Transaction records and other user information are stored in blocks that form a distributed ledger called blockchain. For blockchain to gain widespread adoption, it is crucial that user and transaction data be kept safe. This technology provides cryptographically safe and anonymous financial transactions among the user nodes of the network allowing the transactions to be evaluated and authorized by all the users in a transparent environment. In this paper, we provide a comprehensive overview and classification scheme for all cryptographic ideas currently used in blockchain. Moreover, the current security issues of blockchain are shown, and a future research objective is predicted.