Using crypto/rand for generating permutations with rand.Perm

Go has two packages for random numbers:

• crypto/rand, which provides a way to get random bytes
• math/rand, which has a nice algorithm for shuffling ints

I want to use the Perm algorithm from math/rand, but provide it with high-quality random numbers.

Since the two rand packages are part of the same standard library there should be a way to combine them in a way so that crypto/rand provides a good source of random numbers that is used by math/rand.Perm to generate a permutation.

Here (and on the Playground) is the code I wrote to connect these two packages:

package main
import (
	cryptoRand "crypto/rand"
	"encoding/binary"
	"fmt"
	mathRand "math/rand"
)
    
type cryptoSource struct{}
func (s cryptoSource) Int63() int64 {
	bytes := make([]byte, 8, 8)
	cryptoRand.Read(bytes)
	return int64(binary.BigEndian.Uint64(bytes) >> 1)
}
func (s cryptoSource) Seed(seed int64) {
	panic("seed")
}
func main() {
    rnd := mathRand.New(&cryptoSource{})
    perm := rnd.Perm(52)
	fmt.Println(perm)
}

This code works. Ideally I don't want to define the cryptoSource type myself but just stick together the two rand packages so that they work together. So is there a predefined version of this cryptoSource type somewhere?

That's basically what you need to do. It's not often that you need a cryptographically secure source of randomness for the common usage of math/rand, so there's no adaptor provided. You can make the implementation slightly more efficient by allocating the buffer space directly in the value, rather than allocating a new slice on every call. However in the unlikely event that reading the OS random source fails, this will need to panic to prevent returning invalid results.

type cryptoSource [8]byte
func (s *cryptoSource) Int63() int64 {
	_, err := cryptoRand.Read(s[:])
	if err != nil {
	    panic(err)
    }
	return int64(binary.BigEndian.Uint64(s[:]) & (1<<63 - 1))
}