byte數組和正數BigInteger之間的相互轉換

舊代碼

public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    byte[] key = new byte[16];
    random.nextBytes(key);

    BigInteger bigInteger = new BigInteger(key);
    System.out.println("old:" + Arrays.toString(key));
    System.out.println(bigInteger);
    System.out.println("new:" + Arrays.toString(bigInteger.toByteArray()));

  }

雖然這段代碼能夠進行正常轉換,可是BigInteger不是正數範圍,在密碼學計算中,都要求是正數

指定byte數組爲正數BigInteger

BigInteger m = new BigInteger(1, bytesMessage);

正數BigInteger,會有符號位,去除第一個符號位0,還原獲得原始數組

public static byte[] toByteArray(BigInteger bi) {
    byte[] array = bi.toByteArray();
    if (array[0] == 0) {
      byte[] tmp = new byte[array.length - 1];
      System.arraycopy(array, 1, tmp, 0, tmp.length);
      array = tmp;
    }