The Constrained Application Protocol (CoAP) is a simple and lightweight machine-to-machine (M2M) protocol for constrained devices for use in lossy networks which offers a small memory capacity and limited processing. Designed and developed by the Internet Engineering Task Force (IETF), it functions as an application layer protocol and benefits from reliable delivery and simple congestion control. It is implemented for request/response message exchanges over the User Datagram Protocol (UDP) to support the Internet of Things (IoT). CoAP also provides a basic congestion control mechanism. In dealing with its own congestion, it relies on a fixed interval retransmission timeout (RTO) and binary exponential backoff (BEB). However, the default CoAP congestion control is considered to be unable to effectively perform group communication and observe resources, and it cannot handle rapid, frequent requests. This results in buffer overflow and packet loss. To overcome these problems, we proposed a new congestion control mechanism for CoAP Observe Group Communication, namely Congestion Control Random Early Detection (CoCo-RED), consisting of (1) determining and calculating an RTO timer, (2) a Revised Random Early Detection (RevRED) algorithm which has recently been developed and primarily based on the buffer management of TCP congestion control, and (3) a Fibonacci Pre-Increment Backoff (FPB) algorithm which waits for backoff time prior to retransmission. All the aforementioned algorithms were therefore implemented instead of the default CoAP mechanism. In this study, evaluations were carried out regarding the efficiency of the developed CoCo-RED using a Cooja simulator. The congestion control mechanism can quickly handle the changing behaviors of network communication, and thus it prevents the buffer overflow that leads to congestions. The results of our experiments indicate that CoCo-RED can control congestion more effectively than the default CoAP in every condition.
This study aims to present a modified technique for signing constant messages. In general, intruders may often steal the digital signature of a constant message with relative ease. Assuming there is a constant message that must always be signed by the signer, the digital signature must equally have a constant value. If it is communicated through an insecure channel to the recipient or verifier and is intercepted along the way by attackers, they can assume the identity of the signer and use this signature for authentication. In fact, the proposed method, Digital Signature Algorithm for Constant Message (DSACM) and DSACMV2, are the result of the combination between RSA and OTP. In addition, OTP is selected for signing and validating procedures in which the secret key must be regenerated for each process. Thus, the ciphertext is constantly changing, but the message remains fixed. Moreover, RSA is chosen to protect the transmission of the secret key across an insecure channel. The experimental findings indicate that DSACM and DSACMV2 are suitable for signing a message with a constant value because the signature is an undetermined value. Although it takes two encryption procedures and two decryption processes, the time required to generate the secret key and perform the exclusive or operation increases little. In addition, the proposed methods have the benefit that the constant message is not modified. In fact, it must be combined with an integer such as a timestamp and a random number for the other techniques for changing the ciphertext, and it cannot be signed a single time if its length exceeds the private key.
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