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c++encryptionbit

C++ data type greater than 64 bits

I'm trying to write an RSA implementation and need to work with numbers that are 100 bits and larger. Are there any C++ data types that can allow for this?

Solution

  • If by "C++ data type" you mean "primitive integral type guaranteed by the standard", then no.

    If by "C++ data type" you mean "primitive integral type that actually exists on my platform", then maybe, but you'd have to tell us what your platform is, and you haven't.

    If by "C++ data type" you mean "any type usable in C++", then the answer is trivially of course, since any platform will be able to fit std::array<uint32_t, 4>. You'll have to write some code to use that like a regular integral type, though.

    The more general solution is to use a big integer, arbitrary precision or multiprecision library. For example Boost.multiprecision, but you can find lots of others now you know the correct search terms.

    Note

    Maarten Bodewes makes a good point about security that I completely ignored by just answering the C++ part. You didn't say that your RSA implementation has any security requirements at all, but just in case ...

    If you do care about it actually being safe to use in some real application, consider that

    1. 100 bits is probably much too weak, and
    2. you may have more security concerns than just the correctness of the algorithm (such as side-channel attacks and malicious input attacks).

    These are well outside the scope of this (or any other individual) question, but they deserve some thought and research. Using a multiprecision library indended specifically for cryptographic use is the minimal first step to getting this right.