iotaledger · PhilippGackstatter · May 12, 2023 · May 12, 2023 · May 16, 2023 · Aug 14, 2023
diff --git a/tips/TIP-0052/tip-0052.md b/tips/TIP-0052/tip-0052.md
@@ -0,0 +1,345 @@
+---
+tip: 52
+title: Multi Address
+description: Defines an Address that is owned by multiple other addresses.
+author:
+  Eike Haß (@eike-hass ) <eike.hass@iota.org>, Max Hase (@muXxer) <maximilian.hase@iota.org>, Philipp Gackstatter
+  (@PhilippGackstatter) <philipp.gackstatter@iota.org>
+discussions-to: TODO
+status: Draft
+type: Standards
+layer: Core
+created: 2023-09-07
+---
+
+# Summary
+
+This TIP proposes a new Address type to allow multiple addresses to create complex multi-signature-like unlock
+conditions for outputs. The address is considered unlocked as soon as the cumulative weight of all unlocked contained
+addresses matches or exceeds the configured threshold, where each of the addresses has an assigned weight.
+
+# Motivation
+
+The current outputs in Stardust can be unlocked by single addresses, which does not allow outputs to be unlocked by
+multiple different entities, except with Ed25519 Threshold Signatures. The latter allows to model multi-control
+structures, but it is not transparent who the underlying controlling addresses are and in particular, they do not allow
+different Account or NFT Addresses to control an output. The _Multi Address_ introduced in this TIP allows for any other
+address type to control an output in a multi-control structure, in various configurations. The W3C DID Specification
+distinguishes between [independent control](https://www.w3.org/TR/did-core/#independent-control) and
+[group control](https://www.w3.org/TR/did-core/#group-control) which is a useful definition in this context. With the
+current approach only independent control can be modelled. Allowing for group control by multiple inspectable
+controlling entities might be useful in the following scenarios:
+
+- Modelling multiple W3C DID controllers.
+- Transparent modelling of ownership over funds or assets of entities like organizations.
+- Transparent modelling of committees for anchoring (e.g. L2)
+
+A _Multi Address_ allows for use-cases like:
+
+- Requiring a signature by a hardware security module (HSM) and a signature by one of two trusted devices through
+  software to unlock.
+- Requiring signatures of a supermajority of controlling parties each of which are an Account to unlock.
+- Modelling a 1 of n control structure in addition to a single backup address that has full control.
+- Allowing interested parties to inspect the conditions for unlocking, e.g. majority or supermajority.
+
+# Multi Address
+
+The following table shows the mapping from the address type of the **first byte** to the address type:
+
+| Address       | Type Byte as `uint8` | Bech32 Encoded |
+| ------------- | -------------------- | -------------- |
+| Multi Address | 40                   | iota1**q**...  |
+
+The following table shows the serialization of a _Multi Address_:
+
+<details>
+  <summary>Multi Address</summary>
+  <blockquote>Defines a Multi Address that consists of addresses with weights and a threshold value. The Multi Address can be unlocked if the cumulative weight of all unlocked addresses is equal to or exceeds the threshold.</blockquote>
+</details>
+<table>
+  <tr>
+    <td>
+      <b>Name</b>
+    </td>
+    <td>
+      <b>Type</b>
+    </td>
+    <td>
+      <b>Description</b>
+    </td>
+  </tr>
+  <tr>
+    <td>Address Type</td>
+    <td>uint8</td>
+    <td>Set to <strong>value 40</strong> to denote a <i>Multi Address</i>.</td>
+  </tr>
+  <tr>
+    <td>Addresses Count</td>
+    <td>uint8</td>
+    <td>The number of addresses following.</td>
+  </tr>
+  <tr>
+    <td valign="top">Addresses <code>anyOf</code></td>
+    <td colspan="2">
+      <details>
+        <summary>Weighted Address</summary>
+        <blockquote>An Address with an assigned weight.</blockquote>
+        <table>
+          <tr>
+            <td>
+              <b>Name</b>
+            </td>
+            <td>
+              <b>Type</b>
+            </td>
+            <td>
+              <b>Description</b>
+            </td>
+          </tr>
+          <tr>
+            <td valign="top">Address <code>oneOf</code></td>
+            <td colspan="2">
+              <details>
+                <summary>Ed25519 Address</summary>
+                <blockquote>An Address derived from an Ed25519 Public Key. Defined in <a href='../TIP-0038/tip-0038.md#ed25519-address'>TIP-38 (Ed25519 Address)</a>.</blockquote>
+              </details>
+              <details>
+                <summary>Account Address</summary>
+                <blockquote>An Address derived from an Account ID which can be unlocked by unlocking the corresponding Account. Defined in <a href='../TIP-0038/tip-0038.md#account-address'>TIP-38 (Account Address)</a>.</blockquote>
+              </details>
+              <details>
+                <summary>NFT Address</summary>
+                <blockquote>An Address derived from an NFT ID which can be unlocked by unlocking the corresponding NFT. Defined in <a href='../TIP-0038/tip-0038.md#nft-address'>TIP-38 (NFT Address)</a>.</blockquote>
+              </details>
+              <details>
+                <summary>Anchor Address</summary>
+                <blockquote>An Address derived from an Anchor ID which can be unlocked by unlocking the corresponding Anchor. Defined in <a href='../TIP-0038/tip-0038.md#anchor-address'>TIP-38 (Anchor Address)</a>.</blockquote>
+              </details>
+            </td>
+          </tr>
+          <tr>
+            <td>Weight</td>
+            <td>uint8</td>
+            <td>The weight of the unlocked address.</td>
+          </tr>
+        </table>
+      </details>
+    </td>
+  </tr>
+  <tr>
+    <td>Threshold</td>
+    <td>uint16</td>
+    <td>The threshold that needs to be reached by the unlocked addresses in order to unlock the Multi Address.</td>
+  </tr>
+</table>
+
+A _Multi Address_ is a container for other addresses. Each address has weight and the Multi Address contains a threshold
+`Threshold` that needs to be reached by the unlocked addresses in order to unlock the Multi Address.
+
+## Additional syntactic transaction validation rules
+
+A Multi Address is only valid if all of the following conditions hold:
+
+- `1 <= Addresses Count <= 10`.
+- The addresses must be lexically ordered and unique based on the contained serialized address.
+- `Weight >= 1`, for each `Weight` of contained `Addresses`.
+- `Cumulative Weight >= Threshold`, where `Cumulative Weight` is the sum of weight of all `Weight`s in `Addresses`.
+- `Threshold >= 1`.
+
+## Unlocks
+
+### Multi Unlock
+
+<details>
+  <summary>Multi Unlock</summary>
+  <blockquote>Unlocks a Multi Address with a list of other unlocks.</blockquote>
+</details>
+<table>
+  <tr>
+    <td>
+      <b>Name</b>
+    </td>
+    <td>
+      <b>Type</b>
+    </td>
+    <td>
+      <b>Description</b>
+    </td>
+  </tr>
+  <tr>
+    <td>Unlock Type</td>
+    <td>uint8</td>
+    <td>Set to <strong>value 5</strong> to denote a <i>Multi Unlock</i>.</td>
+  </tr>
+  <tr>
+    <td>Unlocks Count</td>
+    <td>uint8</td>
+    <td>The number of unlocks following.</td>
+  </tr>
+  <tr>
+    <td valign="top">Unlocks <code>anyOf</code></td>
+    <td colspan="2">
+      <details>
+        <summary>Signature Unlock</summary>
+        <blockquote>Unlocks the address derived from the contained Public Key in the transaction in which it is contained in. Defined in <a href='../TIP-0045/tip-0045.md#signature-unlock'>TIP-45 (Signature Unlock)</a>.</blockquote>
+      </details>
+      <details>
+        <summary>Reference Unlock</summary>
+        <blockquote>References a previous unlock to support unlocking multiple inputs owned by the same address. Defined in <a href='../TIP-0045/tip-0045.md#reference-unlock'>TIP-45 (Reference Unlock)</a>.</blockquote>
+      </details>
+      <details>
+        <summary>Account Unlock</summary>
+        <blockquote>Points to the unlock of a consumed Account Output. Defined in <a href='../TIP-0042/tip-0042.md#account-unlock'>TIP-42 (Account Unlock)</a>.</blockquote>
+      </details>
+      <details>
+        <summary>Anchor Unlock</summary>
+        <blockquote>Points to the unlock of a consumed Anchor Output. Defined in <a href='../TIP-0054/tip-0054.md#anchor-unlock'>TIP-54 (Anchor Unlock)</a>.</blockquote>
+      </details>
+      <details>
+        <summary>NFT Unlock</summary>
+        <blockquote>Points to the unlock of a consumed NFT Output. Defined in <a href='../TIP-0043/tip-0043.md#nft-unlock'>TIP-43 (NFT Unlock)</a>.</blockquote>
+      </details>
+      <details>
+        <summary>Empty Unlock</summary>
+        <blockquote>Used to maintain correct index relationship between addresses and signatures when unlocking a Multi Address where not all addresses are unlocked. Defined in <a href='../TIP-0052/tip-0052.md#empty-unlock'>TIP-52 (Empty Unlock)</a>.</blockquote>
+      </details>
+    </td>
+  </tr>
+</table>
+
+#### Additional syntactic transaction validation rules
+
+A Multi Unlock is only valid if all of the following conditions hold:
+
+- `1 <= Unlocks Count <= 10`.
+
+#### Additional semantic transaction validation rules
+
+A transaction containing a Multi Unlock is only valid if all of the following conditions hold:
+
+- The corresponding Address of the to-be-unlocked input is of type _Multi Address_.
+- The `Addresses Count` of the Multi Address and the `Unlocks Count` of the Multi Unlock must match.
+- If `Unlocks` contains Reference, Account or NFT Unlocks, they must point to an Unlock outside of the Multi Unlock.
+- Let the `Cumulative Unlocked Weight` be the sum of weights of the addresses that were successfully unlocked, through
+  Unlocks other than _Empty Unlocks_, which do not add to this sum.
+- `Cumulative Unlocked Weight >= Threshold`.
+
+### Empty Unlock
+
+The indices of _Unlocks_ and `Addresses` in a Multi Address must match when unlocking it since it may be configured to
+be unlocked by less than the number of addresses it contains. In such cases, the Empty Unlock needs to be used to
+maintain correct index relationship between unlocks and addresses in a _Multi Unlock_, to omit the unlocks of the
+addresses that are not unlocked.
+
+For example, consider a Multi Address with 3 addresses with weight 1 and a threshold of 1. The Multi Address can be
+unlocked by providing a Multi Unlock with a Signature Unlock of the first address and two subsequent Empty Unlocks.
+
+<details>
+  <summary>Empty Unlock</summary>
+  <blockquote>Used to maintain correct index relationship between addresses and signatures when unlocking a Multi Address where not all addresses are unlocked.</blockquote>
+</details>
+<table>
+  <tr>
+    <td>
+      <b>Name</b>
+    </td>
+    <td>
+      <b>Type</b>
+    </td>
+    <td>
+      <b>Description</b>
+    </td>
+  </tr>
+  <tr>
+    <td>Unlock Type</td>
+    <td>uint8</td>
+    <td>Set to <strong>value 6</strong> to denote an <i>Empty Unlock</i>.</td>
+  </tr>
+</table>
+
+## Bech32 Representation
+
+To allow querying the outputs owned by a Multi Address, its Bech32 representation is used, which is computed as follows:
+
+- Let `Address Hash` be the BLAKE2b-256 hash of the serialized Multi Address.
+- Bech32-encode the concatenation of the Multi Address type prefix and the `Address Hash`.
+
+# Rationale & Alternatives
+
+Multi Addresses make transparent which other entities control an entity, since they are specified in the address itself.
+For a Digital Identity this allows for inspection of which other identities control it and expresses the relationship of
+L1 entities. Additionally Multi Addresses enhance the security and recoverability of Accounts by adding support for
+multi-factor authentication (e.g. two addresses with weight 1 and a threshold of 2) or backup addresses (e.g. same as
+before plus an address with weight 2 whose private key is in cold storage) for outputs on L1.
+
+**Alternatives**
+
+- One alternative would be to have two sets of addresses, where one lists the mandatory and one the optional addresses,
+  the latter of which contains weights and a threshold. While this allows for similar use cases, it is harder to
+  validate correctness while not being more expressive.
+- Rather than introducing Multi Address as an additional type, it would be possible to introduce it as the only type of
+  address, as it could express all other existing types. However, that exposes every protocol user to the complexity of
+  Multi Addresses, which may not be desirable. Introducing it as a dedicated type confines the complexity to a smaller
+  part of the user-facing surface, while still allowing for the same functionality as if it was the only top-level
+  address type.
+- Another alternative is to simplify towards k of n use cases, avoiding the complexity of weights. This excludes some
+  use cases and a weight-based approach is not significantly more complex than a k of n only approach, which can already
+  be achieved with Ed25519 Threshold signatures.
+
+# Test Vectors
+
+## Bech32
+
+```json
+{
+  "type": 40,
+  "addresses": [
+    {
+      "address": {
+        "type": 0,
+        "pubKeyHash": "0x52fdfc072182654f163f5f0f9a621d729566c74d10037c4d7bbb0407d1e2c649"
+      },
+      "weight": 1
+    },
+    {
+      "address": {
+        "type": 0,
+        "pubKeyHash": "0x53fdfc072182654f163f5f0f9a621d729566c74d10037c4d7bbb0407d1e2c649"
+      },
+      "weight": 1
+    },
+    {
+      "address": {
+        "type": 0,
+        "pubKeyHash": "0x54fdfc072182654f163f5f0f9a621d729566c74d10037c4d7bbb0407d1e2c649"
+      },
+      "weight": 1
+    },
+    {
+      "address": {
+        "type": 8,
+        "accountId": "0x55fdfc072182654f163f5f0f9a621d729566c74d10037c4d7bbb0407d1e2c649"
+      },
+      "weight": 2
+    },
+    {
+      "address": {
+        "type": 16,
+        "nftId": "0x56fdfc072182654f163f5f0f9a621d729566c74d10037c4d7bbb0407d1e2c649"
+      },
+      "weight": 3
+    }
+  ],
+  "threshold": 2
+}
+```
+
+The resulting Bech32 representation must be:
+
+```
+iota19qq0ezu97zl76wqnpdxxleuf55gk0eqhscjtdgqm5sqwav6gcarz6vvesnk
+```
+
+# Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).