code: protected BigInteger RSASP1(RSAPrivateKey key, BigInteger m) {
  
          if (key == null) {
              throw new IllegalArgumentException("key");
          }
  
          if (m == null) {
              throw new IllegalArgumentException("m");
          }
  
          BigInteger n = key.getModulus();
          BigInteger d = key.getPrivateExponent();
  
          if (m.compareTo(BigInteger.ONE) == -1 || m.compareTo(n) != -1) {
              throw new IllegalArgumentException("message representative out of range");
          }
  
          return m.modPow(d, n);
      }
docs: /*
  See https://tools.ietf.org/html/rfc3447#section-5.2.1