Steganography is a solution for covert communication and blockchain is a p2p network for data transmission, so the benefits of blockchain can be used in steganography. In this paper, we discuss the advantages of blockchain in steganography, which include the ability to embed hidden data without manual change in the original data, as well as the readiness of the blockchain platform for data transmission and storage, which eliminates the need for the Steganographer to design and implement a new platform for data transmission and storage. We have proposed two algorithms for steganography in blockchain, the first one is a high-capacity algorithm for the key and the steganography algorithm exchange and switching, and the second one is a medium-capacity algorithm for embedding hidden data. Also, by reviewing the previous three steganography schemes in blockchain, we have examined their drawback and have showed that none of them are practical schemes for steganography in blockchain. Then, we have explained the challenges of steganography in blockchain from the steganographers and steganalyzers point of view.

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Modular exponentiation is considered one of the most widely used, yet costly operations in public-key cryptography, especially for resource-constrained devices in a peer-to-peer networks such as net of IoT or sensor devices. However, it can be computed at a relatively low cost with the help of outsourcing. The first contribution of this paper is investigating the security of the Su et al.’s scheme which, our first attack confirms that the verifiability of this scheme is less than the authors’ claim and our second attack reveals that a malicious server is able to forge user signature. The second contribution of this paper is investigating the verifiability of Ding’s scheme, a scheme previously claimed to provide high verifiability, and we shows that this scheme is not verifiable. The third contribution is centered around proposing two practical and secure outsourcing schemes for outsourc-ing computations of single and composite modular exponentiation, in the single-server model. It is worth noting that both of our proposed schemes offer higher efficiency and security than the previous schemes as their verifiability are equal to one, while the recorded verifiability of the previous schemes decreases by the number of exponentiations utilized.