Double hash in Bitcoin

what exactly is this? Does this mean every TX gets hashed twice? How does it help to filter the content from the block chain (As Dr. Wright said)? Why cant i hash it just once and save the hash of the TX and prune the data (illegal)? Isnt the content now filtered with just a single hash? or am i getting it wrong?

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CSW on double hash - scaleable ASICs

…”If you start to think about it, you will see that where:

A == Hash(X),


B == Hash(A)

allows us to have the value (A) sent to a miner in a way that doesn’t allow them to broadcast a block without individual transactions. In a Merkle tree structure, we could have all of the block sent and solved apart from a single transaction. Doing so would allow us to construct an ASIC mining facility that does not host the data in blocks. The miner is now a distributed function.

As such, the proof-of-work and the verification can be handed off to specialised entities. The ASIC facility can pay the verification facility upon discovery of valid blocks, and yet the system allows them to interact without being able to cheat either party. In fact, they can even have agreements with different miners and pools and allow it to occur without worry: if one party cheats, both lose out.”…

P.S. sha256d does not add to security of the hash as such.

Block Hash

While the previous answer is very detailed, I would like to add that double hash is a reference to the block hashing algorithm. Simplified, you can say that the block header is hashed (which also contains the hash tree reference to all the TXes in the block). This has is then taken and hashed again. This block hash is acceptable as long as it meets the criteria. One of them is difficulty. If the hash is a too large number, it is rejected, so miners also don’t broadcast it.

Transactions are not double hashed according to my knowledge.

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