Move front and back into internal

internal/back/genstring.go

@@ -0,0 +1,27 @@
+package back
+
+import (
+	"bytes"
+	"crypto/rand"
+	"math/big"
+)
+
+var chars = []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
+
+func SecureGenString(length int) (string, error) {
+	charsLength := big.NewInt(int64(len(chars)))
+	var maxN big.Int
+	maxN.Exp(charsLength, big.NewInt(int64(length)), nil)
+	n, err := rand.Int(rand.Reader, &maxN)
+	if err != nil {
+		return "", err
+	}
+
+	var buf bytes.Buffer
+	for n.Cmp(&big.Int{}) == 1 {
+		var charIdx big.Int
+		n.DivMod(n, charsLength, &charIdx)
+		_ = buf.WriteByte(chars[charIdx.Int64()])
+	}
+	return buf.String(), nil
+}