Random bytes generation in c++11

Question

I've written a funtion to prouduce random bytes in a uint8_t type array, unless I seed this with a random_device object it works fine but when I seed there are some compiler errors.

code:

#include <algorithm>
#include <random>
#include <functional>
#include <stdint>

void generateInitVector(uint8_t IV_buff[16])
{
    using bytes_randomizer = std::independent_bits_engine<std::default_random_engine, CHAR_BIT, uint8_t>;
    std::random_device rd;
    bytes_randomizer bytes(rd);

    std::generate(std::begin(IV_buff), std::end(IV_buff), std::ref(bytes));
}

compiler errors:

1. error: no matching function for call to 'begin(uint8_t*&)'|

2. error: request for member 'begin' in '__cont', which is of non-class type 'unsigned char*'|

3. error: request for member 'begin' in '__cont', which is of non-class type 'unsigned char* const'|

4. error: no matching function for call to 'end(uint8_t*&)'|

5. error: request for member 'begin' in '__cont', which is of non-class type 'unsigned char*'|

6. error: request for member 'end' in '__cont', which is of non-class type 'unsigned char* const'|

7. error: 'class std::random_device' has no member named 'generate'|

There's catch that if I define a uint8_t type array within th function

uint8_t data[16];
std::generate(std::begin(data), std::end(data), std::ref(bytes));  //and pass this array in `generate()`.

then only error 7 remains. Any solutions to this?

Answer 1

The issue here is that even though your function looks like

void generateInitVector(uint8_t IV_buff[16])

What it really is, thanks to the array decaying into a pointer is

void generateInitVector(uint8_t * IV_buff)

So, because you have a pointer, you cannot use std::begin and std::end. What you need to do is pass the array by reference to keep it an array. That looks like

void generateInitVector(uint8_t (&IV_buff)[16])

and now you can use std::begin and std::end and you maintain the size information of the array. You can even make it generic for the size using a template like

template<std::size_t N>
void generateInitVector(uint8_t (&IV_buff)[N])
{
    using bytes_randomizer = std::independent_bits_engine<std::default_random_engine, CHAR_BIT, uint8_t>;
    std::random_device rd;
    bytes_randomizer bytes(rd);

    std::generate(std::begin(IV_buff), std::end(IV_buff), std::ref(bytes));
}

You also have an issue with

bytes_randomizer bytes(rd);

std::independent_bits_engine expects a PRNG of the type of the first template parameter. You used std::default_random_engine which is not the same as std::random_device. You will need to change one of them to match the other in order for it to compile. For example:

template<std::size_t N>
void generateInitVector(uint8_t (&IV_buff)[N])
{
    using bytes_randomizer = std::independent_bits_engine<std::default_random_engine, CHAR_BIT, uint8_t>;
    std::default_random_engine rd;
    bytes_randomizer bytes(rd);

    std::generate(std::begin(IV_buff), std::end(IV_buff), std::ref(bytes));
}

int main()
{
    uint8_t data[16];
    generateInitVector(data);
}

Compiles just fine