We went through a pen test recently, and one of the low priority items that fell out of it was the use of Go’s `math/rand`

in a few places instead of the cryptographically secure `crypto/rand`

. I shouldn’t have been surprised – security people hate the use of PRNGs, even when their use is deliberate and secure. In this case I’d been using a PRNG to check whether we should enable or disable random flags, and although it wasn’t a security problem, we’d recently introduced some rand helper utilities, so I switched them over anyway.

I ran into the problem that `crypto/rand`

provides only a very minimal interface – not much more than a reader on top of the system’s version of `/dev/urandom`

. I’d previously been using `math/rand`

’s `Float64`

for flag checks and there was no equivalent.

It wasn’t too hard to put in, but doing so did make me learn a little bit about how floats are implemented. I’d previously written a helper to supplement the built-in `Int`

by providing an easy way of generating a number between 1 an N that doesn’t require diving into `math/big`

:

```
package randutil
import (
"crypto/rand"
"math/big"
)
// Intn is a shortcut for generating a random integer between 0 and
// max using crypto/rand.
func Intn(max int64) int64 {
nBig, err := rand.Int(rand.Reader, big.NewInt(max))
if err != nil {
panic(err)
}
return nBig.Int64()
}
```

We then build on that to implement `Float64`

:

```
// Float64 is a shortcut for generating a random float between 0 and
// 1 using crypto/rand.
func Float64() float64 {
return float64(Intn(1<<53)) / (1 << 53)
}
```

Put simply: generate a number between 0 and 2^{53}, then divide that by 2^{53} to get a float between 0 and 1.

Why 2^{53}? A `float64`

in Go consists of three parts:

- 1 bit sign.
- 53 bits “mantissa”, otherwise known as a
*coefficient*or*significand*. - 10 bits signed integer exponent which modifies the magnitude of the mantissa.

So a `float64`

’s primary value is within those 53 mantissa bits, which is why we use that number as the bound for our float calculation.

I stole the implementation from `math/rand`

’s `Float64`

which notes that the `float64(Intn(1<<53)) / (1 << 53)`

one-liner *would* be its implementation, if not for a concern around backwards compatibility dating back to Go 1, which ties it to a slightly more complicated version.

**Fragment**

Generating a random `float64` with `crypto/rand`

**Published**

June 2, 2022

Find me on Twitter at **@brandur**.

Did I make a mistake? Please consider sending a pull request.