docs: ADR 074: Msg v2

docs/architecture/README.md

@@ -90,6 +90,7 @@ When writing ADRs, follow the same best practices for writing RFCs. When writing
 * [ADR 063: Core Module API](./adr-063-core-module-api.md)
 * [ADR 067: Simulator v2](./adr-067-simulator-v2.md)
 * [ADR 069: `x/gov` modularity, multiple choice and optimisic proposals](./adr-069-gov-improvements.md)
+* [ADR 074: Messages with implicit signers](./adr-074-implicit-msg-signers.md)
 
 ### Draft
 

docs/architecture/adr-074-implicit-msg-signers.md

@@ -0,0 +1,126 @@
+# ADR 074: Messages with Implicit Signers
+
+## Changelog
+
+* 2024-06-10: Initial draft
+
+## Status
+
+PROPOSED Not Implemented
+
+## Abstract
+
+This ADR introduces a new `MsgV2` standard where the signer of the message is implied by the
+credentials of the party sending it, and unlike the current design not part of the message body.
+This can be used for both simple inter-module message passing and simpler messages in transactions.
+
+## Context
+
+Historically operations in the SDK have been modelled with the `sdk.Msg` interface and
+the account signing the message has to be explicitly extracted from the body of `Msg`s.
+Originally this was via a `GetSigners` method on the `sdk.Msg` interface which returned
+instances of `sdk.AccAddress` which itself relied on a global variable for decoding
+the addresses from bech32 strings. This was a messy situation. In addition, the implementation
+for `GetSigners` was different for each `Msg` type and clients would need to do a custom
+implementation for each `Msg` type. These were improved somewhat with the introduction of
+the `cosmos.msg.v1.signer` protobuf option which allowed for a more standardised way of
+defining who the signer of a message was and its implementation in the `x/tx` module which
+extracts signers dynamically and allowed removing the dependency on the global bech32
+configuration.
+
+Still this design introduces a fair amount of complexity. For instance, inter-module message
+passing ([ADR 033](./adr-033-protobuf-inter-module-comm.md)) has been in discussion for years
+without much progress and one of the main blockers is figuring out how to properly authenticate
+messages in a performant and consistent way. With embedded message signers there will always need
+to be a step of extracting the signer and then checking with the module sending is actually
+authorized to perform the operation. With dynamic signer extraction, although the system is
+more consistent, more performance overhead is introduced. In any case why should an inter-module
+message passing system need to do so much conversion, parsing, etc. just to check if a message
+is authenticated? In addition, we have the complexity where modules can actually have many valid
+addresses. How are we to accommodate this? Should there be a lookup into `x/auth` to check if an
+address belongs to a module or not? All of these thorny questions are delaying the delivery of
+inter-module message passing because we do not want an implementation that is overly complex.
+There are many use cases for inter-module message passing which are still relevant, the most
+immediate of which is a more robust denom management system in `x/bank` `v2` which is being explored
+in [ADR 071](https://github.com/cosmos/cosmos-sdk/pull/20316).
+
+## Alternatives
+
+Alternatives that have been considered are extending the current `x/tx` signer extraction system
+to inter-module message passing as defined in [ADR 033](./adr-033-protobuf-inter-module-comm.md).
+
+## Decision
+
+We have decided to introduce a new `MsgV2` standard whereby the signer of the message is implied
+by the credentials of the party sending it. These messages will be distinct from the existing messages
+and define new semantics with the understanding that signers are implicit.
+
+In the case of messages passed internally by a module or `x/account` instance, the signer of a message
+will simply be the main root address of the module or account sending the message. An interface for
+safely passing such messages to the message router will need to be defined.
+
+In the case of messages passed externally in transactions, `MsgV2` instances will need to be wrapped
+in a `MsgV2` envelope:
+```protobuf
+message MsgV2 {
+  string signer = 1;
+  google.protobuf.Any msg = 2;  
+}
+```
+
+Because the `cosmos.msg.v1.signer` annotation is required currently, `MsgV2` types should set the message option
+`cosmos.msg.v2.is_msg` to `true` instead.
+
+Here is an example comparing a v1 an v2 message:
+```protobuf
+// v1
+message MsgSendV1 {
+  string from_address = 1 [(cosmos.msg.v1.signer) = true];
+  string to_address = 2;
+  repeated Coin amount = 3;
+}
+
+// v2
+message MsgSendV2 {
+  option (cosmos.msg.v2.is_msg) = true;
+  // from address is implied by the signer
+  string to_address = 1;
+  repeated Coin amount = 2;
+}
+```
+
+Modules defining handlers for `MsgV2` instances will need to extract the sender from the `context.Context` that is
+passed in. An interface in `core` which will be present on the `appmodule.Environment` will be defined for this purpose:
+```go
+type GetSenderService interface {
+  GetSender(ctx sdk.Context) []byte
+}
+```
+
+Sender addresses that are returned by the service will be simple `[]byte` slices and any bech32 conversion will be
+done by the framework.
+
+## Consequences
+
+### Backwards Compatibility
+
+This design does not depreciate the existing method of embedded signers in `Msg`s and is totally compatible with it.
+
+### Positive
+
+* Allows for a simple inter-module communication design which can be used soon for the `bank` `v2` redesign.
+* Allows for simpler client implementations for messages in the future.
+
+### Negative
+
+* There will be two message designs and developers will need to pick between them.
+
+### Neutral
+
+## Further Discussions
+
+Further discussions should take place on GitHub.
+
+## References
+
+* [ADR 033](./adr-033-protobuf-inter-module-comm.md)