Traditional voting and bidding systems largely rely on paperwork and human resources throughout the voting process, which can incur high costs in terms of both time and money. Electronic voting and electronic bidding systems can be used to reduce costs, and many new systems have been introduced. However, most systems require a powerful and trusted third party to guarantee system integrity and security. With developments in blockchain technology, research has begun to highlight the core concept of decentralization. In this study, we introduce the first decentralized electronic voting and bidding systems based on a blockchain and smart contract. We also use cryptographic techniques such as oblivious transfer and homomorphic encryptions to improve privacy protection. Our proposed systems allow voters and bidders to participate in the opening phase and improve participant anonymity, the privacy of data transmission, and data reliability and verifiability. Moreover, compared with other electronic voting and bidding systems, our systems are safer and more efficient.