Potentially lossy conversion in s_read_wincsp

[mabuchner]

The s_read_wincsp function has a size_t argument n to specify the number of random bytes to generate.

libtommath/s_mp_rand_platform.c

Line 33 in 8355b88

static mp_err s_read_wincsp(void *p, size_t n)

This argument blindly gets converted into a DWORD

libtommath/s_mp_rand_platform.c

Line 46 in 8355b88

return CryptGenRandom(hProv, (DWORD)n, (BYTE *)p) == TRUE ? MP_OKAY : MP_ERR;

According to the documentation, a DWORD stores 32-bits. However, a size_t can potentially store more than 32-bits.

For example, if someone wants to generate more than 4 GB of random data on a 64-bit system, then s_read_wincsp might silently not generate the correct amount of random data.

[sjaeckel]

a fix of this could be done when fixing #513