MathMod256.java

/*
*******************************************************************************    
*   Java Card Bitcoin Hardware Wallet
*   (c) 2015 Ledger
*   
*   This program is free software: you can redistribute it and/or modify
*   it under the terms of the GNU Affero General Public License as
*   published by the Free Software Foundation, either version 3 of the
*   License, or (at your option) any later version.
*
*   This program 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 Affero General Public License for more details.
*
*   You should have received a copy of the GNU Affero General Public License
*   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*******************************************************************************   
*/    

package com.ledger.wallet;

class MathMod256 {

  /** c = a+b mod n 
   *
   *  a,b,n 256 unsigned bits value
   *  precondition: 
   *     0 <= a < n 
   *     0 <= b < n 
   *     n > 0
   *  c shall either NOT partially overlap a or b, or shall be a or b
   *
   */
  public static void addm(byte[] c, short c_off,
                   byte[] a, short a_off,byte[] b, short b_off,
                   byte[] n, short n_off) {
    if ((add(c, c_off,a, a_off, b, b_off) != 0) || 
        (ucmp(c, c_off, n,n_off) > 0)) {
      sub(c, c_off, c, c_off, n, n_off);
    }
  }
  
  /** unsigned compare a and b 
   * return   0  if a == b
   * return < 0  if a < b
   * return > 0  if a > b
   */
   public static short ucmp(byte[] a, short a_off,  byte[] b, short b_off) {
    short ai, bi;
    for (short i =0; i<32; i++) {
      ai = (short)(a[(short)(a_off+i)] & 0x00ff);
      bi = (short)(b[(short)(b_off+i)] & 0x00ff);
      if (ai!=bi) {
        return (short)(ai-bi);
      }
    }
    return 0;
  }
  
  protected static short add(byte[] c, short c_off,
                           byte[] a, short a_off,  byte[] b, short b_off) {

    short ci = 0;
    for (short i = 31 ; i >= 0 ; i--) {
      ci = (short) ((short)(a[(short)(a_off+i)]&0x00FF) + (short)(b[(short)(b_off+i)]&0xFF) + ci);
      c[(short)(c_off+i)] = (byte)ci ;
      ci = (short)(ci >> 8); 
    }
    return ci;
  }
    
   protected static short sub(byte[] c, short c_off,
                            byte[] a, short a_off,  byte[] b, short b_off) {
    short ci = 0;
    for (short i = 31 ; i >= 0 ; i--) {
      ci = (short)  ((short)(a[(short)(a_off+i)]&0xFF) - (short)(b[(short)(b_off+i)]&0xFF) - ci);
      c[(short)(c_off+i)] = (byte)ci ;
      ci = (short)(((ci >> 8) != 0) ? 1:0); 
    }
    return ci;
  }

}