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RC6 Integer operations in modulo 32 between two 32-bit blocks

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tomneil

I am new to cryptography and I am trying to code the RC6 (Rivest cipher 6) algorithm. The algorithm requires addition, subtraction and multiplication in modulo 232. If I am performing these operations between two 32-bit blocks how would this work?

Any help would be appreciated because I can't seem to find any detailed explanation on this which would help me write code on how to execute these operations.

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Daniel S avatar Crypto
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Daniel S

This will depend on the language that you implement in. Java and other C-like languages have a built-in data type to represent unsigned 32-bit integers (this is why RC6 chose to use this form of arithmetic, so that its implementation in these languages is relatively straightforward). In such cases +, -, and * all automatically work mod $2^{32}$.

If you're using python, you can simply use the % operator which returns remainders mod whatever value is specified e.g. a=(b+c)%(2**32).

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A. Hersean avatar
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A. Hersean
Unsigned integers are not supported by Java, except for bytes. However the operations +, -, * and left shift behave the same on signed and unsigned integers.
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dave_thompson_085 avatar
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dave_thompson_085
@A.Hersean: you mean `char`s; Java `byte` is signed, which is a big nuisance in crypto code where you must (remember to) `&0xFF` or `(byte)` on nearly all references --although the JIT-compiler may optimize these into something like MOV.B. (But the types really used in the stack machine, `int` and `long`, are defined as exact-size twos-complement with wraparound, so yes they are equivalent to unsigned for the operations you list.)
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tomneil
@A.Hersean How would I use modulo 2^32 in Verilog?
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tomneil
@dave_thompson_085 Do you know if `&0xFF` is needed in Verilog and how I can implement modulo 2^32 arithmetic?
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dave_thompson_085 avatar
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dave_thompson_085
@tomneil: I know nothing at all about Verilog and cannot help you. If that is important to your question, it should be _in_ your question.
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fgrieu avatar Crypto
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fgrieu

You want to work modulo $2^{32}$, except for shift counts where that should be modulo $32$.

The following is generic and works in Python.

code (result in z) operation
z = (x+y)&0xffffffff 32-bit addition of x and y
z = (x-y)&0xffffffff 32-bit subtraction x minus y
z = (x*y)&0xffffffff 32-bit multiplication of x and y
z = ((x<<(31&y))|(x>>(31&-y)))&0xffffffff 32-bit left-rotation of x by low 5 bits of y

In modern C or C++, use variables of type uint32_t defined in header <stdint.h> or <cstdint>, and optionally remove the &0xffffffff.

In Java, use variables of type int, remove the &0xffffffff, change >> to >>>.

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tomneil
So if I was trying to do this in Verilog, I would also have to use variable of type uint32_t and define <stdint.h>? Then I could use the 32-bit operators by just using conventional x+y, x-y, x*y and ((x<<(31&y))|(x>>(31&-y)))?
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fgrieu avatar
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fgrieu
@tomneil: my guess is if it compiles, it will work, and there's a chance it's not grossly inefficient thanks to automatic optimizations. But then my only contact with Verilog is once helping someone using it.
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