cmd/age-keygen: detect world-readable output file

cmd/age-keygen/keygen.go

@@ -36,6 +36,13 @@ func main() {
 		out = f
 	}
 
+	if fi, err := out.Stat(); err == nil {
+		if fi.Mode().IsRegular() && fi.Mode().Perm()&0004 != 0 {
+			fmt.Fprintf(os.Stderr, "Warning: writing to a world-readable file.\n")
+			fmt.Fprintf(os.Stderr, "Consider setting the umask to 066 and trying again.\n")
+		}
+	}
+
 	generate(out)
 }