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I have a basic understanding of how blockchain technologies works.

I know that in depends on the ledger where are transactions recorded and can be effectivelly verified.

No one should be able to modify the ledger because of the cryptographic hash functions collisions can't be found so fast with current options.

But from that point of view the security of the blokchain technology or use case depends on how many transacations are done through time.

For example, if I will have root record and then 5 transaction recorded in ledger and no other transaction for 1 year, would theoretically someone be able to change the ledger? Possible attacker will have enough time to find collisions.

Is that right or blockchain technologies are resistant for such scenario?

  • The numbers of confirmations matter, not the number of subsequent transactions. Bitcoin does one confirmation every 10 minutes. – paj28 Sep 29 '17 at 21:08
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Such a feat would require the attacker to find a second-order preimage (a special type of collision) in the hash function. This is generally infeasible in most cases, even with many years of computation.

If the target blockchain system were to use a hash that is broken in a way that allows second-order preimage attacks, this may allow forgery. However, most existing cases of this in standard cryptographic hash functions (e.g. SHA-1, MD5) there are specific bounds on how the collision works and little control over the data which produces a collision. As such, not only would the attacker have to find a collision, but they'd also have to find a collision that's useful.

The use of SHA256 in Bitcoin and other cryptocurrencies is interesting in this regard, as it is a Merkle-Damgård construction hash similar to MD5 and SHA-1, both of which have been proven to be broken in a number of ways. However it is important to note that the inherent weakness in Merkle-Damgård constructions (length extension) is length extension attacks, which are not useful for blockchain forgery. However, the shared construction does imply that lessons learned from breaking MD5 and SHA-1 may be transferable to SHA-256. This is largely speculation, though.

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Yes, that is theoretically possible. In fact, all cryptography that is not perfectly secure (one-Time-Pads for example are) is theoretically breakable with enough resources. But for that to happen, an attacker does not only have to build the ledger blocks but also the transactions within, to fork off a separate blockchain from that root, both of which is currently not practically feasible.

Also, changing the ledger has no value when there are no transactions - and timely generation of new blocks is guaranteed by different measures with different block chains.

Please also note that "block chain" is not equal to "bit coin block chain", nor exclusively used in the context of crypto currency transactions and there are other ways of agreement on what the actual leger is outside the bitcoin world other than "longest chain wins". In fact, that Szenario can be easily defeated when a block chain is designed that is to be expecting low traffic.

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theoretically it's possible, but then bear in mind only thing which you can change is last block. after you do that, then you need to gain access to remaining nodes which can be run by different institutions. Bear in mind some of the blockchain might have recovery mechanism build in, when they detect someone is tempering with one of the nodes.

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