diff --git a/programs/pkey/rsa_encrypt.c b/programs/pkey/rsa_encrypt.c
index a3f9a66..8604323 100644
--- a/programs/pkey/rsa_encrypt.c
+++ b/programs/pkey/rsa_encrypt.c
@@ -114,7 +114,7 @@
         goto exit;
     }
 
-    rsa.len = ( mbedtls_mpi_msb( &rsa.N ) + 7 ) >> 3;
+    rsa.len = ( mbedtls_mpi_bitlen( &rsa.N ) + 7 ) >> 3;
 
     fclose( f );