中文摘要：

短消息指标可以度量隐蔽信道的短消息传输能力，是信道容量的必要补充．但指标现有定义中还存在着以下问题：消息长度参数在普通信息系统中不能定量分析；信道限制机制难以同时满足传输时间和保真度两个约束：没有包含消息的敏感度信息．针对这些问题，首先通过引入短消息传输价值的概念，给出了短消息指标的新定义．在该定义中，利用价值阈值统一表示系统对信道短消息传输能力的容忍程度，并且在所采用的价值函数中引入了消息的敏感度因素．其后。基于安全实时数据库应用场景给出了结合短消息指标和信道容量的事务隐蔽信道度量和限制机制．理论分析和模拟结果表明，基于短消息指标的新定义，系统可以对隐蔽信道威胁实施全面的度量和可调节的限制．

英文摘要：

Small Message Criterion （SMC） can measure the capability of the covert channel on transmitting small messages and is a necessary complement to the capacity criterion. However, SMC＇s present definition has deficiencies. The acquirement of message length proved to be hard in the common information system. Mitigating mechanism can not simultaneously satisfy the two restrictions of message transfer time and fidelity. The criterion does not cover information of message＇s sensitivity. At first, the value function of message is introduced to represent the danger of small message transmission. Based on the value function, a new definition of SMC is presented where the threat tolerance standard of system is represented by a threshold of message value. The value function also takes message＇s sensitivity into account. A mechanism for secure real-time database scenario, which combines SMC with the channel capacity, is presented to measure and mitigate the threat of transaction covert channel. Theoretical analysis and experimental results show that with the proposed new SMC, the secure system can perform comprehensive measurement and adjustable mitigation to the threat of covert channel.