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Cigguh

RSA and Elgamal signatures

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Hi. I'm looking for simple RSA and Elgamal sources in c# or c++. Everything i've found on the net was uncommented or just has miles of code. It's for my friend so i don't really need (and don't want) to start from scratch, plus program should be done in few days. Thanx for your help.

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RSA itself isn't that much code.
The problem is the large numbers that you need to use to make it secure.
I'd look for a big-num library that can handle the number of bits that you need, it probably already got the functions needed for RSA if not there's only a few that need to be implemented (and they are not that complicated).
The "problem" with all the implementations out there is that they are based on some quite heavy-weight big-num libraries (often written in C with a C++ wrapper).
I actually wrote my own library in a few days, only supporting basic operations and only unsigned numbers (wich is fine for RSA).

I'm sorry but I'm not allowed to give away my code, but it's got quite a few dependecies on the rest of the code base so it's not that suitable anyway.

But my recommendation is to search for a light-weight big-num libary instead (maybe C# have one built in?).

Once you have that the RSA part is fairly easy, this is my implementation:

class Rsa
{
public:
template <class T> struct Key
{
T m_exponent;
T m_modulo;
};
template <class T> static void genKeys(Key<T>& publicKey, Key<T>& privateKey, Random* const rng = null)
{
static const unsigned int maxBits = (sizeof(T) * 4);
static const unsigned int minBits = ((sizeof(T) * 33) >> 4);
static const unsigned int eBits = ((minBits + 3) >> 2) + 1;
T e = PrimeSearch::find<T>(eBits, maxBits, rng);
T p;
do {
p = PrimeSearch::find<T>(minBits, maxBits, rng);
}while ((p % e) == T(1));
unsigned int bits = p.highestUsedBit();
T q;
do {
q = PrimeSearch::find<T>(maxBits - bits + 1, maxBits - bits + 24, rng);
}while (((q % e) == T(1)) && ((p * q).highestUsedBit() <= maxBits) && ((p * q).highestUsedBit() >= (maxBits + 24)));
T n = p * q;
T m = (q - T(1)) * (p - T(1));
T d = e.inv(m);
T nd = m - d;
T test = q + T(7);
T k = test.powModN(e, n);
T de = k.powModN(d, n);
if (de != test)
d = nd;
publicKey.m_exponent = e;
publicKey.m_modulo = n;
privateKey.m_exponent = d;
privateKey.m_modulo = n;
}
template <class T> static T crypt(const T& m, const Key<T>& key)
{
return (m + (key.m_modulo >> T(2))).powModN(key.m_exponent, key.m_modulo);
}
template <class T> static T decrypt(const T& m, const Key<T>& key)
{
return m.powModN(key.m_exponent, key.m_modulo) - (key.m_modulo >> T(2));
}
};


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Check out the Crypto Toolset for Python. I can't recall, but I guess you'll find the C sources there as well.

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