## 2 Literature Review
After a node has sensed some data, it will generate a message authentication polynomial (MAP) and append it to the measurement to be forwarded to the base station. The scheme only provides end-to-end integrity as it evaluates the integrity at the BS. IA-CTR [23] is an integrity-aware conventional counter mode that is a variant of the conventional counter mode. This mode can provide message integrity and privacy through using a shared counter during block encryption algorithm. This technique does not actually require any additional computational overhead, but using counter mode encryption is not recommended as it is malleable, i.e., the attacker can manipulate the ciphertext so that it decrypts to any message of his choice [24]. Another type of integrity mechanism is using watermarking techniques. There are three types of digital watermarks: robust, fragile, and semi-fragile watermarks [25]. The first two types have been the subject of research in wireless sensor networks. Semi-fragile watermarking is not typically used in WSNs. Work on using watermarking techniques for integrity and authentication in WSNs began in 2003. [PERSON] et al. [26] proposed the first watermarking system that embeds cryptographically encoded signatures into data. Fragile watermarking is used to provide data integrity authentication through embedding a watermark into the original data. [PERSON] et al. [27] have developed a hop-by-hop fragile watermarking technique. They have assumed the network to be homogeneous and formed as a tree with each cluster head to have at most three children. The technique uses data aggregation to reduce communication overhead. The watermark size is only one bit, and the generation process is very simple by using the exclusive or (XOR) operator. The embedding is done at the least significant bit of the data. The technique is simple but not secure. A cross-layer watermarking-based data aggregation (CLWDA) [28] is another fragile watermarking mechanism that adopts two types of watermarks. The first is called a fragile watermark (FW), which is used to validate data sent from the first level of the network. The higher level of the network (aggregator nodes) uses a reinforced fragile watermark (RW), which is more secure as the watermark is encrypted using the asymmetric key. The mechanism adopts a different embedding approach than most of the proposed schemes. The embedding position of the watermark will be dynamic, and it will be calculated based on the wakeup interval used by each node. Although this makes it somewhat random, it can be easily detected as it will form a pattern. The CLWDA mechanism also requires synchronization between nodes to determine wake-up times. A watermarking algorithm based on a one-way hash function was proposed by [PERSON] et al. [29]. The watermark is computed for each byte of data, and then grouped using the XOR function. The final watermark is then divided between the bytes and embedded in predefined redundant spaces. The proposed algorithm is computationally simple. However, it only provides end-to-end integrity. [PERSON] et al. [30] had proposed a zero-watermarking technique (ZWT) that generates a watermark from the properties of the, sensed data. After encrypting the watermark using a shared secret key, the sensor embeds the encrypted watermark at the end of the sensed data before it is sent to the base station. ZWT was tested against other watermarking techniques, and the results showed that it does not introduce any computation overhead as the generation process is simple. ZWT is vulnerable to many security breaches in the case of exposing the shared secret key as it uses only one key for the whole network. Watermarking-LEACH [31] is an integrity-preserving approach that adds a watermark on top of the LEACH routing protocol. It modifies the original approach of LEACH by adding watermarking embedding at the cluster heads and watermark extraction at the BS. This approach involves minimal modification to a very well-known routing protocol.