Add bio base64 verification

Signed-off-by: Asai Neko <sugar@sne.moe>

internal/cryptography/base64.go

@@ -0,0 +1,20 @@
+package cryptography
+
+import (
+	"encoding/base64"
+	"strings"
+)
+
+func IsBase64Std(s string) bool {
+	if s == "" {
+		return false
+	}
+
+	s = strings.TrimSpace(s)
+	if len(s)%4 != 0 {
+		return false
+	}
+
+	_, err := base64.StdEncoding.Strict().DecodeString(s)
+	return err == nil
+}