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Add min_final_expiry_delta to blinded route example #1131

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Jan 30, 2024
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Add min_final_expiry_delta to blinded route example #1131

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31 changes: 16 additions & 15 deletions proposals/route-blinding.md
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Original file line number Diff line number Diff line change
Expand Up @@ -253,11 +253,12 @@ receive slightly less than intended):
* `route_fee_base_msat(n+1) = (fee_base_msat(n+1) * 1000000 + route_fee_base_msat(n) * (1000000 + fee_proportional_millionths(n+1)) + 1000000 - 1) / 1000000`
* `route_fee_proportional_millionths(n+1) = ((route_fee_proportional_millionths(n) + fee_proportional_millionths(n+1)) * 1000000 + route_fee_proportional_millionths(n) * fee_proportional_millionths(n+1) + 1000000 - 1) / 1000000`

This yields the following values:
Alice wants to use a `min_final_cltv_expiry_delta` of 12 blocks, which she adds to the route's
total `cltv_expiry_delta`. This yields the following values:

* `route_fee_base_msat`: 201
* `route_fee_proportional_millionths`: 1001
* `route_cltv_expiry_delta`: 288
* `route_cltv_expiry_delta`: 300

Let's assume the current block height is 1000. Alice wants the route to be used in the next 200
blocks, so she sets `max_cltv_expiry = 1200` and adds `cltv_expiry_delta` for each hop. Alice then
Expand All @@ -269,7 +270,7 @@ transmits the following information to the sender (most likely via an invoice):
* `fee_base_msat`: 201
* `fee_proportional_millionths`: 1001
* `htlc_minimum_msat`: 1000
* `cltv_expiry_delta`: 288
* `cltv_expiry_delta`: 300
* `max_cltv_expiry`: 1200 (may be conveyed via invoice expiration, assuming 10 minute blocks)
* `allowed_features`: empty
* Encrypted data for blinded nodes:
Expand All @@ -281,13 +282,13 @@ transmits the following information to the sender (most likely via an invoice):
* `fee_base_msat`: 100
* `fee_proportional_millionths`: 500
* `htlc_minimum_msat`: 1000
* `max_cltv_expiry`: 1344
* `max_cltv_expiry`: 1356
* `encrypted_payload(carol)`:
* `outgoing_channel_id`: `scid_carol_bob`
* `fee_base_msat`: 100
* `fee_proportional_millionths`: 500
* `htlc_minimum_msat`: 1000
* `max_cltv_expiry`: 1488
* `max_cltv_expiry`: 1500

Note that the introduction point (Carol) uses the real `node_id`, not the blinded one, because the
sender needs to be able to locate this introduction point and find a route to it. The sender will
Expand All @@ -314,7 +315,7 @@ chosen by adding a random cltv offset to the current block height as described i

Erin computes the expiry that should be sent to Carol:

* `expiry = 1100 + 288 = 1388`
* `expiry = 1100 + 300 = 1400`

When a node in the blinded route receives an htlc, the onion will not contain the `amt_to_forward`
or `outgoing_cltv_value`. They will have to compute them based on the fields contained in their
Expand All @@ -323,12 +324,12 @@ or `outgoing_cltv_value`. They will have to compute them based on the fields con
For example, here is how Carol will compute the values for the htlc she relays to Bob:

* `amount = ((100302 - fee_base_msat) * 1000000 + 1000000 + fee_proportional_millionths - 1) / (1000000 + fee_proportional_millionths) = 100152 msat`
* `expiry = 1388 - cltv_expiry_delta = 1244`
* `expiry = 1400 - cltv_expiry_delta = 1256`

And here is how Bob computes the values for the htlc he relays to Alice:

* `amount = ((100152 - fee_base_msat) * 1000000 + 1000000 + fee_proportional_millionths - 1) / (1000000 + fee_proportional_millionths) = 100002 msat`
* `expiry = 1244 - cltv_expiry_delta = 1100`
* `expiry = 1256 - cltv_expiry_delta = 1112`

Note that as the rounding errors aggregate, the recipient will receive slightly more than what was
expected. The sender includes `amt_to_forward` in the onion payload for the recipient to let them
Expand All @@ -342,24 +343,24 @@ The messages exchanged will contain the following values:
| update_add_htlc | update_add_htlc | update_add_htlc | update_add_htlc |
| +--------------------------------+ | +------------------------------------------+ | +------------------------------------------+ | +--------------------------------+ |
| | amount: 100322 msat | | | amount: 100302 msat | | | amount: 100152 msat | | | amount: 100002 msat | |
| | expiry: 1412 | | | expiry: 1388 | | | expiry: 1244 | | | expiry: 1100 | |
| | expiry: 1424 | | | expiry: 1400 | | | expiry: 1256 | | | expiry: 1112 | |
| | onion_routing_packet: | | | onion_routing_packet: | | | onion_routing_packet: | | | onion_routing_packet: | |
| | +----------------------------+ | | | +--------------------------------------+ | | | +--------------------------------------+ | | | +----------------------------+ | |
| --> | | amount_fwd: 100302 msat | | --> | --> | | blinding_eph_key: E(carol) | | --> | --> | | encrypted_data: | | --> | --> | | amount_fwd: 100000 msat | | --> |
| | | outgoing_expiry: 1388 | | | | | encrypted_data: | | | | | +----------------------------------+ | | | | | outgoing_expiry: 1100 | | |
| | | outgoing_expiry: 1400 | | | | | encrypted_data: | | | | | +----------------------------------+ | | | | | outgoing_expiry: 1112 | | |
| | | scid: scid_dave_to_carol | | | | | +----------------------------------+ | | | | | | scid: scid_bob_to_alice | | | | | | encrypted_data: | | |
| | +----------------------------+ | | | | | scid: scid_carol_to_bob | | | | | | | fee_base_msat: 100 | | | | | | +-----------------------+ | | |
| | | blinding_eph_key: E(carol) | | | | | | fee_base_msat: 100 | | | | | | | fee_proportional_millionths: 500 | | | | | | | path_id: preimage | | | |
| | | encrypted_data(carol) | | | | | | fee_proportional_millionths: 500 | | | | | | | htlc_minimum_msat: 1000 | | | | | | | max_cltv_expiry: 1200 | | | |
| | +----------------------------+ | | | | | htlc_minimum_msat: 1000 | | | | | | | cltv_expiry_delta: 144 | | | | | | +-----------------------+ | | |
| | | encrypted_data(bob) | | | | | | cltv_expiry_delta: 144 | | | | | | | max_cltv_expiry: 1344 | | | | | +----------------------------+ | |
| | +----------------------------+ | | | | | max_cltv_expiry: 1488 | | | | | | +----------------------------------+ | | | | tlv_extension | |
| | | encrypted_data(bob) | | | | | | cltv_expiry_delta: 144 | | | | | | | max_cltv_expiry: 1356 | | | | | +----------------------------+ | |
| | +----------------------------+ | | | | | max_cltv_expiry: 1500 | | | | | | +----------------------------------+ | | | | tlv_extension | |
| | | amount_fwd: 100000 msat | | | | | +----------------------------------+ | | | | +--------------------------------------+ | | | +----------------------------+ | |
| | | outgoing_expiry: 1100 | | | | +--------------------------------------+ | | | | amount_fwd: 100000 msat | | | | | blinding_eph_key: E(alice) | | |
| | | encrypted_data(alice) | | | | | encrypted_data(bob) | | | | | outgoing_expiry: 1100 | | | | +----------------------------+ | |
| | | outgoing_expiry: 1112 | | | | +--------------------------------------+ | | | | amount_fwd: 100000 msat | | | | | blinding_eph_key: E(alice) | | |
| | | encrypted_data(alice) | | | | | encrypted_data(bob) | | | | | outgoing_expiry: 1112 | | | | +----------------------------+ | |
| | +----------------------------+ | | | +--------------------------------------+ | | | | encrypted_data(alice) | | | +--------------------------------+ |
| +--------------------------------+ | | | amount_fwd: 100000 msat | | | | +--------------------------------------+ | | |
| | | | outgoing_expiry: 1100 | | | | tlv_extension | | |
| | | | outgoing_expiry: 1112 | | | | tlv_extension | | |
| | | | encrypted_data(alice) | | | | +--------------------------------------+ | | |
| | | +--------------------------------------+ | | | | blinding_eph_key: E(bob) | | | |
| | +------------------------------------------+ | | +--------------------------------------+ | | |
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