## 1 Introduction  
The development of the Internet of Things (IoT) is an important priority in most countries around the world [1, 2]. There are billions of devices that surround our lives and it impacts our society significantly. The IoT devices are furnished with sensors and some processing power deployed in various environments. To connect the unnumbered sensors and modules, communication is indispensable in the IoT. Communication security is a requirement and a secure data stream is significant for the communications of the IoT. Because the communication of the IoT is in real time, a stream cipher with the characteristics of simplicity and high speed is suitable in the real-time communications of IoT for its security. It plays an important role in modern cryptography, specially, in IoT, mobile and wireless communications. Chaotic theorem has been applied in cryptography [3, 4, 5]. The behavior of a chaotic dynamical system has high sensitivity to initial conditions and good statistical characteristics. Because the orbits of low-dimensional chaotic systems have short periodicity, this has caused some defects, such as narrow key space [6, 7, 8]. To resolve this disadvantage, one of the solutions involves a coupled chaotic map lattice (CML). CML has more excellent chaotic characteristics, and is one of many non-linear chaotic systems [9, 10, 11, 12]. The mappings in coupled chaotic map lattices generally demonstrate chaotic behavior both in time and space. However, an original coupled chaotic map lattice also has some hidden danger [13, 14, 15]. Ref. [10] pointed out there is evidence of correlation between the contiguous points. Ref. [16] indicated that CML-based systems still suffer from limited parameter space and local chaotic behavior. The attackers could easier to crack the keystream that is generated by the original coupled chaotic map lattice. Based on CMLs, some literatures have proposed modified CMLs for security applications. [PERSON] and [PERSON] proposed a non-adjacent coupled map lattice (NCML) in which the spatial positions of coupled lattices are dynamically determined through a nonlinear Arnold cat map [17; 18; 19]. [PERSON] et al. used logistic dynamically coupled logistic mapping lattices (LDCML) to improve CMLs in image encryption [6]. Ref. [20] proposed a privacy image encryption algorithm based on a piecewise coupled map lattice with a multi dynamic coupling coefficient to image security. [PERSON] et al. [16] proposed an intermittent jumping coupled map lattice based on multiple chaotic maps for an image cryptosystem. However, most of the improved schemes are applied to image cryptosystems. These schemes require more complicated computation and larger memory than for the applications in real time communications. In this paper, we design a keystream generator for a secure real-time IoT data stream by modifying the original coupled chaotic map lattice. The proposed scheme has the advantage of simplicity, and is easy to implement. It suits the resource constrained IoT environment. To eliminate the weakness of the original coupled chaotic map lattice and promote the confusion, we add a counter as a potential solution to counter some of the shortfalls of the original CML. We implement the proposed improved CML by Matlab and analyze the proposed ciphers against known attacks, such as brute-force attack, chosen ciphertext attack, and guess-and-determine attack, and etc. For the balance performance analysis, the numbers of 0 and 1 in the keystream are almost balanced and the difference rates of differential cryptanalysis show that the proposed schemes have high key sensitivity. The experimental results of the statistical random number tests reveal that the output keystreams have a high pass rate. The proposed scheme overcomes the drawback of the original coupled map lattices and results in strong security strength. The organization of this paper is as follows.