Block Chain
Hashing is key to BlockChain as it generates a unique number for the content in question. One you have know hash of any text/file you can verify whether content has changed or not, because slightest of change will change the hash and hash generated for verification will not match with hash of original content. After stipulated time all the transaction recorded will be converted into a block which will contain transaction and hash of the transaction . Hence if someone tries to change any transaction hashkey will not match. First created block is called Genesis block which will not have address to previous block, anyblock created after first block will point to block after/before block and all blocks backward lead to genesis block. Centralised ledger is notion for centralised database/controlling authority, in this practice if controlling authority goes out of business everyone suffers, also issuing authority can do frauds as no one verifies what they are doing. Centra