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This question details sending a message with Ethereum :How do I send an arbitary message to an Ethereum address?. Say we have two parties P1 & P2 , if P1 sends a message to P2 how can P2 verify the message sent by P1 is what has been stored on the network/blockchain ?

Is this implicitly guaranteed by Ethereum ?

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It's basically implicit.

P1 can provide a transaction hash, block number, or even just the address they sent it from. P2, or anyone else, can then look for that transaction in the canonical blockchain--if it's not there, it didn't happen in the blockchain by definition.

"Canonical blockchain" usually refers to the longest (that is, the one with the most work proved) chain available. There's significant amount of philosophy of what exactly this means (especially when you add hardforks, soft forks, microforks...). For almost all purposes, waiting a sufficient number of blocks and then looking in the chain is definitive.

  • so verifying a message is same philosophy as verifying a payment in bitcoin, wait for multiple nodes to verify the message/payment , as more more nodes provide verification , probability the message/payment is valid exponentially increases ? – blue-sky Dec 7 '16 at 19:37
  • I wouldn't say it's exponetial, but it is the same as in bitcoin. – Matthew Schmidt Dec 7 '16 at 23:49
  • I may be missing something here but what prevents the sender sending a transaction hash with a different message, what binds the message to the transaction hash ? – blue-sky Dec 8 '16 at 12:16
  • The transaction hash comes from hashing the entire transaction. Changing a single byte will result in a different hash. – Matthew Schmidt Dec 8 '16 at 16:53
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    I'm not sure what you mean by message. The link in the question talks about putting a message in a transaction's data field. In that case, that message would be part of the transaction's hash. – Matthew Schmidt Dec 8 '16 at 22:29

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