I don't know how this code work; ;

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i'm studying DES but i have troubles with this part;;
can anyone tell me how this code work??
and why shift bits by 4 16 2 8??

private void des( int[] inInts, int[] outInts, int[] keys )
	{
	int fval, work, right, leftt;
	int round;
	int keysi = 0;

	leftt = inInts[0];
	right = inInts[1];

	work   = ((leftt >>>  4) ^ right) & 0x0f0f0f0f;
	right ^= work;
	leftt ^= (work << 4);

	work   = ((leftt >>> 16) ^ right) & 0x0000ffff;
	right ^= work;
	leftt ^= (work << 16);

	work   = ((right >>>  2) ^ leftt) & 0x33333333;
	leftt ^= work;
	right ^= (work << 2);

	work   = ((right >>>  8) ^ leftt) & 0x00ff00ff;
	leftt ^= work;
	right ^= (work << 8);
	right  = (right << 1) | ((right >>> 31) & 1);

	work   = (leftt ^ right) & 0xaaaaaaaa;
	leftt ^= work;
	right ^= work;
	leftt  = (leftt << 1) | ((leftt >>> 31) & 1);

	for ( round = 0; round < 8; ++round )
	    {
	    work   = (right << 28) | (right >>> 4);
	    work  ^= keys[keysi++];
	    fval   = SP7[ work	       & 0x0000003f ];
	    fval  |= SP5[(work >>>  8) & 0x0000003f ];
	    fval  |= SP3[(work >>> 16) & 0x0000003f ];
	    fval  |= SP1[(work >>> 24) & 0x0000003f ];
	    work   = right ^ keys[keysi++];
	    fval  |= SP8[ work         & 0x0000003f ];
	    fval  |= SP6[(work >>>  8) & 0x0000003f ];
	    fval  |= SP4[(work >>> 16) & 0x0000003f ];
	    fval  |= SP2[(work >>> 24) & 0x0000003f ];
	    leftt ^= fval;
	    work   = (leftt << 28) | (leftt >>> 4);
	    work  ^= keys[keysi++];
	    fval   = SP7[ work	       & 0x0000003f ];
	    fval  |= SP5[(work >>>  8) & 0x0000003f ];
	    fval  |= SP3[(work >>> 16) & 0x0000003f ];
	    fval  |= SP1[(work >>> 24) & 0x0000003f ];
	    work   = leftt ^ keys[keysi++];
	    fval  |= SP8[ work	       & 0x0000003f ];
	    fval  |= SP6[(work >>>  8) & 0x0000003f ];
	    fval  |= SP4[(work >>> 16) & 0x0000003f ];
	    fval  |= SP2[(work >>> 24) & 0x0000003f ];
	    right ^= fval;
	    }

	right  = (right << 31) | (right >>> 1);
	work   = (leftt ^ right) & 0xaaaaaaaa;
	leftt ^= work;
	right ^= work;
	leftt  = (leftt << 31) | (leftt >>> 1);
	work   = ((leftt >>>  8) ^ right) & 0x00ff00ff;
	right ^= work;
	leftt ^= (work << 8);
	work   = ((leftt >>>  2) ^ right) & 0x33333333;
	right ^= work;
	leftt ^= (work << 2);
	work   = ((right >>> 16) ^ leftt) & 0x0000ffff;
	leftt ^= work;
	right ^= (work << 16);
	work   = ((right >>>  4) ^ leftt) & 0x0f0f0f0f;
	leftt ^= work;
	right ^= (work << 4);
	outInts[0] = right;
	outInts[1] = leftt;
	}

What I have tried:

yeah i'm trying this part for 3 weeks. too difficult about permutation;; somebody help me T.T;;

Posted 2-May-18 21:08pm

yogo-yogo

Updated 2-May-18 22:52pm

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OriginalGriff · Answer 1 · 2018-05-02T21:37:00

Do you have any idea how much work explaining code line by line is?
Every single line needs a paragraph of explanation! For example:

int next = r.Next();

Create a new variable called "next" which can hold a integer value. From the previously declared Random instance "r", call the "Next" method to get a new random number, and assign it to the "next" variable.

Can you imagine how long it would take us to explain even a very short code fragment like your example, line by line?

No. It is not going to happen. If you have a specific problem, then ask a question about it. But think first - would you want to sit down for 45 minutes and type up a line-by-line description for no good reason?

So go back to Wiki and read the DES algorithm description again. Then read this: The DES Algorithm Illustrated[^] which explains DES pretty clearly.