Java BigInteger is Prime calculateMPrime(BigInteger n, byte[] message)

Description

Method taken (renamed) from SpongyCastle ECDSASigner class.

License

Apache License

Declaration

public static BigInteger calculateMPrime(BigInteger n, byte[] message) 

Method Source Code

//package com.java2s;
/* Licensed 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
 *
 *    http://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./*from  w  w w.ja  v  a  2  s .  c  o  m*/
 */

import java.math.BigInteger;

public class Main {
    /**
     * Method taken (renamed) from SpongyCastle ECDSASigner class. Cannot call
     * from there since it's private and non static.
     */
    public static BigInteger calculateMPrime(BigInteger n, byte[] message) {
        if (n.bitLength() > message.length * 8) {
            return new BigInteger(1, message);
        } else {
            int messageBitLength = message.length * 8;
            BigInteger trunc = new BigInteger(1, message);

            if (messageBitLength - n.bitLength() > 0) {
                trunc = trunc.shiftRight(messageBitLength - n.bitLength());
            }
            return trunc;
        }
    }
}

Related

1. fnvHash(final byte[] data, final BigInteger fnvOffs, final BigInteger fnvPrime, final BigInteger mod)
2. hasSqrtModPrime(BigInteger r, BigInteger p)
3. isBigPrime(BigInteger number)
4. isCoprime(BigInteger a, BigInteger b)