Explain some questions about bitcoin?

Question

bitcoin

1. All the same, there is a possibility of "forking" (finding 2 blocks) almost simultaneously. Those. it turns out that if the store accepted the payment, and the block with this transaction was thrown out as the shortest, the store will not receive payment? After all, synchronization between pools is not so fast, and if one has found a solution to the block, then the rest do not know about it for some time.
2. Block size = 1 MB ? There is a chance to try all the combinations in the block (I mean the salt) and not get the coveted hash. What happens in such a case? Is salt added? Are new transactions added to the block or something else?
3. How many transactions are in a block?

Answer 1

There is a chance to try all the combinations in the block (I mean the salt) and not get the coveted hash.

The hash is 256 bytes long. It turns out a display of 1Mb -> 256 bytes. For each set of 4194304 collisions (for a perfect hash function). There is a chance, but it is extremely small. If you can’t pick it up, then it’s not necessary to change a lot of transactions. It is enough to change their order.

Answer 2

1. There is a possibility, and such a bifurcation of a branch does not happen rarely. For a store, if it is not attacked, it goes unnoticed. Blocks at the same height have approximately one set of transactions. Well, it will be in the rejected block, and in the winning one. What's up with that store? And even if it is not in the winner, it will be in one of the following, if the transaction is valid.
The risk is only if there is a competing transaction in another block with the return of coins to yourself, or payment to another seller. But this must be done intentionally with a double-spend attack, and this is not something that is simple and cheap.
2. So far, yes, 1 mb, and this does not apply to the question. The described is not a problem, because. in addition to the nonce, the miner's local time also plays a role, at least. That is, even with the same set of transactions and with the same nonce, one miner can find a solution, while the other cannot. And this is without taking into account the fact that transactions are still thrown periodically, there are still fields that can be changed by the pool, that is, the block header itself also changes periodically.
3. When how. Transactions take up a different amount of space. You can artificially make a transaction that will not fit into a block alone, or you can shove several thousand transactions into one block if they take up little space.