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If I put something in the tx-extra field of a transaction, will it be publicly visible and unencrypted? That is, anyone can see it on the public view of the monero transactions? And I assume the tx-extra field content can be of any length?

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Yes, tx-extra can be of any length, as long as the transaction as a whole is not too large to be accepted into a block.

Only the parts of tx-extra that need to be encrypted are encrypted. For example, the transaction public key does not need to be encrypted, so that's just directly in tx-extra. But (the newer style) payment ids do need to be encrypted, so they are encrypted with the transaction shared secret that only the sender and recipient will know, before being put into tx-extra.

If you want to create your own tx-extra subfield and put sensitive information into it, then you'll have to decide for yourself how to encrypt it. If it's a bunch of text, then you'll need some kind of stream cipher. Perhaps use TweetNacl https://tweetnacl.js.org/ to encrypt what you need to store.

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    Suppose I wanted an arbitrary length message in the tx-extra to be publicly visible. Is this possible? – user2233706 Jul 16 '18 at 20:16
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    Yes you can add whatever you want into txextra, encrypted or unencrypted. However, most block explorers display only the hex representation of the data in txextra, so the person that wanted to read the text would have to jump through some hoops to display that hex as ASCII text. You'd also need to write your own software or modify the existing wallet to create a transaction with the extra information in the txextra field. There is no feature in any wallet I know of that will simply let you paste in some text to be sent with a transaction. – knaccc Jul 16 '18 at 20:21
  • Ok, so monero doesn't encrypt the tx-extra by default. – user2233706 Jul 16 '18 at 20:28
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    Monero will only encrypt if necessary the stuff it needs to put in as part of a regular transaction. If you add your own stuff, that's unencrypted unless you find a way to encrypt it. – knaccc Jul 16 '18 at 22:38

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