## 1 Introduction  
In recent decades, computers have been widely adopted in industrial control systems, which lead to an extremely increment of industrial automation. Especially after the proposal of OPC (OLE for Process Control) specification, which builds a bridge between Windows applications and field process control applications [1; 2], it is possible for industrial control networks to communicate with external networks directly, even they can connect to the Internet. However, the OPC specification lacks essential security mechanisms, thus it on one hand makes it convenient for people managing or acquiring data from field devices, on the other hand the security threats from internal and external industrial control networks become much more serious, such as information exposure and control command tampering caused by trojans, viruses or other network attacks [3, 4]. For this reason, OPC Foundation proposed OPC UA (OPC Unified Architecture) specification in August, 2006 which expatiates the security model of OPC UA. This security model mainly guarantees confidentiality, integrity and authentication of transmission in industrial control networks [5, 6, 7, 8, 9]. There are three security profiles included: basic128 rsa15, basic256 and none [10, 11, 12, 13, 14]. Because both basic128 rsa15 and basic256 use AES to encrypt the content of communication in the network, the time of encryption increases along with the increment of message length. However, industrial control systems have high requirement in aspect of real-time. For process control loops, the decrease of real-time will cause delay of control. We take power system as example. The transmission delay time requirement for power system relay protection signals is less than 5 ms, thereby power system is very sensitive to time delay [15, 16]. When the transmission delay time of the control instruction is longer than the specified value, it may result in frequency instability by executing insufficient or excessive load curtailments, and even may cause power failure [17, 18, 19, 20]. For the two security profiles adopted by OPC UA security model, basic128 rsa15 and basic256, according to our experiment shown in Chapter 4, the delay time caused by them are all longer than 5 ms when the data volume transmitted is bigger than 8192 bytes, such a delay time is unacceptable to network communication; therefore, these two security profiles are not efficient enough to be used in practical application [12, 14]. Reference [14] advises the inclusion of a compromising scheme that authenticates data only into OPC UA specification to protect network communication and have a lower overhead at the same time. Both basic128 rsa15 and basic256 adopt SHA-1 to calculate message digest for messages to be transferred. However, security problems lie in this hash function. According to Reference [21], the speed of deciphering is about 2000 times faster than the prospective speed. Moreover, because the computer is becoming more powerful and cheaper, the security of SHA-1 is reducing day by day. Thereby, the security of SHA-1 is questioned by cipher analysts and vendors. Microsoft and Google have announced the plan to abandon SHA-1 and Microsoft will not trust signatures using SHA-1 from January 2016. NIST (National Institute of Standards and Technology) had announced the step-by-step elimination of this hash function. Aims at solving the problem of low communication efficiency and security occurred in OPC UA, this paper proposes a security communication model of OPC UA. This model checks the integrity of communication in networks using the Whirlpool hash function and authenticates the source of communication using X.509 specification. Compared to SHA-1 which is currently used in OPC UA, Whirlpool has a higher calculation speed and lower collision rate.