fix: RegisterLegacyAminoCodec should error if Msg name is >39 chars (#11349)

## Description

Closes: https://github.com/cosmos/cosmos-sdk/issues/10870



---

### Author Checklist

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- [x] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] added `!` to the type prefix if API or client breaking change
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
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- [ ] followed the guidelines for [building modules](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules)
- [x] included the necessary unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [ ] added a changelog entry to `CHANGELOG.md`
- [ ] included comments for [documenting Go code](https://blog.golang.org/godoc)
- [ ] updated the relevant documentation or specification
- [ ] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

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I have...

- [ ] confirmed the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
- [ ] confirmed `!` in the type prefix if API or client breaking change
- [ ] confirmed all author checklist items have been addressed 
- [ ] reviewed state machine logic
- [ ] reviewed API design and naming
- [ ] reviewed documentation is accurate
- [ ] reviewed tests and test coverage
- [ ] manually tested (if applicable)

codec/legacy/amino_msg.go

@@ -0,0 +1,18 @@
+package legacy
+
+import (
+	"fmt"
+
+	"github.com/cosmos/cosmos-sdk/codec"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+)
+
+// RegisterAminoMsg first checks that the msgName is <40 chars
+// (else this would break ledger nano signing: https://github.com/cosmos/cosmos-sdk/issues/10870),
+// then registers the concrete msg type with amino.
+func RegisterAminoMsg(cdc *codec.LegacyAmino, msg sdk.Msg, msgName string) {
+	if len(msgName) > 39 {
+		panic(fmt.Errorf("msg name %s is too long to be registered with amino", msgName))
+	}
+	cdc.RegisterConcrete(msg, msgName, nil)
+}

codec/legacy/amino_msg_test.go

@@ -0,0 +1,38 @@
+package legacy_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/cosmos/cosmos-sdk/codec"
+	"github.com/cosmos/cosmos-sdk/codec/legacy"
+	"github.com/cosmos/cosmos-sdk/testutil/testdata"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRegisterAminoMsg(t *testing.T) {
+	cdc := codec.NewLegacyAmino()
+
+	testCases := map[string]struct {
+		msgName  string
+		expPanic bool
+	}{
+		"all good": {
+			msgName: "cosmos-sdk/Test",
+		},
+		"msgName too long": {
+			msgName:  strings.Repeat("a", 40),
+			expPanic: true,
+		},
+	}
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			fn := func() { legacy.RegisterAminoMsg(cdc, &testdata.TestMsg{}, tc.msgName) }
+			if tc.expPanic {
+				require.Panics(t, fn)
+			} else {
+				require.NotPanics(t, fn)
+			}
+		})
+	}
+}

codec/legacy/doc.go

@@ -1,4 +1,6 @@
 // Package legacy contains a global amino Cdc which is deprecated but
 // still used in several places within the SDK. This package is intended
 // to be removed at some point in the future when the global Cdc is removed.
+// It also contains a util function RegisterAminoMsg that checks a msg name length
+// before registering the concrete msg type with amino.
 package legacy