Offline transactions

[beam-mw]

Since Mimblewimble is inherently an interactive protocol, both the sender and the receiver need to participate in creation of the transaction. This introduces a very different user interface from what we are used to in other crypto currencies. However, using the LelantusMW technology, Beam has built an ability for senders to send funds non interactively even when the receiver is not online. We called this feature offline payments and it comes in two different flavors: using vouchers and using the offline address.

First option, which will be released in 5.1, allows the user to embed a set of special keys called vouchers into an address created in the receive screen. By default, 20 vouchers are created. Since each voucher is about 100 symbols in length this number was chosen to allow the resulting address to be sent through most messengers without breaking into several messages. Each voucher is good for one transaction and when they expire, the wallet will either automatically request a new set from the receiver if it comes online, or prompt the user to ask for a new address with more tokens. The advantage of this, rather complicated, method is due to two properties. Each transaction uses a unique voucher, hence making it impossible to trace the user by trying to monitor which vouchers are used. Also, the sender can not track when funds that were sent using the voucher are spent, i.e extracted from the pool. Thus vouchers provide the maximum anonymity.

The second option, that is currently planned for 5.2 version, uses a single ‘offline’ address, which allows to send any amount of offline transactions to the same address. Due to details of how LelantusMW pool works, there can only be a very limited (and definitely capped) number of such addresses created for each wallet since each new address would require the wallet to rescan the pool for UTXOs sent to it. This option is less secure since it allows deanonymization by comparing offline addresses sent out to different senders and also the sender can see when the funds are extracted from the pool. It is however much simpler for the user and more consistent with how most other crypto currencies work, which is why we recommend it for specific user cases like donations and non-interactive pool payouts, rather than general confidential transactions.