Rotating bits during a left bit shift?

AARRRGH

Hello

I've googled this and I cannot find anything.

If I want to perform one left shift on the following:

10001111

How do I make the 1 rotate to the end of the byte so it looks like:

00011111

Any ideas?

Thank you.

Webmonkey

Take it x86 ASM?
Of course you could embed it into C

http://en.wikibooks.org/wiki/X86_Assembly/Shift_and_Rotate wrote:

Roll Instructions
In a roll instruction, the bits that slide off the end of the register are fed back into the spaces.

ror arg
roll to the right

rol arg
roll to the left

AARRRGH

Thanks webmonkey.

I'm actually doing it in C.

When you say, "embed it in C", how exactly would I do that?

I'm back new with C after years of PHP, so I'm still getting my head around actual real logic

Thanks

Webmonkey

You can embed assembly into C by using the

asm
{
//ASM Code
}

Being a while with C now but what exactly are you trying to do, If its an array, you could just temporairly copy the 1st bit somewhere, then copy the next bit over the current bit and then at the end place the copied bit to the end.

If its a register on the processor or a known memory location you could do it low level with the assembly.

AARRRGH

It's an array.

I have a four byte array. I want to left shift some bits from the first byte onto the end of the last byte.

So...

11111111 11111111 11111111 00000000

Shifted twice:

11111111 11111111 11111100 00000011

I'll try your suggestion (keeping a temporary copy of the bit, shifting the bit to it's right, and then copying the bit to the end.)

Thank you.

AARRRGH

OK, problem.

How do I shift all the bits in the array at once?

I've tried this:

*key_ptr << 1; // key_ptr points to my array

But that only changes this:

11110000 11001100 10101010 11110000

to this:

11110000 11001100 10101010 11110000

In other words, it does nothing. (I've tried changing it to *key_ptr <<= 1; but it makes no difference.)

Shifting an individual byte in the array works fine, but I then have to individually shift all the other bytes, which is quite complicated.

Dos anyone have any ideas on this?

Thank you

AARRRGH

Right, I've developed a rather complicated system where I shift each byte seperately and then alter the relevant bits which should have changed due to bits coming from the byte to its right.

Ugh.

Complicated.

Cantab.

dublindude wrote: »

Right, I've developed a rather complicated system where I shift each byte seperately and then alter the relevant bits which should have changed due to bits coming from the byte to its right.

Ugh.

Complicated.

Ugh.

Boring.

DublinDilbert

Sounds like your trying to implement DES in "C", that's why its so inefficient in a micro-controller.... in the hardware implementations (which was what is was designed for) the shifts are just hardwired at each stage....

As others have said you need to use assembly to do proper rotate instructions.

In C for each rotate you'll need to store the MSB status, perform the left shift, then re-instate the LSB based on the stored value of MSB. Just pop that into a while/for loop and robert is your mothers brother!

AARRRGH

Hi DublinDilbert

Yep, that's what I'm doing.

What complicates it is the fact that DES breaks a 56 bit key into two 28 bit halves, so when I'm doing my shifting, I have to be aware of the extra space and other things like that!

For example, when working on the right half of the first subkey, I have to firstly shift the subkey four places to the right, and then add the missing four bits (bits 29, 30, 31, 32) from the left subkey, and then shift to the left...

It's not amazingly difficult, but it's tricky enough and it's easy to make mistakes with all the bitmasks, etc.

It's good fun though, and I'm learning a lot

Thanks everyone for your help. I'll probably be back with more questions later on

DublinDilbert

dublindude wrote: »

Hi DublinDilbert

Yep, that's what I'm doing.

What complicates it is the fact that DES breaks a 56 bit key into two 28 bit halves, so when I'm doing my shifting, I have to be aware of the extra space and other things like that!

For example, when working on the right half of the first subkey, I have to firstly shift the subkey four places to the right, and then add the missing four bits (bits 29, 30, 31, 32) from the left subkey, and then shift to the left...

It's not amazingly difficult, but it's tricky enough and it's easy to make mistakes with all the bitmasks, etc.

It's good fun though, and I'm learning a lot

Thanks everyone for your help. I'll probably be back with more questions later on

Yea the key schedule is a bitch, when you try go from the text description to the implementation....

Here's a java script implementation (http://people.eku.edu/styere/Encrypt/JS-DES.html) which will let you check if your generating the correct round keys, it also displays the data after each round so you can debug your application.

My advise would be to write a function that takes a 32 bit number and prints it to std-out as a 32 binary string, then you can compare the values from your code to the values from the java script implementation above, this is essential for debugging.

lynchie

dublindude wrote: »

Hello

I've googled this and I cannot find anything.

If I want to perform one left shift on the following:

10001111

How do I make the 1 rotate to the end of the byte so it looks like:

00011111

Any ideas?

Thank you.

Quick google shows this link which has a c function to implement the rol and ror asm functions.