plugin: always canonicalize plugin names

Reported by Joe Doyle of Trail of Bits.

plugin/client.go

@@ -193,7 +193,7 @@ func NewIdentityWithoutData(name string, ui *ClientUI) (*Identity, error) {
 		return nil, fmt.Errorf("invalid plugin name: %q", name)
 	}
 	return &Identity{
-		name: name, encoding: s, ui: ui,
+		name: strings.ToLower(name), encoding: s, ui: ui,
 	}, nil
 }
 

plugin/client_test.go

@@ -84,6 +84,26 @@ func TestIdentityV1NegativeIndex(t *testing.T) {
 	}
 }
 
+func TestPluginNameCase(t *testing.T) {
+	p, err := New("MixedCase")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if p.Name() != "mixedcase" {
+		t.Errorf("Plugin.Name = %q", p.Name())
+	}
+	id, err := NewIdentityWithoutData("MixedCase", &ClientUI{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if id.Name() != "mixedcase" {
+		t.Errorf("Identity.Name = %q", id.Name())
+	}
+	if _, err := New("\u212a"); err == nil {
+		t.Error("New accepted a non-ASCII name")
+	}
+}
+
 func TestLabels(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skip("Windows support is TODO")

plugin/plugin.go

@@ -16,6 +16,7 @@ import (
 	"os"
 	"slices"
 	"strconv"
+	"strings"
 
 	"filippo.io/age"
 	"filippo.io/age/internal/format"
@@ -44,11 +45,14 @@ type Plugin struct {
 	broken bool
 }
 
-// New creates a new Plugin with the given name.
+// New creates a new Plugin with the given case-insensitive name.
 //
 // For example, a plugin named "frood" would be invoked as "age-plugin-frood".
 func New(name string) (*Plugin, error) {
-	return &Plugin{name: name, stdin: os.Stdin,
+	if !validPluginName(name) {
+		return nil, fmt.Errorf("invalid plugin name: %q", name)
+	}
+	return &Plugin{name: strings.ToLower(name), stdin: os.Stdin,
 		stdout: os.Stdout, stderr: os.Stderr}, nil
 }