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EMSA_PSS.java

package gnu.crypto.sig.rsa;

// ----------------------------------------------------------------------------
// $Id: EMSA_PSS.java,v 1.7 2003/09/27 00:00:30 raif Exp $
//
// Copyright (C) 2001, 2002, 2003 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2, or (at your option)
// any later version.
//
// GNU Crypto is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; see the file COPYING.  If not, write to the
//
//    Free Software Foundation Inc.,
//    59 Temple Place - Suite 330,
//    Boston, MA 02111-1307
//    USA
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library.  Thus, the terms and
// conditions of the GNU General Public License cover the whole
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// As a special exception, the copyright holders of this library give
// you permission to link this library with independent modules to
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// library, you may extend this exception to your version of the
// library, but you are not obligated to do so.  If you do not wish to
// do so, delete this exception statement from your version.
// ----------------------------------------------------------------------------

import gnu.crypto.hash.HashFactory;
import gnu.crypto.hash.IMessageDigest;
import gnu.crypto.util.Util;

import java.io.PrintWriter;
import java.util.Arrays;

/**
 * <p>An implementation of the EMSA-PSS encoding/decoding scheme.</p>
 *
 * <p>EMSA-PSS coincides with EMSA4 in IEEE P1363a D5 except that EMSA-PSS acts
 * on octet strings and not on bit strings. In particular, the bit lengths of
 * the hash and the salt must be multiples of 8 in EMSA-PSS. Moreover, EMSA4
 * outputs an integer of a desired bit length rather than an octet string.</p>
 *
 * <p>EMSA-PSS is parameterized by the choice of hash function Hash and mask
 * generation function MGF. In this submission, MGF is based on a Hash
 * definition that coincides with the corresponding definitions in IEEE Std
 * 1363-2000, PKCS #1 v2.0, and the draft ANSI X9.44. In PKCS #1 v2.0 and the
 * draft ANSI X9.44, the recommended hash function is SHA-1, while IEEE Std
 * 1363-2000 recommends SHA-1 and RIPEMD-160.</p>
 *
 * <p>References:</p>
 * <ol>
 *    <li><a href="http://www.cosic.esat.kuleuven.ac.be/nessie/workshop/submissions/rsa-pss.zip">
 *    RSA-PSS Signature Scheme with Appendix, part B.</a><br>
 *    Primitive specification and supporting documentation.<br>
 *    Jakob Jonsson and Burt Kaliski.</li>
 * </ol>
 *
 * @version $Revision: 1.7 $
 */
00078 public class EMSA_PSS implements Cloneable {

   // Debugging methods and variables
   // -------------------------------------------------------------------------

   private static final String NAME = "emsa-pss";
   private static final boolean DEBUG = false;
   private static final int debuglevel = 5;
   private static final PrintWriter err = new PrintWriter(System.out, true);
   private static void debug(String s) {
      err.println(">>> "+NAME+": "+s);
   }

   // Constants and variables
   // -------------------------------------------------------------------------

   /** The underlying hash function to use with this instance. */
00095    private IMessageDigest hash;

   /** The output size of the hash function in octets. */
00098    private int hLen;

   // Constructor(s)
   // -------------------------------------------------------------------------

   /**
    * <p>Trivial private constructor to enforce use through Factory method.</p>
    *
    * @param hash the message digest instance to use with this scheme instance.
    */
00108    private EMSA_PSS(IMessageDigest hash) {
      super();

      this.hash = hash;
      hLen = hash.hashSize();
   }

   // Class methods
   // -------------------------------------------------------------------------

   /**
    * <p>Returns an instance of this object given a designated name of a hash
    * function.</p>
    *
    * @param mdName the canonical name of a hash function.
    * @return an instance of this object configured for use with the designated
    * options.
    */
00126    public static EMSA_PSS getInstance(String mdName) {
      IMessageDigest hash = HashFactory.getInstance(mdName);
      return new EMSA_PSS(hash);
   }

   // Instance methods
   // -------------------------------------------------------------------------

   // Cloneable interface implementation --------------------------------------

   public Object clone() {
      return getInstance(hash.name());
   }

   // own methods -------------------------------------------------------------

   /**
    * <p>The encoding operation EMSA-PSS-Encode computes the hash of a message
    * <code>M</code> using a hash function and maps the result to an encoded
    * message <code>EM</code> of a specified length using a mask generation
    * function.</p>
    *
    * @param mHash the byte sequence resulting from applying the message digest
    * algorithm Hash to the message <i>M</i>.
    * @param emBits the maximal bit length of the integer OS2IP(EM), at least
    * <code>8.hLen + 8.sLen + 9</code>.
    * @param salt the salt to use when encoding the output.
    * @return the encoded message <code>EM</code>, an octet string of length
    * <code>emLen = CEILING(emBits / 8)</code>.
    * @exception IllegalArgumentException if an exception occurs.
    *
    */
00158    public byte[] encode(byte[] mHash, int emBits, byte[] salt) {
      int sLen = salt.length;

      // 1. If the length of M is greater than the input limitation for the hash
      // function (2**61 - 1 octets for SHA-1) then output "message too long"
      // and stop.
      // 2. Let mHash = Hash(M), an octet string of length hLen.
      if (hLen != mHash.length) {
         throw new IllegalArgumentException("wrong hash");
      }
      // 3. If emBits < 8.hLen + 8.sLen + 9, output 'encoding error' and stop.
      if (emBits < (8*hLen + 8*sLen + 9)) {
         throw new IllegalArgumentException("encoding error");
      }
      int emLen = (emBits + 7) / 8;
      // 4. Generate a random octet string salt of length sLen; if sLen = 0,
      // then salt is the empty string.
      // ...passed as argument to accomodate JCE
      // 5. Let M0 = 00 00 00 00 00 00 00 00 || mHash || salt;
      // M0 is an octet string of length 8 + hLen + sLen with eight initial zero
      // octets.
      // 6. Let H = Hash(M0), an octet string of length hLen.
      byte[] H;
      int i;
      synchronized (hash) {
         for (i = 0; i < 8; i++) {
            hash.update((byte) 0x00);
         }
         hash.update(mHash, 0, hLen);
         hash.update(salt, 0, sLen);
         H = hash.digest();
      }
      // 7. Generate an octet string PS consisting of emLen - sLen - hLen - 2
      // zero octets. The length of PS may be 0.
      // 8. Let DB = PS || 01 || salt.
      byte[] DB = new byte[emLen - sLen - hLen - 2 + 1 + sLen];
      DB[emLen - sLen - hLen - 2] = 0x01;
      System.arraycopy(salt, 0, DB, emLen - sLen - hLen - 1, sLen);
      // 9. Let dbMask = MGF(H, emLen - hLen - 1).
      byte[] dbMask = MGF(H, emLen - hLen - 1);
      if (DEBUG && debuglevel > 8) {
         debug("dbMask (encode): "+Util.toString(dbMask));
         debug("DB (encode): "+Util.toString(DB));
      }
      // 10. Let maskedDB = DB XOR dbMask.
      for (i = 0; i < DB.length; i++) {
         DB[i] = (byte)(DB[i] ^ dbMask[i]);
      }
      // 11. Set the leftmost 8emLen - emBits bits of the leftmost octet in
      // maskedDB to zero.
      DB[0] &= (0xFF >>> (8*emLen - emBits));
      // 12. Let EM = maskedDB || H || bc, where bc is the single octet with
      // hexadecimal value 0xBC.
      byte[] result = new byte[emLen];
      System.arraycopy(DB, 0, result, 0,                emLen - hLen - 1);
      System.arraycopy(H,  0, result, emLen - hLen - 1, hLen);
      result[emLen - 1] = (byte) 0xBC;
      // 13. Output EM.
      return result;
   }

   /**
    * <p>The decoding operation EMSA-PSS-Decode recovers the message hash from
    * an encoded message <code>EM</code> and compares it to the hash of
    * <code>M</code>.</p>
    *
    * @param mHash the byte sequence resulting from applying the message digest
    * algorithm Hash to the message <i>M</i>.
    * @param EM the <i>encoded message</i>, an octet string of length
    * <code>emLen = CEILING(emBits/8).
    * @param emBits the maximal bit length of the integer OS2IP(EM), at least
    * <code>8.hLen + 8.sLen + 9</code>.
    * @param sLen the length, in octets, of the expected salt.
    * @return <code>true</code> if the result of the verification was
    * <i>consistent</i> with the expected reseult; and <code>false</code> if the
    * result was <i>inconsistent</i>.
    * @exception IllegalArgumentException if an exception occurs.
    */
00236    public boolean decode(byte[] mHash, byte[] EM, int emBits, int sLen) {
      if (DEBUG && debuglevel > 8) {
         debug("mHash: "+Util.toString(mHash));
         debug("EM: "+Util.toString(EM));
         debug("emBits: "+String.valueOf(emBits));
         debug("sLen: "+String.valueOf(sLen));
      }
      if (sLen < 0) {
         throw new IllegalArgumentException("sLen");
      }

      // 1. If the length of M is greater than the input limitation for the hash
      //    function (2**61 ? 1 octets for SHA-1) then output 'inconsistent' and
      //    stop.
      // 2. Let mHash = Hash(M), an octet string of length hLen.
      if (hLen != mHash.length) {
         if (DEBUG && debuglevel > 8) {
            debug("hLen != mHash.length; hLen: "+String.valueOf(hLen));
         }
         throw new IllegalArgumentException("wrong hash");
      }
      // 3. If emBits < 8.hLen + 8.sLen + 9, output 'decoding error' and stop.
      if (emBits < (8*hLen + 8*sLen + 9)) {
         if (DEBUG && debuglevel > 8) {
            debug("emBits < (8hLen + 8sLen + 9); sLen: "+String.valueOf(sLen));
         }
         throw new IllegalArgumentException("decoding error");
      }
      int emLen = (emBits + 7) / 8;
      // 4. If the rightmost octet of EM does not have hexadecimal value bc,
      //    output 'inconsistent' and stop.
      if ((EM[EM.length - 1] & 0xFF) != 0xBC) {
         if (DEBUG && debuglevel > 8) {
            debug("EM does not end with 0xBC");
         }
         return false;
      }
      // 5. Let maskedDB be the leftmost emLen ? hLen ? 1 octets of EM, and let
      //    H be the next hLen octets.
      // 6. If the leftmost 8.emLen ? emBits bits of the leftmost octet in
      //    maskedDB are not all equal to zero, output 'inconsistent' and stop.
      if ((EM[0] & (0xFF << (8 - (8*emLen - emBits)))) != 0) {
         if (DEBUG && debuglevel > 8) {
            debug("Leftmost 8emLen - emBits bits of EM are not 0s");
         }
         return false;
      }
      byte[] DB = new byte[emLen - hLen - 1];
      byte[] H = new byte[hLen];
      System.arraycopy(EM, 0,                DB, 0, emLen - hLen - 1);
      System.arraycopy(EM, emLen - hLen - 1, H,  0, hLen);
      // 7. Let dbMask = MGF(H, emLen ? hLen ? 1).
      byte[] dbMask = MGF(H, emLen - hLen - 1);
      // 8. Let DB = maskedDB XOR dbMask.
      int i;
      for (i = 0; i < DB.length; i++) {
         DB[i] = (byte)(DB[i] ^ dbMask[i]);
      }
      // 9. Set the leftmost 8.emLen ? emBits bits of DB to zero.
      DB[0] &= (0xFF >>> (8*emLen - emBits));
      if (DEBUG && debuglevel > 8) {
         debug("dbMask (decode): "+Util.toString(dbMask));
         debug("DB (decode): "+Util.toString(DB));
      }
      // 10. If the emLen -hLen -sLen -2 leftmost octets of DB are not zero or
      //     if the octet at position emLen -hLen -sLen -1 is not equal to 0x01,
      //     output 'inconsistent' and stop.
      // IMPORTANT (rsn): this is an error in the specs, the index of the 0x01
      // byte should be emLen -hLen -sLen -2 and not -1! authors have been
      // advised
      for (i = 0; i < (emLen - hLen - sLen - 2); i++) {
         if (DB[i] != 0) {
            if (DEBUG && debuglevel > 8) {
               debug("DB["+String.valueOf(i)+"] != 0x00");
            }
            return false;
         }
      }
      if (DB[i] != 0x01) { // i == emLen -hLen -sLen -2
            if (DEBUG && debuglevel > 8) {
               debug("DB's byte at position (emLen -hLen -sLen -2); i.e. "
                  +String.valueOf(i)+" is not 0x01");
            }
         return false;
      }
      // 11. Let salt be the last sLen octets of DB.
      byte[] salt = new byte[sLen];
      System.arraycopy(DB, DB.length - sLen, salt, 0, sLen);
      // 12. Let M0 = 00 00 00 00 00 00 00 00 || mHash || salt;
      //     M0 is an octet string of length 8 + hLen + sLen with eight initial
      //     zero octets.
      // 13. Let H0 = Hash(M0), an octet string of length hLen.
      byte[] H0;
      synchronized (hash) {
         for (i = 0; i < 8; i++) {
            hash.update((byte) 0x00);
         }
         hash.update(mHash, 0, hLen);
         hash.update(salt, 0, sLen);
         H0 = hash.digest();
      }
      // 14. If H = H0, output 'consistent.' Otherwise, output 'inconsistent.'
      return Arrays.equals(H, H0);
   }

   // helper methods ----------------------------------------------------------

   /**
    * <p>A mask generation function takes an octet string of variable length
    * and a desired output length as input, and outputs an octet string of the
    * desired length. There may be restrictions on the length of the input and
    * output octet strings, but such bounds are generally very large. Mask
    * generation functions are deterministic; the octet string output is
    * completely determined by the input octet string. The output of a mask
    * generation function should be pseudorandom, that is, it should be
    * infeasible to predict, given one part of the output but not the input,
    * another part of the output. The provable security of RSA-PSS relies on
    * the random nature of the output of the mask generation function, which in
    * turn relies on the random nature of the underlying hash function.</p>
    *
    * @param Z a seed.
    * @param l the desired output length in octets.
    * @return the mask.
    * @exception IllegalArgumentException if the desired output length is too
    * long.
    */
00362    private byte[] MGF(byte[] Z, int l) {
      // 1. If l > (2**32).hLen, output 'mask too long' and stop.
      if (l < 1 || (l & 0xFFFFFFFFL) > ((hLen & 0xFFFFFFFFL) << 32L)) {
         throw new IllegalArgumentException("mask too long");
      }
      // 2. Let T be the empty octet string.
      byte[] result = new byte[l];
      // 3. For i = 0 to CEILING(l/hLen) ? 1, do
      int limit = ((l + hLen - 1) / hLen) - 1;
      IMessageDigest hashZ = null;
      hashZ = (IMessageDigest) hash.clone();
      hashZ.digest();
      hashZ.update(Z, 0, Z.length);
      IMessageDigest hashZC = null;
      byte[] t;
      int sofar = 0;
      int length;
      for (int i = 0; i < limit; i++) {
      //    3.1 Convert i to an octet string C of length 4 with the primitive
      //        I2OSP: C = I2OSP(i, 4).
      //    3.2 Concatenate the hash of the seed Z and C to the octet string T:
      //        T = T || Hash(Z || C)
         hashZC = (IMessageDigest) hashZ.clone();
         hashZC.update((byte)(i >>> 24));
         hashZC.update((byte)(i >>> 16));
         hashZC.update((byte)(i >>>  8));
         hashZC.update((byte) i        );
         t = hashZC.digest();
         length = l - sofar;
         length = (length > hLen ? hLen : length);
         System.arraycopy(t, 0, result, sofar, length);
         sofar += length;
      }
      // 4. Output the leading l octets of T as the octet string mask.
      return result;
   }
}

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