Add Skipping Decryption Error Message

Closes gh-10220

saml2/saml2-service-provider/src/opensaml4Main/java/org/springframework/security/saml2/provider/service/authentication/OpenSaml4AuthenticationProvider.java

@@ -491,6 +491,10 @@ public final class OpenSaml4AuthenticationProvider implements AuthenticationProv
 		if (responseSigned) {
 			this.responseElementsDecrypter.accept(responseToken);
 		}
+		else if (!response.getEncryptedAssertions().isEmpty()) {
+			result = result.concat(new Saml2Error(Saml2ErrorCodes.INVALID_SIGNATURE,
+					"Did not decrypt response [" + response.getID() + "] since it is not signed"));
+		}
 		result = result.concat(this.responseValidator.convert(responseToken));
 		boolean allAssertionsSigned = true;
 		for (Assertion assertion : response.getAssertions()) {

saml2/saml2-service-provider/src/opensaml4Test/java/org/springframework/security/saml2/provider/service/authentication/OpenSaml4AuthenticationProviderTests.java

@@ -280,7 +280,7 @@ public class OpenSaml4AuthenticationProviderTests {
 		Saml2AuthenticationToken token = token(response, decrypting(verifying(registration())));
 		assertThatExceptionOfType(Saml2AuthenticationException.class)
 				.isThrownBy(() -> this.provider.authenticate(token))
-				.satisfies(errorOf(Saml2ErrorCodes.MALFORMED_RESPONSE_DATA));
+				.satisfies(errorOf(Saml2ErrorCodes.INVALID_SIGNATURE, "Did not decrypt response"));
 	}
 
 	@Test