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The hash of a block consists of three parts :
Previous Header: The hash value of the previous block guarantees that the blocks are connected to each other and cannot be changed, because every block is based on the previous block.
Merkle Root: The Merkle Root is the result of bundled transactions that have been encrypted by the SHA-256 algorithm.
Nonce: The Nonce is an 8-character numerical sequence that must be discovered by the miner to create the block. The nonce must be chosen in a way that the resulting hash starts with a certain number of zeros.
A hash function is a one-way function. With the knowledge of the three parts (Previous Header, Merkle Root, Nonce) it is easy to create a hash. However, if the nonce is unknown, it is only possible to find a suitable nonce by generating random combinations of numbers .