cmd/age: add -e and support encrypting with -i

This will come in handy for symmetric plugins, but make it require an
explicit -e so that missing a -d can't cause a mistaken encryption.

agessh/agessh.go

@@ -102,6 +102,13 @@ func NewRSAIdentity(key *rsa.PrivateKey) (*RSAIdentity, error) {
 	return i, nil
 }
 
+func (i *RSAIdentity) Recipient() age.Recipient {
+	return &RSARecipient{
+		sshKey: i.sshKey,
+		pubKey: &i.k.PublicKey,
+	}
+}
+
 func (i *RSAIdentity) Unwrap(stanzas []*age.Stanza) ([]byte, error) {
 	return multiUnwrap(i.unwrap, stanzas)
 }
@@ -303,6 +310,13 @@ func ed25519PrivateKeyToCurve25519(pk ed25519.PrivateKey) []byte {
 	return out[:curve25519.ScalarSize]
 }
 
+func (i *Ed25519Identity) Recipient() age.Recipient {
+	return &Ed25519Recipient{
+		sshKey:         i.sshKey,
+		theirPublicKey: i.ourPublicKey,
+	}
+}
+
 func (i *Ed25519Identity) Unwrap(stanzas []*age.Stanza) ([]byte, error) {
 	return multiUnwrap(i.unwrap, stanzas)
 }

agessh/agessh_test.go

@@ -11,6 +11,7 @@ import (
 	"crypto/ed25519"
 	"crypto/rand"
 	"crypto/rsa"
+	"reflect"
 	"testing"
 
 	"filippo.io/age/agessh"
@@ -36,6 +37,11 @@ func TestSSHRSARoundTrip(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// TODO: replace this with (and go-diff) with go-cmp.
+	if !reflect.DeepEqual(r, i.Recipient()) {
+		t.Fatalf("i.Recipient is different from r")
+	}
+
 	fileKey := make([]byte, 16)
 	if _, err := rand.Read(fileKey); err != nil {
 		t.Fatal(err)
@@ -74,6 +80,11 @@ func TestSSHEd25519RoundTrip(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// TODO: replace this with (and go-diff) with go-cmp.
+	if !reflect.DeepEqual(r, i.Recipient()) {
+		t.Fatalf("i.Recipient is different from r")
+	}
+
 	fileKey := make([]byte, 16)
 	if _, err := rand.Read(fileKey); err != nil {
 		t.Fatal(err)

agessh/encrypted_keys.go

@@ -55,6 +55,10 @@ func NewEncryptedSSHIdentity(pubKey ssh.PublicKey, pemBytes []byte, passphrase f
 
 var _ age.Identity = &EncryptedSSHIdentity{}
 
+func (i *EncryptedSSHIdentity) Recipient() (age.Recipient, error) {
+	return ParseRecipient(string(ssh.MarshalAuthorizedKey(i.pubKey)))
+}
+
 // Unwrap implements age.Identity. If the private key is still encrypted, and
 // any of the stanzas match the public key, it will request the passphrase. The
 // decrypted private key will be cached after the first successful invocation.