# Tag Info

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You don't need to assume block hashes will be unique for a given blocks chain. If you are fearful it could cause a collision, then assume block hashes will be unique for a given blocks chain and for a block mining date and time. That's going to fix your problem, which is to correctly model blocks and block chains, in your application's data layer. Then, when ...

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"What you're really asking about is the collision resistance of SHA-256. A collision has never been found ..." Correct me if I am wrong, but in my opinion although extremely unlikely, collisions must exist because the header is 80 byte = 640 bits, which is less than the hash length of 256 bits.

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Yes, I believe there can be many nonce values that produce a hash whose numeric value is less than the current target. Particularly as other data can be varied too. The probability obviously depends on the target. The order in which a miner tries nonce values is not prescribed but is any order chosen by the miner. See also How does solving a block work in ...

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Security levels First of all, the relation between security level and the number of bits involved is non-trivial, and depends on the sort of attack we're talking about. For what follows, there are roughly 3 ones that matter: preimage resistance of a hash function: This is how hard it is for an attacker to find a piece of data that hashes to a given hash. So,...

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In Bitcoin's consensus rules (block and transaction validity rules): Transactions hashes (txids and wtxids) are computed as double-SHA256 of their serialization (without and with witness data, respectively). Block Merkle roots are computed by repeatedly pairwise double-SHA256 hashing, starting from the list of txids in a block, until only a single element ...

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If the number of leaves is exactly a power of two (i.e., n = 2k), then the number of hashes performed is exactly n-1. This is easy to see: every hash operation takes two inputs, and reduces them to a single one. After n-1 operations, that means n nodes are reduced to 1, and that one is the Merkle root. When the number of leaves is not a power of two, it's a ...

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