FLIP-0200: Application of BIP 44 in Flow Wallets (#200)

* First draft of FLIP: "Application of BIP 44 in Flow Wallets"

* Update 20201125-bip-44-multi-account.md

* Update FLIP 200 based on discussion feedback

* Update account discovery to cover deleted accounts

* Update 20201125-bip-44-multi-account.md

* Update 20201125-bip-44-multi-account.md

* Update prior art

* Update flips/20201125-bip-44-multi-account.md

Co-authored-by: Rob Myers <rob@robmyers.org>

* Updates FLIP 200 with new proposal updates (#727)

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Updates FLIP 200 with new proposal updates

* Update 20201125-bip-44-multi-account.md

* mention SLIP-10

* Specify that Key & Account index gap limits are flexible (#832)

* Update 20201125-bip-44-multi-account.md

Co-authored-by: Rob Myers <rob@robmyers.org>
Co-authored-by: Jeff Doyle <jeffrey.doyle@dapperlabs.com>
Co-authored-by: Tarak Ben Youssef <50252200+tarakby@users.noreply.github.com>
Co-authored-by: Tarak Ben Youssef <tarak.benyoussef@dapperlabs.com>

flips/20201125-bip-44-multi-account.md

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+# Application of BIP 44 in Flow Wallets
+
+| Status        | Accepted |
+:-------------- |:---------|
+| **FLIP #**    | [200](https://github.com/onflow/flow/pull/200) |
+| **Author(s)** | Peter Siemens (peter@dapperlabs.com), Jeffery Doyle (jeffrey.doyle@dapperlabs.com) |
+| **Sponsor**   | Peter Siemens (peter@dapperlabs.com) |
+| **Updated**   | 2022-03-03 |
+
+## Objective
+
+This document aims to define a standard pattern for the application of [Bitcoin
+Improvement Proposal
+44](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki) (BIP 44) in
+Flow-compatible hardware and software wallets.
+
+## Motivation
+
+This proposal was prompted by the recent development of the first Flow hardware
+wallet application (Ledger) and its companion host software (FCL-Ledger-Web among
+others).
+
+In the near term, this proposal affects users of the Flow Ledger application. In
+the long term, it affects Flow wallet developers and their users.
+
+## User Benefit
+
+By following this standard, developers of existing BIP 44 crypto wallets will be
+able to reduce friction for users who wish to create accounts on Flow.
+
+By standardizing BIP 44 usage across wallets, users should be able to use their
+BIP 39 mnemonic codes across wallets to manage access to the same keys and accounts.
+
+## Design Proposal
+
+### Path Levels
+
+BIP 44 defines an implementation of [Bitcoin Improvement Proposal
+32](https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki) (BIP 32)
+that standardizes a format for hierarchical deterministic (HD) key derivation
+paths that support multiple coins and multiple accounts per coin.
+
+Flow accounts support keys on two different elliptic curves (secp256k1 and NIST P-256).
+While BIP 32 describes the key derivation process for the secp256k1 curve, [SLIP-0010]
+(https://github.com/satoshilabs/slips/blob/master/slip-0010.md) generalizes the derivation 
+process to the NIST P-256 curve. In the rest of this document, BIP 44 is applicable for 
+both elliptic curves. 
+
+The BIP 44 path format is as follows:
+
+```
+m / purpose' / coin_type' / account' / change / address_index
+```
+
+When using this format to construct or discover a Flow user wallet, the value(s)
+of each path parameter should follow the specifications below.
+
+#### Purpose
+
+Purpose is always the constant 44' (0x8000002C), as described in [BIP
+44](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#purpose).
+
+#### Coin Type
+
+Coin type is always the constant 539' (0x8000021b), the unique identifier for
+the Flow protocol, as defined in [SatoshiLabs Improvement Proposal
+44](https://github.com/satoshilabs/slips/blob/master/slip-0044.md) (SLIP 44).
+
+#### Account
+
+Account is the index of a distinct account address owned by the wallet user. No
+two account indices should be used to generate child public keys that are
+assigned to the same Flow account address. Furthermore, a single account index
+should not be used to generate keys for more than one account address.
+
+The motivation for these accounts is the same set forth in BIP 44:
+
+> Users can use these accounts to organize the funds in the same fashion as bank
+> accounts; for donation purposes (where all addresses are considered public),
+> for saving purposes, for common expenses etc.
+
+When generating a key pair to be added on a new account, use the smallest 
+account index that has not already been used to generate keys for an existing account.
+
+Unlike [BIP 44](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#account),
+new accounts should be able to be created even if previously created accounts
+have no transaction history.
+
+#### Change
+
+Unlike Bitcoin, Flow does not utilize change addresses. Change is always the
+constant 0.
+
+#### Index
+
+In BIP 44, each index generates a public key that is used to derive a unique
+address for an account. Although Flow does not derive account addresses from
+public keys, index can still be used to generate multiple public keys for a
+single account.
+
+When applying BIP 44 to Flow, each subsequent value for index should be used to
+generate a new key pair for the same account, starting from 0 and incrementing
+by 1.
+
+This index should not be confused with the on-chain account key index. It must
+not be assumed that this index will reflect the on-chain account key index for
+any key.
+
+When generating a new key pair for an account, the key index to use in the path to 
+generate the new key should be the smallest unused key index available which 
+has not yet been used to generate a key on the account.
+
+### Legacy Path
+
+There exist Flow accounts with keys assigned to them which have been generated
+by wallets using the legacy path.
+
+The legacy path has been previously used by FCL-Ledger-Web and is what has been used to
+generate keys for Ledger Flow users. As far as we are aware, this is the only
+application which has used the legacy path.
+
+The legacy path is _deprecated_ by this FLIP, and should not be used to
+generate new keys.
+
+The legacy path is:
+
+```
+m / 44' / 1' / 769 / 0 / 0
+```
+
+All keys generated with the legacy path as of today (Dec 2021) have been done so using the
+NIST P-256 elliptic curve.
+
+### Account Discovery
+
+Wallet developers should use a modified version of the original account
+discovery procedure [described in BIP
+44](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#account-discovery).
+
+Modified constants from BIP 44 include:
+
+The key index gap limit should be `5`.
+The account index gap limit should be `2`.
+(The key index gap limit and account index gap limit are flexible.
+Developers should use their best judgement when choosing a key index gap limit
+or account index gap limit.)
+
+The prescribed account discovery procedure is as follows:
+
+1. Derive the key pair using the legacy path and P-256 curve, 
+   checking its use with the public key registry. If an address is found, 
+   query the Flow network to fetch the account's details. If an account 
+   is found, remember the relationship between the path used to generate 
+   this key and the account's details.
+2. Derive the key pair(s) (starting with account index = 0 and key index = 0) 
+   using the path specified in this FLIP and each elliptic curve that
+   both you and Flow support (Flow currently supports keys generated on the
+   P-256 and secp256k1 curves).
+   Note: If you are unable to derive keys for a certain elliptic curve, 
+   you may fail to discover an account / key that the user has generated 
+   previously.
+3. Scan for each keys usage by querying the public key registry with the key and
+   elliptic curve used to generate it.
+   If the gap limit has been reached for both key index and account index, stop
+   discovery.
+    - 3.1. If no addresses are found in the registry
+        - 3.1.1 If the key index gap limit has been reached without finding any
+        addressed in the registry, then go to step 2, incrementing the account index
+        by one and starting with key index = 0 again.
+        - 3.1.2 If the key index gap limit has not been reached, go to step 2 and
+        increment the key index by one.
+    - 3.2. If one or more addresses are found, query the Flow network to fetch each
+      account's details.
+        - 3.2.1. If no accounts are found<sup>1</sup>, go to step 2, incrementing
+        the account index by one.
+        - 3.2.2. For each account found<sup>1</sup>, remember the relationship between the path
+        used to generate this key, the curve used to generate this key,
+        and the hash algorithm corresponding to this key and the account's details.
+        Then go to step 2, incrementing the key index by one.
+
+<sup>1</sup>Flow supports account deletion, meaning that an address found in the
+registry may refer to a nonexistent account. In this case the address should be
+skipped but discovery should continue.
+
+#### Account Discovery Conflict Resolution
+
+It is possible for a wallet to incorrectly apply the specifications in this FLIP
+to generate keys.
+
+Should this scenario occur, compliant wallets may be required to work around any
+such previous incorrect usage.
+
+##### Conflict Resolution: Incorrect Use of Account Index
+
+If a wallet has used the account index field incorrectly to generate keys for a
+user, conflict resolution logic may be required when attempting to generate new
+keys.
+
+When generating additional keys, the correct account index to use in the path to
+generate the additional key should be:
+
+- If adding a key to a new account:
+  - The smallest account index which has not yet been used to generate keys
+    set on any account.
+- If adding a key to an existing account:
+  - The largest account index that account discovery determines has been
+    previously used to generate existing keys set on the account, which has also
+    not been used incorrectly to generate keys set on another account.
+  - Otherwise, the smallest account index which has not yet been used to
+    generate keys set on any account.
+
+##### Conflict Resolution: Incorrect Use of Key Index
+
+When generating additional keys for an account, the correct key index to
+use in the path to generate the new key should be the smallest unused key index
+available which has not yet been used to generate a key on the account.
+
+##### Conflict Resolution: Multiple Signature & Hashing Algorithms Used for Keys Generated Using the Same Path
+
+The same derivation path should not be used with multiple combinations of signature and
+hashing algorithms to generate keys set on an account.
+
+The prescribed account discovery algorithm will find keys generated using
+all combinations of derivation paths, supported signature algorithms and supported hashing
+algorithms. However, wallets should not use the same path twice to apply
+it to multiple signature and hashing algorithms to generate keys.
+
+When generating a new key pair for an account, a new path, which has not yet been used in
+combination with any signature or hashing algorithm to generate keys set on the account,
+should be used.
+
+### Drawbacks
+
+#### Mapping Public Keys to Addresses
+
+A Flow account address is not derived from a public key and is instead assigned
+during an on-chain transaction. Flow supports a many-to-many relationship
+between accounts and public keys; multiple public keys can be added to a single
+account, and a single public key can be added to multiple accounts. 
+
+The Flow protocol maintains an index from `address => [publicKey]`, but there is
+currently no on-chain reverse mapping from `publicKey => [address]`.
+
+Implementers of this standard will require a separate source of truth, the
+public key registry, to return the associated user address for a given public
+key. This requirement is mentioned in the [related issues](#related-issues)
+section below.
+
+#### Account Key Index Alignment
+
+The Flow protocol assigns an index to each account key that is registered on
+chain. When discussing this proposal, Flow contributors considered a variation
+that would require the path index to always align with the on-chain index for
+the corresponding account.
+
+For example, a user may create an account from a seed phrase and generate public
+keys at indices 0, 1 and 2. If added to the account in order, their on-chain
+indices would also be 0, 1, 2, resulting in a clear mapping from each on-chain
+key to its corresponding derivation path.
+
+The authors of this proposal chose to omit this restriction after considering
+that, in many cases, a user may add additional public keys from other sources,
+thus breaking the perfect alignment between on-chain and key path indices. Even
+without outside keys, it is still possible for keys to be added out of order.
+
+Wallet developers or users should therefore not rely on the false assumption
+that the path index of a public key always matches its on-chain account key
+index, as doing so will cause confusion and errors.
+
+#### Potential Misuse of BIP 44
+
+One could argue that this is a misuse of the original BIP 44, BIP 43 and BIP 32
+standards based on the fact that Flow does not derive addresses directly from
+cryptographic public keys.
+
+#### Overdependence on BIP 44
+
+This proposal may also promote an overdependence on the BIP 44 standard,
+especially for wallets that do not already implement it. In some cases it may be
+better to use a new implementation or adaptation of BIP 32 that is better suited
+to the Flow account model.
+
+### Dependencies
+
+* Dependent projects: Flow Ledger application, Flow Port, FCL-Ledger-Web
+
+### Tutorials and Examples
+
+TODO: add examples 
+
+### Compatibility
+
+This proposal does not change any existing APIs. The chosen path structure is
+meant to maximize forward compatibility in a variety of use cases.
+
+The convention proposed in this document is intended for use by any wallet that
+supports the BIP 44 standard.
+
+In particular, this proposal impacts wallet software that is compatible with the
+Flow Ledger application, such as FCL-Ledger-Web.
+
+## Related Issues
+
+### Mapping Public Keys to Addresses
+
+As mentioned in the [drawbacks](#drawbacks) section, this proposal necessitates
+a public key registry that maps public keys to account addresses.
+
+## Prior Art
+
+This document outlines a specific application of the standard described by [BIP
+44](https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki). This
+standard has been widely adopted by Bitcoin, Ethereum and many other
+cryptocurrency projects and communities.
+
+[SLIP-0010](https://github.com/satoshilabs/slips/blob/master/slip-0010.md) 
+is a document that generalizes the BIP 32 key derivation process
+to other curves, in particular the NIST P-256 curve supported by Flow accounts.
+
+[SLIP 48](https://github.com/satoshilabs/slips/blob/master/slip-0048.md)
+describes an alternative to BIP 44 for Graphene-based networks which, similar to
+Flow, do not derive account addresses directly from cryptographic public keys.
+
+## Questions and Discussion Topics
+
+Should the index path parameter be used?