diff --git a/programs/pkey/rsa_decrypt.c b/programs/pkey/rsa_decrypt.c
index de16de5..2a6c15f 100644
--- a/programs/pkey/rsa_decrypt.c
+++ b/programs/pkey/rsa_decrypt.c
@@ -120,7 +120,7 @@
         goto exit;
     }
 
-    rsa.len = ( mbedtls_mpi_msb( &rsa.N ) + 7 ) >> 3;
+    rsa.len = ( mbedtls_mpi_bitlen( &rsa.N ) + 7 ) >> 3;
 
     fclose( f );