From: Roland Schmitt <Roland.Schmitt@...>
Date: 2004-08-30T19:56:44+09:00
Subject: Re:Réf. : ASCII, RSA AND BIGNUM

Tad wrotes:

> First, note that 100 base 10  = 1100100 base 2,
> 
> so that 100**100 = (100**64)  * (100**32) * (100**4)
> 
> Now calculate,
> 
> 25800**1 mod 257 = 100
> 100**2 mod 257 = 100*100 mod 257 = 235
> 100**4 mod 257 = 235*235 mod 257 = 15
> 100**8 mod 257 =  15*15  mod 257 = 225
> 100**16 mod 257 = 225*225 mod 257 = 253
> 100**32 mod 257 = 253*253 mod 257 = 16
> 100**64 mod 257 = 16 * 16 mod 257 = 256
> 
> Then,
> 
> A = ((256 * 16) mod 257) * (15 mod 257)
> 
> A= ( 241 * 15) mod 257
> 
> So,  the answer is A=17

I think that is exactly what the method ** in class BN of OpenSSL does.
When implementing RSA there other useful methods like mod_mul(), prime_fasttest() and gcd() in this class.

Regards,
Roland

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