diff --git a/programs/pkey/rsa_sign.c b/programs/pkey/rsa_sign.c
index 118394d..e77dc92 100644
--- a/programs/pkey/rsa_sign.c
+++ b/programs/pkey/rsa_sign.c
@@ -54,7 +54,7 @@
     size_t i;
     rsa_context rsa;
     unsigned char hash[20];
-    unsigned char buf[512];
+    unsigned char buf[POLARSSL_MPI_MAX_SIZE];
 
     ret = 1;