Verify Signature of SAML Response

Question

I'm trying to follow the following spec to verify the signature of the SAML response: https://www.w3.org/TR/xmldsig-core/#sec-PKCS1

Here is my workflow: I get the SAML Response. I get rid of the signature envelope, I canonicalize it, I check the digest and then I check the signature. I was able to successfully calculate SHA1 digest of the transformed SAML response and verify it. However, RSA-SHA1 signature checking still eludes me.

SAML Response included signature method algorithm: http://www.w3.org/2000/09/xmldsig#rsa-sha1

I have this method to check the signature:

public static boolean verifySignature(String signatureType, PublicKey publicKey, byte[] contentBytes, byte[] sigBytes) {
    try {
        Signature sig = Signature.getInstance(signatureType);
        sig.initVerify(publicKey);
        sig.update(contentBytes);
        return sig.verify(sigBytes);
    } catch (Exception e) {
        Logger.log(e);
        return false;
    }
}

I do something like this to call it:

String publicKeyStr =  "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";
PublicKey publicKey = Security.getPublicKey(publicKeyStr); // uses Certificate class
String sig = "QfAcRJM2P65JucyBpqn6j48/zd1oSLPBJ0lYI1grH5/xenwBEh0D4Eh0S1J3535OUmldpW7D+G7DW6eAT5N8TdoqUvMXuIAUpFHHDR45KZykPXJPUtli+z2rwlCCHypZWnniT/wrcQYdpp1zzNJBBtKdkaqQg0NMktPSQ/0ti+ruMI3qwfTaL9kDQ3Zyi/a2J3RCAPA0RfviLnDLlid0PthiV1NEbs0AvnguDi57fWXAILk0L1Cx20sliQckxlFQ9u4OaHeMscXdjh3SfESkM9m0Y9PppisZWTrCYzGmvDwsZTCBPD3f/ZFIit+Smgh2fi1u8/gZq0yizPyacR3Y/A==";
String newXMLToOperateOn = "<saml2:Assertion xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:xs="http://www.w3.org/2001/XMLSchema" ID="_420cc1a25a3890bd5df1f4c04bd7e986" ...";

boolean sign = Security.verifySignature("SHA1withRSA", pk, newXMLToOperateOn.getBytes(), Util.base64DecodeAsBytes(sig));
// SIGN IS FALSE!!!!

newXMLToOperateOn is the correct canonicalized string XML of the SAMLResponse. This is what I use to generate the correct SHA1 digest. Security.verify() works correctly since I use to for OpenID SSO checking and that works.

I also tried to substitute the data to check instead of the entire XML to the bytes of the SHA1 digest, but didn't work either. E.G. verify(algo, pk, sha1Digest.getBytes(), sig.getBytes().

Can someone tell me if there is something that I'm doing horribly wrong when checking the signature? The spec linked above tells me the signing method does this:

   CRYPT (PAD (ASN.1 (OID, DIGEST (data))))

Which doesn't help me a ton when I'm verifying the signature. I cannot recreate the signature since I can't find what private key they use in their examples.

Thanks for any help.

Answer 1

You only need to verify the 'SignedInfo' portion of the XML