyyyjBjyCvewnTerkXAomwf92xV77siOn1VZD2/YwWarv1Hk/0WtW1c6QGj0VNd4EbzUqvMtnkzY3301hz5MRqvPiPNez9tWMaawDzbMrGGkwbF2ivVAW3LHQ==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue></KeyValue></KeyInfo></Signature></root>

To test both results or for more details, just copy them here and click "Verify Signature".

Answer 1

I solved the problem. If anyone wants to know, the canonicalized SignedInfo was wrong, it was missing the namespace definition, which should come from the outer Signature element.

So, the canonicalized SignedInfo element now looks like this (without new lines):

<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
    ...
</SignedInfo>

So after signing this SignedInfo element, the signature value is correct.