Protocol specifications and service definitions for distributed open system applications are defined using ASN.1. Therefore, to implement an open system application likes MHS, it is necessary to have well defined encoding/decoding modules which translate ASN.1 protocol specifications into their transfer syntaxes. However, that work is usually tedius, time consuming, and error prone. In this paper, we designed and implemented a new ASN.1 tool set which includes a new ASN.1 run-time library, called HY BER/DER, and an enhanced ASN.1-to-C compiler, called HYASNC$^{+}$. HYASNC$^{+}$ automatically generates C language encoder/decoder stub files and heder files for basic ASN.1 types and subtypes defiend in X.208 recommandation, and all X400 MHS system macro definitions. And, we evaluated the performance of HYASNC$^{+}$ compiler and HY BER/DER run-tiem library, and tested the interoperability of ASN.1 run-time library.