Re: IBM press release - encryption and authentication

[Nikita Borisov]

Greg Rose writes:
>At 05:14 PM 12/11/2000 -0800, Nikita Borisov wrote:
>>But in his examples, addition mod 2^128 - 159 can be implemented rather
>>quickly:
>>
>>S_i = S_{i-1} + b [regular 128-bit addition]
>>if (b > S_i) S_i += 159
>
>Ahhh, yes, a classical example of premature optimisation. This is, of 
>course, a different definition of modular arithmetic than most people would 
>use.

Well, it _does_ find a number congruent to S_{i-1} + b mod (2^128-159),
which is one definition of modular addition.  But you're right -- unless
both sides are using this version of the algorithm, a final reduction is
necessary to find a representation in the range [0,2^128-159).  I should
have looked at his slides more carefully...

- Nikita