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# Timechain and Proof-of-Work
*The network timestamps transactions by hashing them into an ongoing chain of hash-based proof-of-work, forming a record that cannot be changed without redoing the proof-of-work.*
\- Satoshi Nakamoto, Bitcoin Whitepaper
The **timechain** is a term that encapsulates the nature of the Bitcoin blockchain as **a sequential chain of timestamped events in history**. As transactions are received into the network, nodes capture and collate them into logs, known as **blocks**. Each block contains a timestamp applied to a sequential list of transactions, representing a consensus agreement on the existence and validity of all transactions contained within.
**Proof-of-work** is the mechanism that determines who gets to update transactions on the Bitcoin blockchain. To gain the right to update the next block of transactions, **a node must provide proof that it has solved a computational challenge that is difficult yet easily verifiable by the network**. This process is akin to completing a jigsaw puzzle; while it may be challenging to solve, once completed, it is straightforward for others to validate.
As new transactions are received, **nodes compile them into a block template containing all accepted transactions** that have not yet been included in a valid block. They then perform h**ash-based work on a difficulty puzzle** that must be solved to form a valid block. The hashing process involves taking an input of data of any length and transforming it into a repeatable yet essentially random output of a fixed length. In Bitcoin, the hashing algorithm used is **SHA-256**, which produces a fixed-length output of 256 bits.
The network **adjusts the puzzle's difficulty** to maintain an average block time of **approximately 10 minutes**. As more nodes contribute their CPU power to the pool performing proof-of-work, the puzzle becomes increasingly challenging to solve. Consequently, **altering blocks that have had several subsequent blocks built on top of them becomes nearly impossible due to the accumulation of Proof-of-work.**
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