diff --git a/boot/bootutil/src/image_rsa.c b/boot/bootutil/src/image_rsa.c
index d437add..79eb61b 100644
--- a/boot/bootutil/src/image_rsa.c
+++ b/boot/bootutil/src/image_rsa.c
@@ -85,6 +85,8 @@
         return -3;
     }
 
+    ctx->len = mbedtls_mpi_size(&ctx->N);
+
     if (*p != end) {
         return -4;
     }