Validate rand key length before 3des operations

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/session/ghome.go

@@ -1,6 +1,7 @@
 package session
 
 import (
+	"errors"
 	ghomemodel "honoka-chan/internal/model/ghome"
 )
 
@@ -37,3 +38,11 @@ func (ss *Session) SetRandKey(key string) {
 		return
 	}
 }
+
+func (ss *Session) Get3DESRandKey() ([]byte, error) {
+	randKey := ss.GetRandKey()
+	if len(randKey) < 24 {
+		return nil, errors.New("invalid rand key")
+	}
+	return randKey[:24], nil
+}