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August 19th, 2004, 08:37 AM
 ajm235 Registered User Join Date: Aug 2004 Posts: 3 Thanks: 0 Thanked 0 Times in 0 Posts
RSA

Hi, i've written a programme to decrypt rsa and it works fine for small values, for example when i chose my two primes to be 3 and 7 and therefore n=3*7=21 and totient function=12.

But i have to decrypt rsa with much larger primes, 167 and 233. So n=38911, totient function =38512
This means i have to calculate 18^8423 (mod(38911))!!
which is way too big for my computer to remember, isn't it?

i'm using double variables, what kind of variables would allow me to do this calculation?

August 19th, 2004, 01:41 PM
 davekw7x Authorized User Join Date: Feb 2004 Posts: 76 Thanks: 0 Thanked 0 Times in 0 Posts

Well if you do some simple estimation, you can see that 18 to the power 8423 (which is what I think you mean by 18^8423) has a value of something like 10 to the 10573 (over 10000 decimal digits --- over 30000 bits). I think it is safe to say that no implementation of C that you are likely to have access to will have data types with this many bits. Therefore a simple calculation using built-in data types and operators is not likely.

You may find (on the web, using a search engine) C-language packages or libraries that have ways of manipulating arbitrarily large numbers (limited only by the amount of memory in your system). You may find ways of solving problems like this that use some algorithmic approach that doesn't require direct calculation of all quantities (but still require manipulation of large numbers).

Good Luck!!!!!!
Dave

Variables

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