Question about verifying signatureValue for SAML2 redirect initiated by Shibboleth SP

[Maassen, Helma]

hi,

Sorry in advance for this long post.......

I have the following issue;
I'm using Shibboleth SP 2.5.1. to start a session.
So the SP generates a AuthnRequest, and sends it to the configured IDP.

This is done by sending a GET request to the IDP url:
https://idp.net/broker/sso/1.5?SAMLRequest=fVPBbuIwEL3vV0S%2B..........dkqoARc%2B90gdKL3TUJ%2FcXmS76D254qN8q57%2FeOV%2F&RelayState=ss%3Amem%3A2a145986d479c5694e3e4a101422d756fa75361700b557bb4aca7b6bfe1660d5&SigAlg=http%3A%2F%2Fwww.w3.org%2F2000%2F09%2Fxmldsig%23rsa-sha1
&Signature=MUuiVGl8X0w74jG...........9732NWviy6FdPOBugDX%2BtF2iySz7g5XIb3wUE6QMAC6bbijf16hEIx1nhghg2hHGI3aKVCwq8maGmS5BwFl3f85GPBT0R9ouCA%3D%3D

AFAIK the IDP should be able to verify the Signature, using the Public Key we did share.

Now I get to the real problem; The IDP I'm trying to connect to, does actually perform such a Signature verification, but it FAILS!

So what I tried, is to perform the verification myself, using a small Java program (source below).
I've based my program on the functionality SAML2RedirectEncoder.cpp, I find that the Signature is calculated over the complete string "SAMLRequest=.....&RelayState=......&SigAlg=......"
So I need to verify the Signature value to that string.

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.UnsupportedEncodingException;
import java.net.URLDecoder;
import java.security.GeneralSecurityException;
import java.security.KeyFactory;
import java.security.PublicKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.KeySpec;
import java.security.spec.X509EncodedKeySpec;

import org.apache.commons.codec.binary.Base64;

public class TestURLEncoded2 {

	public static String signedValue = "SAMLRequest=fVPBbuIwEL3vV0S%2Bkzihoa1FIrFwW.....76D254qN8q57%2FeOV%2F&RelayState=ss%3Amem%3A2a145986d479c5694e......56fa75361700b557bb4aca7b6bfe1660d5&SigAlg=http%3A%2F%2Fwww.w3.org%2F2000%2F09%2Fxmldsig%23rsa-sha1";
	public static String signatureValue = "Signature=MUuiVGl8X0w74jGghui23XKyuSiYXeogvV..........3aKVCwq8maGmS5BwFl3f85GPBT0R9ouCA%3D%3D";

	public static void main(String[] args) throws GeneralSecurityException, IOException {
		TestURLEncoded2 test = new TestURLEncoded2();
		String signatureValueDecoded = test.urlDecode(signatureValue);

		byte[] signedValueInBytes = test.base64Decode(signedValue);
		byte[] signatureValueInBytes = test.base64Decode(signatureValueDecoded);
		
		
        PublicKey pubKey = getPublicKeyFromPem(retrievePEMCertificate());
        Signature sig = Signature.getInstance("SHA1withRSA");
        sig.initVerify(pubKey);
        byte[] sigToVerify = signatureValueInBytes;
        
        sig.update(signedValueInBytes);
		try {
			if (sig.verify(sigToVerify)) {
				System.out.println("SUCCES!!!");
			} else {
				System.out.println("Something wrong");
			}
		} catch (SignatureException e) {
			e.printStackTrace();
		}

	}

	private String urlDecode(String stringValue) throws UnsupportedEncodingException {
		return URLDecoder.decode(stringValue, "UTF-8");
	}
	
	private byte[] base64Decode(String stringValue) {
		return Base64.decodeBase64(stringValue);
	}
.....
}

When I feed the program with the Shibboleth-SP created request, it seems to fail.
When I feed the program with a self constructed and signed request, it works fine, so the part of reading and loading the certificate seems to be working fine.

=======>> Can anyone find the flaw in my program? (Did I use the correct classes?)


Eventually;
I need to find out how to convince the IDP-guys (so how to prove), that the Signature value in the request sent is valid for the content from the request.
or
I need to find a way to improve my SP config so Signature verification will succeed :-(


Best regards!
Helma Maassen.