CIP-1855: add derivation path standard for forging tokens

CIP-1855/CIP-1855.md

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+---
+CIP: 1855
+Title: Forging policy keys for HD Wallets
+Authors: Samuel Leathers <samuel.leathers@iohk.io>, John Lotoski <john.lotoski@iohk.io>, Michael Bishop <michael.bishop@iohk.io>
+Comments-Summary: Multi-party transaction signing and key management for HD wallets.
+Comments-URI: https://github.com/cardano-foundation/CIPs/wiki/Comments:CIP-1855
+Status: Draft
+Type: Standards
+Created: 2021-06-02
+License: CC-BY-4.0
+---
+
+# Abstract
+
+This document describes how to derive forging policy keys used for minting/burning tokens.
+
+## Glossary
+
+
+Term     | Definition
+---      | ---
+HD       | Hierarchical Deterministic, refers to wallets as described in [BIP-0032].
+
+# Motivation
+
+## Overview 
+
+Forging tokens is derived from a script policy. The script policy includes hashes of keys needed to forge new tokens and must be witnessed by the keys with hashes listed. 
+
+# Specification
+
+## HD Derivation
+
+We consider the following HD derivation paths similarly to [CIP-1852]:
+
+```
+m / purpose' / coin_type' / account_ix' / role / index
+```
+
+
+To associate policy keys to a wallet, we reserve however `purpose=1855'` to reserve for policy keys for forging tokens. The coin type remains `coin_type=1815'` to identify Ada as registered in [SLIP-0044]. We use a hardened index for each policy key as derivation is not needed.
+
+We can summarize the various paths and their respective domain in the following table:
+
+| `purpose` | `coin_type` | `policy_ix`        |
+| ---       | ---         | ---                |
+| `1855'`   | `1815'`     | `[2^31 .. 2^32-1]'` |
+
+
+### Rationale
+
+- ERC20 Converter IOHK is developing needs to keep track of policy keys. Rather than having randomly generated policy keys, a policy key can be associated with a mnemonic which is easier to backup.
+- A 3rd party may want to have multiple tokens tied to same mnemonic, so we allow an index to specify the token.
+
+- We use a different purpose for mainly two reasons:
+
+  - It prevents mixing up standard wallets with policy keys used for forging.
+
+  - Using a different purpose also fits well the use-case on hardware wallets who can still rely on a single root seed to manage many types of wallets. 
+
+### Examples
+
+- `m/1855’/1815’/0’`
+- `m/1854’/1815’/1’`
+- `m/1854’/1815’/2’`
+
+
+# Backwards Compatibility
+
+N/A (no preceding implementation or design).
+
+# Reference Implementation
+
+None yet.
+
+# Related Work
+
+Description                                  | Link
+---                                          | ---
+BIP-0032 - HD Wallets                        | https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
+CIP-5 - Common Bech32 Prefixes               | https://github.com/cardano-foundation/CIPs/tree/master/CIP-0005
+CIP-1852 - Cardano HD Wallets                | https://github.com/cardano-foundation/CIPs/tree/master/CIP-1852
+A Formal Specification of the Cardano Ledger | https://hydra.iohk.io/job/Cardano/cardano-ledger-specs/shelleyLedgerSpec/latest/download-by-type/doc-pdf/ledger-spec
+
+
+# Copyright
+
+CC-BY-4.0
+
+[BIP-0032]: https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki
+[CIP-5]: https://github.com/cardano-foundation/CIPs/tree/master/CIP5
+[CIP-1852]: https://github.com/cardano-foundation/CIPs/blob/master/CIP-1852/CIP-1852.md
+[ledger-spec.pdf]: https://hydra.iohk.io/job/Cardano/cardano-ledger-specs/shelleyLedgerSpec/latest/download-by-type/doc-pdf/ledger-spec
+[SLIP-0044]: https://github.com/satoshilabs/slips/blob/master/slip-0044.md