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As I was reading Bitcoin hashing algorithm, I see that a Block Header contains hashes in the form of fields like hashPrevBlock , hashMerkleRoot . I have following questions:

1) Are the transactions 100% secure? If yes, then I am wondering if Hash is the reason behind it? Since moving forward in a hash is easy than backwards.

2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated based on the Hashes ?

Please elaborate.

  • 2 identical transactions is impossible. A transaction by definition includes inputs and amounts and outputs and signatures. If all of that is identical then it's actually one and the same transaction. But yes as the transactions won't be identical the hashes won't be either so they are easily differentiated by the hashes. – Jannes Apr 30 '15 at 14:51
  • 2 identical transactions is both possible and has happened in the live blockchain before. for example, block 91842 and block 91812 both have the exact same coinbase tx. once one of these is spent then the other becomes unspendable. – mulllhausen May 20 '15 at 1:29
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1) Are the transactions 100% secure?

No.

2) Suppose there are two identical transactions happening at the same time, are those transactions are going to get differentiated based on the Hashes?

Yes. It's difficult, though, because each transaction must include the hash of the previous transaction, and so on.

It used to be much easier. A while back, it was pointed out that two coinbases could have identical hashes, and so create identical transactions down the line. See also: Why block height is required in coinbase?

  • Can you explain why its not 100% secure? Any example? – John Apr 30 '15 at 4:13
  • @John For example, a transaction that has a confirmation can lose that confirmation if a longer blockchain that does not include it appears. – Nick ODell Apr 30 '15 at 4:26

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