plugin: build tag EncodeX25519Recipient which uses crypto/ecdh

plugin/encode.go

@@ -5,7 +5,6 @@
 package plugin
 
 import (
-	"crypto/ecdh"
 	"fmt"
 	"strings"
 
@@ -54,13 +53,3 @@ func ParseRecipient(s string) (name string, data []byte, err error) {
 	name = strings.TrimPrefix(hrp, "age1")
 	return name, data, nil
 }
-
-// EncodeX25519Recipient encodes a native X25519 recipient from a
-// [crypto/ecdh.X25519] public key. It's meant for plugins that implement
-// identities that are compatible with native recipients.
-func EncodeX25519Recipient(pk *ecdh.PublicKey) (string, error) {
-	if pk.Curve() != ecdh.X25519() {
-		return "", fmt.Errorf("wrong ecdh Curve")
-	}
-	return bech32.Encode("age", pk.Bytes())
-}

plugin/encode_go1.20.go

@@ -0,0 +1,24 @@
+// Copyright 2023 The age Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.20
+
+package plugin
+
+import (
+	"crypto/ecdh"
+	"fmt"
+
+	"filippo.io/age/internal/bech32"
+)
+
+// EncodeX25519Recipient encodes a native X25519 recipient from a
+// [crypto/ecdh.X25519] public key. It's meant for plugins that implement
+// identities that are compatible with native recipients.
+func EncodeX25519Recipient(pk *ecdh.PublicKey) (string, error) {
+	if pk.Curve() != ecdh.X25519() {
+		return "", fmt.Errorf("wrong ecdh Curve")
+	}
+	return bech32.Encode("age", pk.Bytes())
+}