cmd/age-plugin-tag,cmd/age-plugin-tagpq: new backward compatibility plugins

cmd/age-plugin-tag/plugin-tag.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+
+	"filippo.io/age"
+	"filippo.io/age/plugin"
+	"filippo.io/age/tag"
+)
+
+const usage = `age-plugin-tag is an age plugin for P-256 tagged recipients. These are supported
+natively by age v1.3.0 and later, but this plugin can be placed in $PATH to add
+support to any version and implementation of age that supports plugins.
+
+Usually, tagged recipients are the public side of private keys held in hardware,
+where the identity side is handled by a different plugin.`
+
+func main() {
+	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
+
+	p, err := plugin.New("tag")
+	if err != nil {
+		log.Fatal(err)
+	}
+	p.HandleRecipient(func(b []byte) (age.Recipient, error) {
+		return tag.NewClassicRecipient(b)
+	})
+	os.Exit(p.Main())
+}

cmd/age-plugin-tagpq/plugin-tagpq.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"os"
+
+	"filippo.io/age"
+	"filippo.io/age/plugin"
+	"filippo.io/age/tag"
+)
+
+const usage = `age-plugin-tagpq is an age plugin for ML-KEM-768 + P-256 post-quantum hybrid
+tagged recipients. These are supported natively by age v1.3.0 and later, but
+this plugin can be placed in $PATH to add support to any version and
+implementation of age that supports plugins.
+
+Usually, tagged recipients are the public side of private keys held in hardware,
+where the identity side is handled by a different plugin.`
+
+func main() {
+	flag.Usage = func() { fmt.Fprintf(os.Stderr, "%s\n", usage) }
+
+	p, err := plugin.New("tagpq")
+	if err != nil {
+		log.Fatal(err)
+	}
+	p.HandleRecipient(func(b []byte) (age.Recipient, error) {
+		return tag.NewHybridRecipient(b)
+	})
+	os.Exit(p.Main())
+}

doc/age.1.ronn

@@ -237,6 +237,20 @@ instruct the user to perform encryption with the `-e`/`--encrypt` and
 doesn't make sense (such as a password-encryption plugin) may instruct the user
 to use the `-j` flag.
 
+#### Tagged recipients
+
+`age` can natively encrypt to recipients starting with `age1tag1` (using P-256
+ECDH) or `age1tagpq1` (using the ML-KEM-768 + P-256 post-quantum hybrid). These
+are intended to be the public side of private keys held in hardware.
+
+They are directly supported to avoid the need to install the plugin, which may
+be platform-specific, on the encrypting side.
+
+The tag reduces privacy, by allowing an observer to correlate files with a
+recipient (but not files amongst them without knowledge of the recipient),
+but this is also a desirable property for hardware keys that require user
+interaction for each decryption operation.
+
 ## EXIT STATUS
 
 `age` will exit 0 if and only if encryption or decryption are successful for the