Argthtjtr

I am working on encryption with DES, which uses a 56-bit effective key (after discarding the least significant bits) to encrypt a 64-bit plaintext. I want to set the first 20-bits of the key to random bits, and the last 36-bits to 0. I have been trying to do it with BitSet where I have set up an array with with 64-bits where all the values are false in the beginning. Then I have set up a temp array of 20-bits, and I have been trying to use bitset.set(Random.nextInt(n), true) within a for loop, which goes from 0-20 - The idea is that I get exactly 20 random bits.

In order to discard the least significant bit, I have a for loop where I go from 0 to 20. Within this for loop, I have an if statement which discards every 8th element for the first 20 elements

static BitSet key1 = new BitSet(64);

static BitSet key2 = new BitSet(64);



    public static void generate() {



        BitSet temp1 = new BitSet(20);

        BitSet temp2 = new BitSet(20);



        Random r = new Random();

        for (int i = 0; i < 20; i++) {

            temp1.set(r.nextInt(60), true);

            temp2.set(r.nextInt(60), true);

        }

for (int i = 0; i < 20; i++) {

            key1.set(i, temp1.get(i));

            key2.set(i, temp2.get(i));

        }

            System.out.println(k1temp);



        for (int i = 0; i < temp1.length(); i++) {



        if (i % 8 == 0) {

                key1.clear(i);

                key2.clear(i);

            }

        }    

    }

So, the problem I have is that my BitSet does not always consist of 20 elements, which leads to that the key I generate is wrong. I have been through the code several times, but I cannot see what is wrong.

EDIT:

What I mean by first and last is that the first bits are the Most significant bits and the last are the Least significant bits.

You can use bitmasks with the & (bitwise and) operator:

Random r = new SecureRandom(); // WARNING - use SecureRandom when generating cryptographic keys!

long v1 = r.nextLong() & 0xfffff; // Low order 20 bits are random, rest zero

long v2 = r.nextLong() & 0xfffff00000000000L; // High order 20 bits are random