Good morning list,
Most of you already know and love lightning addresses [1].
I wanted to revisit that protocol, to see how we could improve it and
fix its privacy drawbacks, while preserving the nice UX improvements
that it brings.
I have prepared a gist with three different designs that achieve those
goals [2]. I'm attaching the contents of that gist below. I'd like to
turn it into a bLIP once I collect enough feedback from the community.
I don't think we should select and implement all three options. They
show that we have a large enough design space, but I think we should
aim for simplicity of implementation and deployment. My personal choice
would be to do options 1 and 3: clients (mobile wallets) would first
make a DNS request corresponding to option 3, and if that fails, they
would fallback to option 1. Domain owners would implement only one of
those two options, depending on their DNS capabilities.
Curious to hear your thoughts!
Many thanks to Rusty and Matt who reviewed early drafts of that gist.
[1] https://lightningaddress.com/
[2] https://gist.github.com/t-bast/78fd797a7da570d293a8663908d3339b
# Lightning Address
[Lightning Address](https://lightningaddress.com/) is a very popular
protocol that brings UX improvements that users love.
We'd like to provide those UX benefits without its privacy and
security drawbacks.
## Issues with the current lightning address protocol
As described
[here](https://github.com/andrerfneves/lightning-address/blob/master/README.md),
the lightning address protocol requires payment senders to make an
HTTP request to the recipient's domain owner.
This has some inconvenient side effects:
1. The payment sender reveals their IP address to the recipient's
domain owner, who knows both the sender and the recipient.
2. The domain owner can swap invoices to steal some of the payment.
3. It introduces a dependency on DNS servers and the need for an
HTTP stack on the sender side.
We can do better and fix or mitigate some of these issues, without
compromising on UX.
We need two somewhat distinct mechanisms:
1. A way to privately obtain the `node_id` associated with a given
domain.
2. A way to privately contact that domain to obtain the recipient's
payment details.
## User story
Alice wants to pay `b...@domain.com` without any other prior
information.
She doesn't want to reveal:
* her identity to Bob (payment sender privacy)
* her identity to the manager of `domain.com <http://domain.com>`
(payment sender privacy)
* the fact that she wants to pay `b...@domain.com` to her LSP
(payment recipient privacy)
## Option 1: use DNS records to link domains to nodes
A first proposal would be to use a DNS record to obtain the
`node_id` associated with a given domain.
### Obtain a blinded path to the node associated with a domain
Domain owners add a DNS `TXT` record for their domain containing a
blinded path to their node.
They may include an empty path if they wish to directly reveal their
`node_id`.
| hostname | record type | value |
TTL |
|------------------------|-------------|---------------------|-------------|
| _lnaddress.domain.com <http://lnaddress.domain.com>. | TXT
| path:<blinded_path> | path expiry |
Alice can then make a DNS query to obtain that blinded path.
```text
Alice DNS server
| |
| dig TXT _lnaddress.domain.com <http://lnaddress.domain.com> |
|-------------------------------------------------------------------->|
| _lnaddress.domain.com
<http://lnaddress.domain.com>. IN TXT "path:c3056fb73aa623..." |
|<--------------------------------------------------------------------|
```
:question: What encoding should we use for the blinded path option?
Bech32m with the `lnp` prefix?
:warning: Alice should query that DNS record using
[DoH](https://datatracker.ietf.org/doc/html/rfc8484) for privacy.
She should also query multiple DoH servers to protect from malicious
ones.
:warning: Alice should check the AD flag is correctly set (DNSSEC).
### Obtain a Bolt 12 offer from the recipient
Now that Alice has a way to reach the node that owns Bob's domain,
she needs to contact them to obtain a Bolt 12 offer from Bob.
We use an `onion_message` for that, which has the following benefits:
* Alice doesn't reveal her identity (IP address or `node_id`) to Bob
or Bob's domain
* Alice doesn't reveal Bob's identity (IP address or `node_id`) to
her LSP
* Alice doesn't even need to know the IP address for Bob's domain's
lightning node
```text
Alice Alice's
LSP Bob's LSP Bob
| | | |
| onion_message | | |
|-------------------------------->| onion_message
| |
| | get_offer_from
=b...@domain.com | |
| |---------------------------------->| |
| | | wake_up |
| | |-------------------------------->|
| | | offer |
| | |<--------------------------------|
| | onion_message
| |
| |<----------------------------------| |
| onion_message | | |
| bob's bolt12 offer | | |
| bob's LSP signature |
| |
|<--------------------------------| | |
```
Note that Alice cannot verify that the offer she receives is really
from Bob: she has to TOFU (trust on first use).
But that's something we fundamentally cannot fix if the only
information Alice has is `b...@domain.com`.
However, Alice obtains a signed statement from Bob's LSP that
attests that `b...@domain.com` is associated with the Bolt12 offer
she receives.
If she later discovers that this was invalid, she can publish that
proof to show the world that Bob's LSP is malicious.
Otherwise, since there needs to be some out-of-band communication
where the recipient advertizes their lightning address (e.g. on
social media), some kind of verification code could be attached
(hash of the `node_id`?).
The sender's wallet could optionally add a manual verification step
of that verification code.
This would only need to be done once, since Alice can then reuse the
same offer to fetch new invoices.
### Advantages and drawbacks
The main advantage of this proposal is that it is simple,
inexpensive and relies on standard mechanisms.
Its drawback is that domain owners need to be able to publish DNS
`TXT` records, but that is widely supported.
## Option 2: use `node_announcement` to link nodes to specific domains
This proposal is only based on the lightning network, without any
dependency on DNS or HTTP stacks (apart from certificate validation).
### Obtain the `node_id` associated with a domain
We add fields to `node_announcement` to let nodes advertize which
domains they own.
Those fields would typically contain a signature of the `node_id`
using the private key for the corresponding domain TLS certificate,
along with its certificate chain.
Alice can then simply sync `node_announcement`s that contain domain
links with her LSP:
```text
Alice LSP
| |
| node_announcement(foobar.com
<http://foobar.com>) |
|<-----------------------------------------------|
| node_announcement(domain.xyz
<http://domain.xyz>) |
|<-----------------------------------------------|
| node_announcement(ln.stuff) |
|<-----------------------------------------------|
| ... |
|<-----------------------------------------------|
```
### Obtain a Bolt 12 offer from the recipient
This uses exactly the same onion message mechanism as the previous
proposal.
### Advantages and drawbacks
The main advantage of this proposal is that it relies entirely on
lightning protocol messages.
Its drawback is that Alice needs to sync some `node_announcement`s
to obtain the domain owner's `node_id`.
Alice also needs to validate the certificate chain, which is old
school annoying crypto.
It also doesn't allow domain owners to keep their `node_id` private
(which may be useful for small community-based nodes).
## Option 3: use DNS records to directly store Bolt 12 offers
Another option would be to make domain owners create one DNS `TXT`
record for each of their user, directly containing their Bolt 12 offer:
| hostname | record type | value |
TTL |
|----------------------------|-------------|---------------------|--------------|
| bob._lnaddress.domain.com <http://lnaddress.domain.com>. |
TXT | lno1qqx2n6mw2fh2... | offer expiry |
```text
Alice DNS server
| |
| dig TXT bob._lnaddress.domain.com
<http://lnaddress.domain.com> |
|-------------------------------------------------------------------->|
| bob._lnaddress.domain.com
<http://lnaddress.domain.com>. IN TXT "lno1qqx2n6mw2fh2..." |
|<--------------------------------------------------------------------|
```
:warning: Alice should query that DNS record using
[DoH](https://datatracker.ietf.org/doc/html/rfc8484) for privacy.
She should also query multiple DoH servers to protect from malicious
ones.
:warning: Alice should check the AD flag is correctly set (DNSSEC).
### Advantages and drawbacks
The main advantage of this proposal is that it is straightforward
for the sender and doesn't require any addition to the lightning
protocol.
There are some drawbacks though, mostly for the domain owner,
because they will need to create a lot of DNS records (one per user).
If they're using a cloud provider, there will be limitations in the
number of records they are allowed to create.
They may not have programmatic access to perform that operation
automatically (when a user creates their lightning address).
Cheers,
Bastien
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