# Hash that defines previous block in Blockchain vs mathematical solution

Is the hash that defines the previous block (block A) in a new block (block B), the same as the hash the miner calculated to submit block A?

As far as I know, you need to find a nonce so that the hash of the entire block A starts with zeroes. If you find a fitting nonce is this hash with the zeroes used by the next miner in his block b when he links to your block with the previous hash?

To clarify, if person A found a nonce so that the hash of his block is 000231 is the hash in the previous field of the next block also 000231?

Thanks

## 1 Answer

Yes, it is. That is the hash of the block header, so it is used as an identifier for that block, and is used to reference it in the next block in the same way that a hash of a transaction (txid) is used to refer to that transaction.

• thanks but doest this gimp the strength of the hash? since the hash of every block starts with zeroes doesnt this mean that there are less bits to define a block and doesnt this increase the likelihood of a hashcollision? Let's assume that in the future because of rising computing power the first 240 bits have to be zeroes. This would mean that there are only 2^16 hashes a block can have – Peter111 Sep 15 '17 at 14:10
• @Peter111 If we had that much hashing power, the hash would be worthless anyway. The hash isn't being gimped by the hashing power -- the zeroes are just a symptom. – David Schwartz Sep 15 '17 at 17:07
• @Peter111, all hashes are equally as hard to find a collision with. This answer might help bitcoin.stackexchange.com/a/57661/51948 – MeshCollider Sep 15 '17 at 21:00