docs: binary interoperable addresses explainer

docs/addresses/interoperable-address-explainer.md

@@ -1,54 +1,123 @@
 Introducing: Interoperable Addresses
 ====
 
-*Specs in the formal format are written [here](/specs/addresses/cross-chain-interoperable-addresses-spec.md)*.
+*Specs in the formal, normative format can be found [here](/specs/addresses/cross-chain-interoperable-addresses-spec.md)*.
 
 Interoperable Addresses is a standard defining address formats for a multi-chain world.
 
-For a normal user, they look like this:
+They contain the following information:
 
-<p>
-    <code>
-        <span style="color:grey">3::</span><span style="color: blue">vitalik.eth</span>@<span style="color: magenta">eth</span>#<span style="color:grey">5966be0f</span>
-    </code>
-</p>
+```solidity
+    struct InteroperableAddress {
+        bytes targetChainId;
+        bytes targetAddress;
+        bytes resolverChainId;
+        bytes resolverAddress;
+    }
+```
 
-- <code><span style="color:grey">3::</span></code>: the Interoperable Address Resolver version, which your wallet may very well omit.
-- <code><span style="color: blue">vitalik.eth</span></code>: A human-readable name, specific to a chain. Supported by (anything looking like) ENS.
-- <code><span style="color: magenta">eth</span></code>: A human-readable chain name, defined by either https://github.com/ethereum-lists/chains if available (same registry for ERC-3770) or CAIP-2 in all other cases.
-- <code><span style="color:grey">5966be0f</span></code>: A checksum so you can at a glance validate what you see matches what you expect to be under the hood.
+Let's take an example:
 
----
+<pre>
+InteroperableAddress addy = InteroperableAddress({
+    targetChainId: hex'<span style="color:green">0000</span><span style="color:orange">01</span>',
+    targetAddress: hex'<span style="color:blue">D8DA6BF26964AF9D7EED9E03E53415D37AA96045</span>',
+    resolverChainId: hex'<span style="color:black">0000<span style="color:pink">01</span>',
+    resolverAddress: hex'<span style="color:magenta">00000000000C2E074EC69A0DFB2997BA6C7D2E1E</span>'
+})
+</pre>
 
-For nerds and computers, however, they look like this:
+Would be seen by users like this:
 
-<p>
-    <code>
-        <span style="color:grey">3:</span><span style="color:green">eip155:1</span>:<span style="color: red">0x00000000000C2E074eC69A0dFb2997BA6C7d2e1e</span>::<span style="color:magenta">eip155:1</span>:<span style="color: blue">0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045</span>#<span style="color:grey">5966be0f</span>
-    </code>
-</p>
+<pre>
+    <span style="color:blue">vitalik.eth</span>@<span style="color:green">eip:155</span>:<span style="color:orange">1</span>#<span style="color:grey">618AD0D1</span>
+</pre>
 
-- <code><span style="color:grey">3:</span></code>: Interoperable Address version.
-- <code><span style="color:green">eip155:1</span></code>: CAIP-2 id of chain where the naming registry is located
-- <code><span style="color: red">0x00000000000C2E074eC69A0dFb2997BA6C7d2e1e</span></code>: Address where to find the naming registry
-- <code><span style="color:magenta">eip155:1</span></code>: CAIP-2 id of chain we are referring to
-- <code><span style="color: blue">0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045</span></code>: Chain-specific address we are targeting in that chain
-- <code><span style="color:grey">5966be0f</span></code>: Checksum of all previous fields, to make sure nothing got lost in transit
+...and in memory, it would actually be laid out like this:
 
----
+<pre>
+C000618AD0D10000000000000000000000000000000000000000000000000001
+<span style="color:green">0000</span><span style="color:orange">01</span><span style="color: grey">000000000000000000000000000000000000000000000000000000000C</span>
+<span style="color:blue">D8DA6BF26964AF9D7EED9E03E53415D37AA96045</span><span style="color: grey">000000000000000000000028</span>
+<span style="color:black">0000</span><span style="color:pink">01</span><span style="color: grey">000000000000000000000000000000000000000000000000000000000C</span>
+<span style="color:magenta">00000000000C2E074EC69A0DFB2997BA6C7D2E1E</span><span style="color: grey">000000000000000000000028</span>
+</pre>
 
-The Interoperable Address is a format for strings to:
-- Fully specify an address on a particular chain (using CAIP-10), supporting any EVM-ecosystem chain and any chain on SLIP-044.
-- Also include the information to display it to a human in a readable way, as displayed above.
+It includes:
+- A full specification of what (address,chain) pair we're referring to
+- A full specification of on which (address,chain) pair the contract responsible for resolving addresses to names lives
+- ... and a full 32-byte word before any of that?
 
-Some of its features are:
-- Uses mature technologies such as CAIP-10 and ENS.
-- Is extensible for future resolving mechanisms (eg: ERC-7785, when/if it reaches production).
-- Resolution of Interoperable Addresses to human readable names is fully deterministic.
+Said word is what enables us to make this standard extensible, let's take a closer look:
+
+<pre>
+MSB                                                          LSB
+<span style="color: red">C000</span><span style="color:green">6164DA1</span><span style="color:blue">00000000000000000000000000000000000000000000000000001</span>
+^^^^-------------------------------------------------------------- 255-240 <span style="color:red">Interoperable Address Version</span>
+    ^^^^^^^^ ----------------------------------------------------- 239-208 <span style="color:green">Checksum</span>
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - 207-0   <span style="color:blue">Resolver Interface Key</span>
+</pre>
+
+- <span style="color:red">Interoperable Address Version</span>: First two bytes of the payload allows us to discern how we should interpret it, more on this later
+- <span style="color:green">Checksum</span>: Covers only the (address,chain) we are referring to -> is independent of the method used to display the address, or how it is serialized -> makes naming collisions evident 
+- <span style="color:blue">Resolver Interface Key</span>: in addresses version `0xC000`, instructs us on what to expect from `nameContractAddress` and `nameContractChain`. In particular, this value means to expect an ENSIP-11 contract.
+
+But wait! I actually want...
+- A canonical representation: use Interoperable Address Version `0x8000`
+- To be as space-efficient as possible for EVM chains: use Interoperable Address Version `0x0000` or `0x0001`
+- To display human-readable chain names to users, but not rely on centralized lists: define a new 'Resolver Interface Key' as soon as ERC-7785 reaches production
+
+What some other resolvers look like:
+
+`0x0000`:
+<pre>
+MSB                                                            LSB
+0x<span style="color: red">0000</span><span style="color: green">618ad0d1</span><span style="color: blue">00000000000</span><span style="color: magenta">1D8DA6BF26964AF9D7EED9E03E53415D37AA96045</span>
+  ^^^^------------------------------------------------------------- 255-240 <span style="color: red">Interoperable Address version</span>
+      ^^^^^^^^----------------------------------------------------- 239-208 <span style="color: green">Checksum</span>
+              ^^^^^^^^^^^^----------------------------------------- 207-160 <span style="color: blue">Chainid</span>
+                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- 159-0   <span style="color: magenta">Address</<span>
+</pre>
+Is represented as:
+<pre>
+<span style="color: magenta">0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045</span>@eip155:<span style="color: blue">1</span>#<span style="color: green">618AD0D1</span>
+</pre>
+
+To fit in a single word, it sacrifices:
+* Ability to refer to CAIP-2 namespaces other than eip155 (it's implied by the version number)
+* Ability to represent addresses longer than 20 bytes
+* Ability to refer to chains with chainids longer than 48 bits (rules out ERC-7785)
+
+`0x8000`:
+```
+MSB                                                          LSB
+8000618AD0D10000000000000000000000000000000000000000000000000000
+^^^^-------------------------------------------------------------- 255-240 Interoperable Address Version
+    ^^^^^^^^ ----------------------------------------------------- 239-208 Checksum
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ - 207-0   not used, always zero
+MSB                                                          LSB
+00000100000000000000000000000000000000000000000000000000000000C0
+^^^^^^------------------------------------------------------------ 255-208 bytes array payload
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ --- 9-207   padding
+                                                              ^^ - 0-8    2*6 (payload length)
+MSB                                                          LSB
+D8DA6BF26964AF9D7EED9E03E53415D37AA96045000000000000000000000028
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  ------------------------ 255-97 bytes array payload
+                                        ^^^^^^^^^^^^^^^^^^^^^^ --- 9-96   padding
+                                                              ^^ - 0-8    2*20 (payload length)
+```
+Is represented as:
+`0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045@eip155:1#618AD0D1`
+
+Some properties of this system:
+- Interoperable addresses of any version can always be converted to format `0x8000` -> forwards-compatibility and graceful degradation of interfaces.
+- Interoperable addresses of any version can always be casted to a valid CAIP-10.
+- Addresses of any length in chains of any supported CAIP-2 namespace can be represented
+- Future-proofing for developments on future resolving mechanisms.
+- Implementable right now with current tools & standards.
 - Edge cases are securely abstractable to a library/SDK for wallets.
-- Does not enshrine a particular ENS contract, making it flexible for a future naming-only rollup, instances on other rollups or even an ENS fork.
+- Does not enshrine a particular naming contract, making it flexible for a future naming-only rollup, instances on other rollups or even an ENS competitor.
 
-And, in all honesty, it has some drawbacks as well:
-- Exposes a version number to users (although that can be mitigated by wallet UX)
+And, in all honesty, some drawbacks as well:
 - Supporting different name resolution contracts mean human-readable name -> machine address resolution can produce different results (which is mitigated by the checksum)
 - Wallets will have to maintain a set of name resolving contracts considered trustworthy, with a trust model similar to RPC urls (can be taken care of by a library/SDK, but user overrides should be supported).