diff --git a/include/mbedtls/pk_internal.h b/include/mbedtls/pk_internal.h
index 592eb4b..4823294 100644
--- a/include/mbedtls/pk_internal.h
+++ b/include/mbedtls/pk_internal.h
@@ -82,7 +82,10 @@
     /** Interface with the debug module */
     void (*debug_func)( const void *ctx, mbedtls_pk_debug_item *items );
 
+    /** Signature size */
+    size_t (*signature_size_func)( const void *ctx );
 };
+
 #if defined(MBEDTLS_PK_RSA_ALT_SUPPORT)
 /* Container for RSA-alt */
 typedef struct