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How would an offline transactions look like on a mobile phone? In my wallet I can not even log in with my seed if I am not online... So how can I send funds?

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Offline transaction must be understood as "not connected to a full node".

Offline transaction can occurs using the "flash channel" technology. "Flash channel" are currently not supported by the wallet and I don't see any reason to support it in the wallet.

Flash channel make sense in a specific applicative context, where one device sell a real service to another.

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There are a few approaches to achieve offline transactions:

Flash Channels

Connected to an offline full node

In this scenario Alice and Bob are not completely offline, they either have full nodes on their mobile phones or are connected any other devices that are full nodes. But they have no connection the the rest of the network.

  • Alice's and Bob's host full nodes loose connection to the rest of the network
  • Alice and Bob transact with each other as usual (with POW)
    • if at this point somebody who is online has access to Alice's or Bob's seed, they could perform a double spend to make the offline transactions made by Alice and Bob invalid
  • Alice's or Bob's host full node connects to the rest of the network again and broadcasts their transactions.

This already works today. You just have to change your host to the "offline full node".

Exchanging signatures

I'm not 100% sure about this but I think it would also work.
Please edit the question and remove these 2 lines/the whole section if you know more and I am right/wrong.

  • Alice and Bob loose connection to any full nodes but are still connected with each other
  • now Alice can build a transaction/bundle without attaching it to the Tangle (not doing POW, not doing tip selection, just inserting Bob's address, the amount and the signature)
  • Alice sends this incomplete transaction to Bob
  • once Bob connects to a full node again, he does the tip selection, the POW and publishes the transaction
    • again: if Alice performs a double spend and publishes the transaction before Bob does the offline transaction is most likely invalid

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