add captcha util

utils/rand.go

@@ -0,0 +1,20 @@
+package utils
+
+import (
+	"crypto/rand"
+)
+
+// RandomCreateBytes generate random []byte by specify chars.
+func RandomCreateBytes(n int, alphabets ...byte) []byte {
+	const alphanum = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+	var bytes = make([]byte, n)
+	rand.Read(bytes)
+	for i, b := range bytes {
+		if len(alphabets) == 0 {
+			bytes[i] = alphanum[b%byte(len(alphanum))]
+		} else {
+			bytes[i] = alphabets[b%byte(len(alphabets))]
+		}
+	}
+	return bytes
+}