+.TP
+.B mac
+This uses a symmetric message-authentication algorithm rather than a
+digital signature. The precise message-authentication scheme used is
+determined by the
+.B mac
+attribute on the key, which defaults to
+.IB hash -hmac
+if unspecified. Use the
+.B binary
+algorithm of the
+.B key add
+command (see
+.BR key (1))
+to generate the key.