extern void exim_dkim_init(void);
extern gstring * exim_dkim_data_append(gstring *, uschar *);
-extern const uschar * exim_dkim_signing_init(uschar *, es_ctx *);
+extern const uschar * exim_dkim_signing_init(const uschar *, es_ctx *);
extern const uschar * exim_dkim_sign(es_ctx *, hashmethod, blob *, blob *);
extern const uschar * exim_dkim_verify_init(blob *, keyformat, ev_ctx *);
extern const uschar * exim_dkim_verify(ev_ctx *, hashmethod, blob *, blob *);