C++ handling very large integers


I am using the RSA Algorithm for encryption/decryption, and in order to decrypt the files you have to deal with some pretty big values. More specifically, things like

P = C^d % n
  = 62^65 % 133

Now that is really the only calculations that ill be doing. I have tried using Matt McCutchen's BigInteger Library, but I am getting a lot of compiler errors during linking, such as:

encryption.o(.text+0x187):encryption.cpp: undefined reference to `BigInteger::BigInteger(int)'

encryption.o(.text+0x302):encryption.cpp: undefined reference to `operator<<(std::ostream&, BigInteger const&)'

encryption.o(.text$_ZNK10BigIntegermlERKS_[BigInteger::operator*(BigInteger const&) const]+0x63):encryption.cpp: undefined reference to `BigInteger::multiply(BigInteger const&, BigInteger const&)'

So I was wondering what would be the best way to go about handling the really big integers that come out of the RSA Algorithm.

I heard that a possibility would be to declare your variables as a double long, so…

long long decryptedCharacter;

but I'm not sure exactly how big of an integer that can store.

Well for example, I try to compile and run the following program using dev C++:

#include iostream

#include "bigint\BigIntegerLibrary.hh"

using namespace std;

int main()
    BigInteger a = 65536;
    cout << (a * a * a * a * a * a * a * a);
    return 0;

then I get those errors.

Derek, I thought that by including the BigIntegerLibrary.hh file, that the compiler would go through and compile all the necessary files that it will use.

How should I try and compile the program above in order to resolve the linking errors?

Best Answer

Tomek, it sounds like you aren't linking to the BigInteger code correctly. I think you should resolve this problem rather than looking for a new library. I took a look at the source, and BigInteger::BigInteger(int) is most definitely defined. A brief glance indicates that the others are as well.

The link errors you're getting imply that you are either neglecting to compile the BigInteger source, or neglecting to include the resulting object files when you link. Please note that the BigInteger source uses the "cc" extension rather than "cpp", so make sure you are compiling these files as well.

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