emsa_pss.java

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    // 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.
   */
  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.
   */
  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;
  }
}