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   /*
    * Copyright 2017 The Netty Project
    *
    * The Netty Project licenses this file to you 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:
    *
    * https://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 io.netty.example.ocsp;
  
  import java.io.OutputStream;
  
  import org.bouncycastle.asn1.ASN1ObjectIdentifier;
  import org.bouncycastle.asn1.oiw.OIWObjectIdentifiers;
  import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
  import org.bouncycastle.cert.ocsp.OCSPReqBuilder;
  import org.bouncycastle.crypto.Digest;
  import org.bouncycastle.crypto.digests.SHA1Digest;
  import org.bouncycastle.crypto.digests.SHA256Digest;
  import org.bouncycastle.crypto.io.DigestOutputStream;
  import org.bouncycastle.operator.DigestCalculator;
  
  /**
   * BC's {@link OCSPReqBuilder} needs a {@link DigestCalculator} but BC doesn't
   * provide any public implementations of that interface. That's why we need to
   * write our own. There's a default SHA-1 implementation and one for SHA-256.
   * Which one to use will depend on the Certificate Authority (CA).
   */
  public final class Digester implements DigestCalculator {
  
      public static DigestCalculator sha1() {
          Digest digest = new SHA1Digest();
          AlgorithmIdentifier algId = new AlgorithmIdentifier(
                  OIWObjectIdentifiers.idSHA1);
  
          return new Digester(digest, algId);
      }
  
      public static DigestCalculator sha256() {
          Digest digest = new SHA256Digest();
  
          // The OID for SHA-256: http://www.oid-info.com/get/2.16.840.1.101.3.4.2.1
          ASN1ObjectIdentifier oid = new ASN1ObjectIdentifier(
                  "2.16.840.1.101.3.4.2.1").intern();
          AlgorithmIdentifier algId = new AlgorithmIdentifier(oid);
  
          return new Digester(digest, algId);
      }
  
      private final DigestOutputStream dos;
  
      private final AlgorithmIdentifier algId;
  
      private Digester(Digest digest, AlgorithmIdentifier algId) {
          this.dos = new DigestOutputStream(digest);
          this.algId = algId;
      }
  
      @Override
      public AlgorithmIdentifier getAlgorithmIdentifier() {
          return algId;
      }
  
      @Override
      public OutputStream getOutputStream() {
          return dos;
      }
  
      @Override
      public byte[] getDigest() {
          return dos.getDigest();
      }
  }