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  package java.security.cert;
  
  import java.util.Set;
  
  /**
   * Interface for an X.509 extension.
   *
   * <p>The extensions defined for X.509 v3
   * {@link X509Certificate Certificates} and v2
   * {@link X509CRL CRLs} (Certificate Revocation
   * Lists) provide methods
   * for associating additional attributes with users or public keys,
   * for managing the certification hierarchy, and for managing CRL
   * distribution. The X.509 extensions format also allows communities
   * to define private extensions to carry information unique to those
   * communities.
   *
   * <p>Each extension in a certificate/CRL may be designated as
   * critical or non-critical.  A certificate/CRL-using system (an application
   * validating a certificate/CRL) must reject the certificate/CRL if it
   * encounters a critical extension it does not recognize.  A non-critical
   * extension may be ignored if it is not recognized.
   * <p>
   * The ASN.1 definition for this is:
   * <pre>
   * Extensions  ::=  SEQUENCE SIZE (1..MAX) OF Extension
   *
   * Extension  ::=  SEQUENCE  {
   *     extnId        OBJECT IDENTIFIER,
   *     critical      BOOLEAN DEFAULT FALSE,
   *     extnValue     OCTET STRING
   *                   -- contains a DER encoding of a value
   *                   -- of the type registered for use with
   *                   -- the extnId object identifier value
   * }
   * </pre>
   * Since not all extensions are known, the {@code getExtensionValue}
   * method returns the DER-encoded OCTET STRING of the
   * extension value (i.e., the {@code extnValue}). This can then
   * be handled by a <em>Class</em> that understands the extension.
   *
   * @author Hemma Prafullchandra
   */
  
  public interface X509Extension {
  
      /**
       * Check if there is a critical extension that is not supported.
       *
       * @return {@code true} if a critical extension is found that is
       * not supported, otherwise {@code false}.
       */
      public boolean hasUnsupportedCriticalExtension();
  
      /**
       * Gets a Set of the OID strings for the extension(s) marked
       * CRITICAL in the certificate/CRL managed by the object
       * implementing this interface.
       *
       * Here is sample code to get a Set of critical extensions from an
       * X509Certificate and print the OIDs:
       * <pre>{@code
       * X509Certificate cert = null;
       * try (InputStream inStrm = new FileInputStream("DER-encoded-Cert")) {
       *     CertificateFactory cf = CertificateFactory.getInstance("X.509");
       *     cert = (X509Certificate)cf.generateCertificate(inStrm);
       * }
       *
       * Set<String> critSet = cert.getCriticalExtensionOIDs();
       * if (critSet != null && !critSet.isEmpty()) {
       *     System.out.println("Set of critical extensions:");
       *     for (String oid : critSet) {
       *         System.out.println(oid);
       *     }
       * }
       * }</pre>
       * @return a Set (or an empty Set if none are marked critical) of
       * the extension OID strings for extensions that are marked critical.
       * If there are no extensions present at all, then this method returns
       * null.
       */
      public Set<String> getCriticalExtensionOIDs();
  
      /**
       * Gets a Set of the OID strings for the extension(s) marked
       * NON-CRITICAL in the certificate/CRL managed by the object
       * implementing this interface.
       *
       * Here is sample code to get a Set of non-critical extensions from an
       * X509CRL revoked certificate entry and print the OIDs:
       * <pre>{@code
       * CertificateFactory cf = null;
       * X509CRL crl = null;
       * try (InputStream inStrm = new FileInputStream("DER-encoded-CRL")) {
       *     cf = CertificateFactory.getInstance("X.509");
       *     crl = (X509CRL)cf.generateCRL(inStrm);
       * }
       *
       * byte[] certData = <DER-encoded certificate data>
       * ByteArrayInputStream bais = new ByteArrayInputStream(certData);
       * X509Certificate cert = (X509Certificate)cf.generateCertificate(bais);
       * X509CRLEntry badCert =
       *              crl.getRevokedCertificate(cert.getSerialNumber());
       *
       * if (badCert != null) {
       *     Set<String> nonCritSet = badCert.getNonCriticalExtensionOIDs();
       *     if (nonCritSet != null)
       *         for (String oid : nonCritSet) {
       *             System.out.println(oid);
       *         }
       * }
       * }</pre>
       *
       * @return a Set (or an empty Set if none are marked non-critical) of
       * the extension OID strings for extensions that are marked non-critical.
       * If there are no extensions present at all, then this method returns
       * null.
       */
      public Set<String> getNonCriticalExtensionOIDs();
  
      /**
       * Gets the DER-encoded OCTET string for the extension value
       * (<em>extnValue</em>) identified by the passed-in {@code oid}
       * String.
       * The {@code oid} string is
       * represented by a set of nonnegative whole numbers separated
       * by periods.
       *
       * <p>For example:<br>
       * <table border=groove summary="Examples of OIDs and extension names">
       * <tr>
       * <th>OID <em>(Object Identifier)</em></th>
       * <th>Extension Name</th></tr>
       * <tr><td>2.5.29.14</td>
       * <td>SubjectKeyIdentifier</td></tr>
       * <tr><td>2.5.29.15</td>
       * <td>KeyUsage</td></tr>
       * <tr><td>2.5.29.16</td>
       * <td>PrivateKeyUsage</td></tr>
       * <tr><td>2.5.29.17</td>
       * <td>SubjectAlternativeName</td></tr>
       * <tr><td>2.5.29.18</td>
       * <td>IssuerAlternativeName</td></tr>
       * <tr><td>2.5.29.19</td>
       * <td>BasicConstraints</td></tr>
       * <tr><td>2.5.29.30</td>
       * <td>NameConstraints</td></tr>
       * <tr><td>2.5.29.33</td>
       * <td>PolicyMappings</td></tr>
       * <tr><td>2.5.29.35</td>
       * <td>AuthorityKeyIdentifier</td></tr>
       * <tr><td>2.5.29.36</td>
       * <td>PolicyConstraints</td></tr>
       * </table>
       *
       * @param oid the Object Identifier value for the extension.
       * @return the DER-encoded octet string of the extension value or
       * null if it is not present.
       */
      public byte[] getExtensionValue(String oid);
  }
  