[bitcoin-dev] BIP draft: Memo server

[Chris Priest via bitcoin-dev]

I'm currently working on a wallet called multiexplorer. You can check
it at https://multiexplorer.com/wallet

It supports all the BIPs, including the ones that lets you export and
import based on a 12 word mnemonic. This lets you easily import
addresses from one wallet to the next. For instance, you can
copy+paste your 12 word mnemonic from Coinbase CoPay into
Multiexplorer wallet and all of your address and transaction history
is imported (except CoPay doesn't support altcoins, so it will just be
your BTC balance that shows up). Its actually pretty cool, but not
everything is transferred over.

For instance, some people like to add little notes such as "paid sally
for lunch at Taco Bell", or "Paid rent" to each transaction they make
through their wallet's UI. When you export and import into another
wallet these memos are lost, as there is no way for this data to be
encoded into the mnemonic.

For my next project, I want to make a stand alone system for archiving
and serving these memos. After it's been built and every wallet
supports the system, you should be able to move from one wallet by
just copy+pasting the mnemonic into the next wallet without losing
your memos. This will make it easier for people to move off of old
wallets that may not be safe anymore, to more modern wallets with
better security features. Some people may want to switch wallets, but
since its much harder to backup memos, people may feel stuck using a
certain wallet. This is bad because it creates lock in.

I wrote up some details of how the system will work:

https://github.com/priestc/bips/blob/master/memo-server.mediawiki

Basically the memos are encrypted and then sent to a server where the
memo is stored. An API exists that allows wallets to get the memos
through an HTTPS interface. There isn't one single memo server, but
multiple memo servers all ran by different people. These memo servers
share data amongst each other through a sync process.

The specifics of how the memos will be encrypted have not been set in
stone yet. The memos will be publicly propagated, so it is important
that they are encrypted strongly. I'm not a cryptography expert, so
someone else has to decide on the scheme that is appropriate.

[Luke Dashjr via bitcoin-dev]

First of all, and most importantly, I like the idea/concept.

The first issue I see is that this scheme exposes private information in the
form of which inputs/outputs are related to the user. But IMO this information
should also be private and kept encrypted, so memo servers don't have anything
at all to leak.

Note this necessarily means you can't reuse the keys for the blockchain UTXOs
for memos. But such key reuse is also a risk that should be avoided anyway.
Instead, I suggest encrypting all the memos to an arbitrary key which is
derived from the HD seed and shouldn't ever be used for UTXOs.

Ideally, the memo server shouldn't be tied to a specific wallet schema. So the
next step is to not tell the memo server anything except your memo-specific
identifier (which can be a hash of a pubkey, or really anything at all -
there's no reason the memo server needs to know ANYTHING about the user's
wallet). Using an arbitrary identifier of sufficient length allows for future
wallet schemas to continue to use the same memo servers. (The specifics on how
to derive the identifier can be specified in a separate BIP to ensure wallets
can be compatible with each other.)

I don't think there is a real need for memo servers to sync data. It should be
sufficient for users to decide on two or more memo servers they wish to
entrust their memos with, or possibly trust only their own memo server(s).

There should probably also be a way for memos of different types. Some wallets
might only support simple memos, but others might associate more data for (eg)
proof-of-existence schemas. What types are used *might* be desirable to
encrypt as well, so this should probably be in the second "how wallets use it"
BIP.

IIRC, Electrum already has some kind of "memo server" interface in a plugin.
Have you looked at how it works, and considered its features (and/or flaws)
for your proposal?

Finally, using "?data&data&data" doesn't follow the standard
"?key=value&key=value" scheme; simple to fix.

Luke

Post by Chris Priest via bitcoin-dev
I'm currently working on a wallet called multiexplorer. You can check
it at https://multiexplorer.com/wallet
It supports all the BIPs, including the ones that lets you export and
import based on a 12 word mnemonic. This lets you easily import
addresses from one wallet to the next. For instance, you can
copy+paste your 12 word mnemonic from Coinbase CoPay into
Multiexplorer wallet and all of your address and transaction history
is imported (except CoPay doesn't support altcoins, so it will just be
your BTC balance that shows up). Its actually pretty cool, but not
everything is transferred over.
For instance, some people like to add little notes such as "paid sally
for lunch at Taco Bell", or "Paid rent" to each transaction they make
through their wallet's UI. When you export and import into another
wallet these memos are lost, as there is no way for this data to be
encoded into the mnemonic.
For my next project, I want to make a stand alone system for archiving
and serving these memos. After it's been built and every wallet
supports the system, you should be able to move from one wallet by
just copy+pasting the mnemonic into the next wallet without losing
your memos. This will make it easier for people to move off of old
wallets that may not be safe anymore, to more modern wallets with
better security features. Some people may want to switch wallets, but
since its much harder to backup memos, people may feel stuck using a
certain wallet. This is bad because it creates lock in.
https://github.com/priestc/bips/blob/master/memo-server.mediawiki
Basically the memos are encrypted and then sent to a server where the
memo is stored. An API exists that allows wallets to get the memos
through an HTTPS interface. There isn't one single memo server, but
multiple memo servers all ran by different people. These memo servers
share data amongst each other through a sync process.
The specifics of how the memos will be encrypted have not been set in
stone yet. The memos will be publicly propagated, so it is important
that they are encrypted strongly. I'm not a cryptography expert, so
someone else has to decide on the scheme that is appropriate.
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