diff --git a/library/rsa.c b/library/rsa.c
index 616f5d9..98d7770 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -149,7 +149,7 @@
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     if( mpi_msb( &ctx->E ) < 2 ||
-        mpi_msb( &ctx->E ) > 64 )
+        mpi_cmp_mpi( &ctx->E, &ctx->N ) >= 0 )
         return( POLARSSL_ERR_RSA_KEY_CHECK_FAILED );
 
     return( 0 );