Blockchain relies on cryptography for its security, specifically using techniques like hash functions and digital signatures to ensure the integrity and authenticity of data. Each block in a blockchain contains a unique hash of the previous block, forming an unchangeable chain. When transactions are validated, they are encrypted and added to the blockchain, making it resistant to tampering and fraud. This decentralized approach not only enhances security but also fosters trust, as all participants in the network can verify transactions without a central authority.