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Background: I want to implement sign message feature in Trezor.

Is there a standard for signing text messages?

So far I found out that etherscan.io signs messages without the prefix, i.e. it signs keccak(msg), but also accepts signatures "with Geth prefix" of keccak("\x19Ethereum Signed Message\n32"+keccak(msg)). Signed messages of the form keccak("\x19Ethereum Signed Message\n"+strlen(msg)+msg) are not accepted.

MyEtherWallet only accepts messages signed without any prefix.

Without the prefix one can misuse the sign message feature to sign a transaction, though. Is this intentional?

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There's no official standard (at the time of this writing) and has been debated for a while in this EIP thread EIP #683

According to issue #3731:

Geth prepends the string \x19Ethereum Signed Message:\n<length of message> to all data before signing it (https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_sign). If you want to verify such a signature from Solidity, you'll have to prepend the same string in solidity before doing the ecrecovery.

Related

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I think it's not save to always use the same prefix when signing. For example if you have deployed the smart-contract multiple times, signatures can be replayed on each instance. You should always use a custom prefix and also add the contracts address to the hashing before you sign something. For javascript you could use eth-crypto to sign signs with a custom prefix, like described in the tutorial.

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