code: public static BigInteger fromUnsignedByteArray(byte[] buf, int off, int length) {
  		byte[] mag = buf;
  		if (off != 0 || length != buf.length) {
  			mag = new byte[length];
  			System.arraycopy(buf, off, mag, 0, length);
  		}
  		return new BigInteger(1, mag);
  	}
docs: 无符号bytes转{@link BigInteger}
  
  @param buf 无符号bytes
  @param off 起始位置
  @param length 长度
  @return {@link BigInteger}