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What happens on hash collisions for, e.g., transactions, blocks, and contracts?

What happens if, say, two transactions hash to the same value? In this case, we say that a hash-collision occured.

I've tried looking at the yellowpaper.io for an explanation, but can't really find the details.

Since the transaction hash serves as the transaction's identity, I don't see how Ethereum should allow multiple transactions with the same hash?

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Transaction hashes are 64 hexadecimal characters. This means there are 16 ^ 64 = 115792089237316195423570985008687907853269984665640564039457584007913129639936 possible transaction hashes.

Currently, 77752349 Ethereum transactions have occurred. If you create a new transaction now, the chance of its hash colliding with a previous transaction is:

77752349 / 16^64 = 0.000000000000000000000000000000000000000000000000000000000000000000067148 %

You are more likely to win the jackpot in a national lottery 8 times in a row.

This doesn't really answer your question, but just looking at the probability we have better things to worry about :-)

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    Thanks, so if a collision occur, the EVM specification doesn't mention what should happen?
    – Shuzheng
    Nov 5, 2017 at 10:25
  • @Shuzheng It doesn't have to mention anything because the chance is so small. If you want to know for sure, you will have to read the source code, or ask a wallet developer. It may vary by implementation: geth may do something different from Parity. I would guess that the transaction would fail. In that situation, you would have to send a different transaction first to increment the nonce of your address. Then, you can do the transaction you wanted to do because it will have a different hash.
    – Jesbus
    Nov 5, 2017 at 10:35
  • That makes sense to increase the nonce and try again. I don't agree that it doesn't have to mention it, since the probability is small. Would you like the whole blockchain to go into a corrupted state with small probability?
    – Shuzheng
    Nov 5, 2017 at 10:37
  • @Shuzheng There is a much larger chance that a huge electromagnetic eruption on the sun will wipe the blockchain from all hard drives. The specification doesn't have to mention this either.
    – Jesbus
    Nov 5, 2017 at 10:38
  • OK! - you get a +1, and I will accept the answer, if no others can say what the specification suggests :-)
    – Shuzheng
    Nov 5, 2017 at 10:41

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