This paper proposes an enhancement model of blockchain transactions that uses a cryptographic protocol for private and secure smart contracts. The framework is dependent on the security of homomorphic encryption (HE) and the power of zero-knowledge proofs (ZKPs), which ensure that all data related to the blockchain network is confidential, integrity-rich, and authentic in its use for smart contract operation on the network. We implement these protocols on the private Ethereum testnet and provide a performance-based evaluation of several smart contract operations. Our results indicate that although cryptographic protocols incur an average increase in computational overhead of 10% for homomorphic encryption and 30% for zero-knowledge proofs, they achieve significant improvements in security and privacy. The framework achieves near-complete security against attacks with a 100% success rate in prevention for replay attacks and man-in-the-middle attacks. It increases privacy preservation with significant data exposure reduction from very high in traditional systems to almost negligible in the proposed system. Besides that, the times taken for privacy-preserving operations like encrypted token transfers (0.32 s vs. 0.25 s without cryptography) and private voting (1.25 s vs. 0.55 s without cryptography) are not more than slightly higher than in the case of the traditional smart contracts; thus, this is a possible solution for secure blockchain transactions. The work discusses the importance of cryptographic protocols in the protection of decentralised applications while presenting insights into the future developments on blockchain privacy and security.