feat: add invite funding key derivation path

dip-0009/assignments.md

@@ -2,12 +2,11 @@
 
 Here is a table of current feature paths and any associated DIP. Future DIPs may introduce more types.
 
-
 | Feature Index * | Feature | DIP Number and Name | Note |
 | ------------------ | ------------ | ------------------- | ---- |
 | `3'` | Masternode Keys | [DIP 0003: Deterministic Masternode List](https://github.com/dashpay/dips/blob/master/dip-0003.md) | The masternode related keys are located in the following sub-paths:  <br>`0'/*` - _Reserved_<br>`1'/*` - Voting Key<br>`2'/*` - Owner Key<br>`3'/*` - Operator Key<br>`4'/*` - [Platform P2P Key](../dip-0028.md#calculating-the-platform-node-id) (ED25519)<br><br>For example, the first voting key for Dash would be at `m/9'/5'/3'/1'/0` |
 | `4'` | CoinJoin Keys | CoinJoin | The masternode related keys are located in the following sub-paths:  <br>`0'/0/*` - Account 0 (external)<br>`0'/1/*` - Account 0 (change, not typically used)<br><br>For example, the first key of Account 0 for CoinJoin would be at `m/9'/5'/4'/0'/0/0`.<br><br>The main purpose of this using this path is to segregate CoinJoin-related keys from the BIP44 paths that are used to receive coins and change.  CoinJoin derivation paths may require different lookup or gap values than the BIP44 paths that have much less transaction activity.  Additionally, it may be possible to prune used keys from the BIP37 bloom filters used by mobile apps.|
-| `5'` | Identity Keys | [DIP 0013: Identities in Hierarchical Deterministic wallets](../dip-0013.md) | The related keys are located in the following sub-paths: <br>`0'/key type'/identity index'/key index'/*` - Identity Authentication ([details](../dip-0013.md#identity-authentication-keys))<br>`1'/*` - Identity Registration Funding ([details](../dip-0013.md#identity-registration-funding-keys))<br>`2'/*` - Identity Topup Funding ([details](../dip-0013.md#identity-top-up-funding-keys))<br><br>For example, the first Identity Registration Funding key for Dash would be at `m/9'/5'/5'/1'/0` |
+| `5'` | Identity Keys | [DIP 0013: Identities in Hierarchical Deterministic wallets](../dip-0013.md) | The related keys are located in the following sub-paths: <br>`0'/key type'/identity index'/key index'/*` - Identity Authentication ([details](../dip-0013.md#identity-authentication-keys))<br>`1'/*` - Identity Registration Funding ([details](../dip-0013.md#identity-registration-funding-keys))<br>`2'/*` - Identity Topup Funding ([details](../dip-0013.md#identity-top-up-funding-keys))<br>`3'/*` - Identity Invitation Funding ([details](../dip-0013.md#identity-invitatation-funding-keys))<br><br>For example, the first Identity Registration Funding key for Dash would be at `m/9'/5'/5'/1'/0` |
 | `15'` | DashPay - Incoming Funds | [DIP 0015: DashPay](../dip-0015.md#dashpay-incoming-funds-derivation-path) | The related keys are located in the following sub-paths: `/0'/account'/*`<br><br>For example, incoming funds for the first identity would be at `m/9'/5'/15'/0'/*` |
 | `16'` | DashPay - Auto Accept Proof | [DIP 0015: DashPay](../dip-0015.md#auto-accept-proof-autoacceptproof) | The related keys are located in the following sub-paths: `16'/expiration timestamp'`<br><br>For example, the key for a proof expiring at a Unix epoch time of `1605927033` would be at `m/9'/5'/16'/1605927033'` |
 

dip-0013.md

@@ -20,6 +20,7 @@
 1. [Identity Authentication keys](#identity-authentication-keys)
 1. [Identity Registration Funding keys](#identity-registration-funding-keys)
 1. [Identity Top Up Funding keys](#identity-top-up-funding-keys)
+1. [Identity Invitation Funding keys](#identity-invitation-funding-keys)
 1. [Copyright](#copyright)
 
 # Abstract
@@ -158,6 +159,25 @@ It is recommended that 30 address hashes of these keys be added to the bloom fil
 If transactions containing 25 address hashes or more have been found, the filter should be
 regenerated with 30 more.
 
+# Identity Invitation Funding keys
+
+Identity Invitation Funding keys used to fund invites are the fourth sub-feature for identities. They are always derived as ECDSA keys.
+
+We define the following levels for them:
+
+`m / purpose' / coin_type' / feature' / sub feature' / funding index' /`
+
+Sub feature is set to `3'`.
+
+If a wallet recovers an unused invitation funding transaction, the wallet should allow the user to
+attempt reusing it. Dash Platform can be queried to see if the invitation funding transaction has
+been used. It will be up to the user to determine if the invite should be resent to the original
+recipient or a new recipient.
+
+It is recommended that 30 address hashes of these keys be added to the bloom filter sent to peers.
+If transactions containing 25 address hashes or more have been found, the filter should be
+regenerated with 30 more.
+
 # Copyright
 
 Copyright (c) 2020 Dash Core Group, Inc. [Licensed under the MIT