On Thursday, August 29, 2013 4:08:46 PM UTC-4, Stu wrote:
>
> The RSA keysize is the length of the modulus. If I generate a new key
> using the GenerateRandomWithKeySize function, then I know what the keySize
> is because I just set it. However, if I load the RSA key from a file or
> some other location, I need a way to determine what the keysize is.
>
> The most direct way would be as follows:
>
> privateKey.GetModulus().ByteCount();
>
GetModulus() returns a Crypto++ Integer. Integer has a class method
BitCount().
unsigned int BitCount () const
number of significant bits = floor(log2(abs(*this))) + 1
http://www.cryptopp.com/docs/ref/class_integer.html.
Jeff
AutoSeededRandomPool pool;
RSA::PrivateKey k1, k2, k3;
k1.GenerateRandomWithKeySize(pool, 1023);
k2.GenerateRandomWithKeySize(pool, 1024);
k3.GenerateRandomWithKeySize(pool, 1025);
cout << k1.GetModulus().BitCount() << endl;
cout << k2.GetModulus().BitCount() << endl;
cout << k3.GetModulus().BitCount() << endl;
$ ./t.exe
1023
1024
1025
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