Securely shuffling an array with 75 numbers in Go

Question

I have a SHA512 hex string consisting of my random input and that of the user.

Since rand.Seed only accepts a int64 I can never generate enough possibilities to shuffle an array with numbers 1-75: 75! = 2.480914e+109.

So my plan was to loop over the hex string 11 times, each time converting 10 hex characters to decimal and keep on shuffling the numbers.

func Shuffle(t []int, hex string) {
    for j := 0; j < 11; j++ {
        shuffleSeed, _ := strconv.ParseInt(hex[j*10:(j+1)*10], 16, 64)
        rand.Seed(shuffleSeed)
        for i := 1; i < len(t); i++ {
            r := rand.Intn(i + 1)
            if i != r {
                t[r], t[i] = t[i], t[r]
            }
        }
    }
}

items := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,
    16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
    31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45,
    46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
    61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75}

hex := "099b5721eaa9b5bcd87a07366aa175a1fb5b6fa6e5671b93ef443df927474198ec2ae959928f95c46f9fe9621575aa4e1e6f70780b059044da17737fcc99a322"
Shuffle(items, hex)

What do you think about this? I'm not a cryptography expert, not a math expert and certainly not a Go expert (I started yesterday).

Taken from another post I shuffled an array with [1,2,3] and here are the occurrences of each after 1 million times:

167136
166403
166877
166520
166925
166145

Answer 1

Initialization

A simple loop would be easier to maintain than hard-coding an array of 75 values.

items := make([]int, 75, 75)
for i := range items {
    items[i] = i + 1
}

Seeding

The PRNG should be seeded just once.

You are correct that an Int64 seed is "only" capable of producing 2⁶⁴ ≈ 10¹⁹ distinct sequences, which is well under 75! ≈ 10¹⁰⁹.

However, you should also consider the magnitude of the numbers involved. The age of the Universe is well under 10¹⁸ seconds. There is therefore no way you could remotely come close to producing a small fraction of the 75! possible permutations, just due to lack of time. A 64-bit seed is more than enough to generate as many unreproducible sequences as you could ever want.

So, calling rand.Seed(time.Now().UnixNano()) just once in func init() will be sufficient.