import (
    "crypto/rand"
    "encoding/binary"
)

func SecureShuffleDeck(d *Deck) error {
    *d = append([]Card{}, DefaultDeck...)
    n := len(*d)

    randomBytes := make([]byte, n*8)
    // crypto/rand pulls entropy from the OS CSPRNG, making shuffles unpredictable.
    _, err := rand.Read(randomBytes)
    if err != nil { return err }

    for i := n - 1; i > 0; i-- {
        offset := (n - 1 - i) * 8
        randVal := binary.BigEndian.Uint64(randomBytes[offset : offset+8])
        // Fisher–Yates: each swap index is chosen uniformly from the remaining range.
        j := int(randVal % uint64(i+1))
        (*d)[i], (*d)[j] = (*d)[j], (*d)[i]
    }
    return nil
}

Fisher–Yates shuffle powered by OS entropy via Go's crypto/rand. Read the docs