/*
    * Copyright 2024–2026 Michael Osipov
    *
    * Licensed under the Apache License, Version 2.0 (the "License");
    * you may not use this file except in compliance with the License.
    * You may obtain a copy of the License at
    *
    *     http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package net.sf.michaelo.tomcat.pac;
  
  import java.security.Key;
  import java.security.MessageDigest;
  import java.security.SignatureException;
  import java.util.ArrayList;
  import java.util.Base64;
  import java.util.List;
  import java.util.stream.Collectors;
  
  import net.sf.michaelo.tomcat.pac.PacSignatureData.SignatureType;
  
  import sun.security.krb5.Checksum;
  import sun.security.krb5.EncryptionKey;
  
  /**
   * A PAC signature verifier based on private Sun classes from Java's Kerberos implementation.
   *
   * <p><strong>Note:</strong> This implementation is far from ideal because it uses private classes which can break
   * anytime. If you are running on Java 17 or newer you <em>must</em> pass
   * {@code --add-exports=java.security.jgss/sun.security.krb5=ALL-UNNAMED} to your JVM. A better solution would be to use
   * the <a href="https://directory.apache.org/kerby/">Apache Kerby</a> library.
   */
  public class PrivateSunPacSignatureVerifier extends PacSignatureVerifierBase {
  
  	@Override
  	protected void verifyInternal(PacSignatureData signatureData, byte[] data, Key[] keys) throws SignatureException {
  		SignatureType type = signatureData.getType();
  		byte[] expectedSignature = signatureData.getSignature();
  		List<byte[]> actualFailedSignatures = new ArrayList<>();
  		for (int i = 0; i < keys.length; i++) {
  			Key key = keys[i];
  			EncryptionKey encKey = new EncryptionKey(type.getEType(), key.getEncoded());
  			try {
  				Checksum checksum;
  				try {
  					checksum = new Checksum(type.getValue(), data, encKey, KU_KERB_NON_KERB_CKSUM_SALT);
  				} catch (Exception e) {
  					throw new SignatureException("Failed to calculate signature", e);
  				}
  
  				byte[] actualSignature = checksum.getBytes();
  				if (MessageDigest.isEqual(expectedSignature, actualSignature)) return;
  				else actualFailedSignatures.add(actualSignature);
  			} finally {
  				encKey.destroy();
  			}
  		}
  
  		String actualFailedSignaturesStr = actualFailedSignatures.stream()
  				.map(Base64.getEncoder()::encodeToString)
  				.collect(Collectors.joining(",", "[", "]"));
  		throw new SignatureException("Calculated signatures " + actualFailedSignaturesStr
  				+ " do not match expected signature '"
  				+ Base64.getEncoder().encodeToString(expectedSignature) + "'");
  	}
  }