Allow specifying user and host when adding keys

The username and hostname for new keys can be specified with the new
--user and --host flags, respectively. The flags are used only by the
`key add` command and are otherwise ignored.

This allows adding keys with for a desired user and host without having
to run restic as that particular user on that particular host, making
automated key management easier.

Co-authored-by: James TD Smith <ahktenzero@mohorovi.cc>

changelog/unreleased/issue-2175

@@ -0,0 +1,9 @@
+Enhancement: Allow specifying user and host when creating keys
+
+When adding a new key to the repository, the username and hostname for the new
+key can be specified on the command line. This allows overriding the defaults,
+for example if you would prefer to use the FQDN to identify the host or if you
+want to add keys for several different hosts without having to run the key add
+command on those hosts.
+
+https://github.com/restic/restic/issues/2175

cmd/restic/cmd_key.go

@@ -32,13 +32,19 @@ Exit status is 0 if the command was successful, and non-zero if there was any er
 	},
 }
 
-var newPasswordFile string
+var (
+	newPasswordFile string
+	keyUsername     string
+	keyHostname     string
+)
 
 func init() {
 	cmdRoot.AddCommand(cmdKey)
 
 	flags := cmdKey.Flags()
 	flags.StringVarP(&newPasswordFile, "new-password-file", "", "", "the file from which to load a new password")
+	flags.StringVarP(&keyUsername, "user", "", "", "the username for new keys")
+	flags.StringVarP(&keyHostname, "host", "", "", "the hostname for new keys")
 }
 
 func listKeys(ctx context.Context, s *repository.Repository, gopts GlobalOptions) error {
@@ -120,7 +126,7 @@ func addKey(gopts GlobalOptions, repo *repository.Repository) error {
 		return err
 	}
 
-	id, err := repository.AddKey(gopts.ctx, repo, pw, repo.Key())
+	id, err := repository.AddKey(gopts.ctx, repo, pw, keyUsername, keyHostname, repo.Key())
 	if err != nil {
 		return errors.Fatalf("creating new key failed: %v\n", err)
 	}
@@ -151,7 +157,7 @@ func changePassword(gopts GlobalOptions, repo *repository.Repository) error {
 		return err
 	}
 
-	id, err := repository.AddKey(gopts.ctx, repo, pw, repo.Key())
+	id, err := repository.AddKey(gopts.ctx, repo, pw, "", "", repo.Key())
 	if err != nil {
 		return errors.Fatalf("creating new key failed: %v\n", err)
 	}

cmd/restic/integration_test.go

@@ -639,6 +639,28 @@ func testRunKeyAddNewKey(t testing.TB, newPassword string, gopts GlobalOptions)
 	rtest.OK(t, runKey(gopts, []string{"add"}))
 }
 
+func testRunKeyAddNewKeyUserHost(t testing.TB, gopts GlobalOptions) {
+	testKeyNewPassword = "john's geheimnis"
+	defer func() {
+		testKeyNewPassword = ""
+		keyUsername = ""
+		keyHostname = ""
+	}()
+
+	cmdKey.Flags().Parse([]string{"--user=john", "--host=example.com"})
+
+	t.Log("adding key for john@example.com")
+	rtest.OK(t, runKey(gopts, []string{"add"}))
+
+	repo, err := OpenRepository(gopts)
+	rtest.OK(t, err)
+	key, err := repository.SearchKey(gopts.ctx, repo, testKeyNewPassword, 1, "")
+	rtest.OK(t, err)
+
+	rtest.Equals(t, "john", key.Username)
+	rtest.Equals(t, "example.com", key.Hostname)
+}
+
 func testRunKeyPasswd(t testing.TB, newPassword string, gopts GlobalOptions) {
 	testKeyNewPassword = newPassword
 	defer func() {
@@ -681,6 +703,8 @@ func TestKeyAddRemove(t *testing.T) {
 	t.Logf("testing access with last password %q\n", env.gopts.password)
 	rtest.OK(t, runKey(env.gopts, []string{"list"}))
 	testRunCheck(t, env.gopts)
+
+	testRunKeyAddNewKeyUserHost(t, env.gopts)
 }
 
 func testFileSize(filename string, size int64) error {

internal/repository/key.go

@@ -58,7 +58,7 @@ var (
 // createMasterKey creates a new master key in the given backend and encrypts
 // it with the password.
 func createMasterKey(s *Repository, password string) (*Key, error) {
-	return AddKey(context.TODO(), s, password, nil)
+	return AddKey(context.TODO(), s, password, "", "", nil)
 }
 
 // OpenKey tries do decrypt the key specified by name with the given password.
@@ -199,7 +199,7 @@ func LoadKey(ctx context.Context, s *Repository, name string) (k *Key, err error
 }
 
 // AddKey adds a new key to an already existing repository.
-func AddKey(ctx context.Context, s *Repository, password string, template *crypto.Key) (*Key, error) {
+func AddKey(ctx context.Context, s *Repository, password, username, hostname string, template *crypto.Key) (*Key, error) {
 	// make sure we have valid KDF parameters
 	if Params == nil {
 		p, err := crypto.Calibrate(KDFTimeout, KDFMemory)
@@ -213,24 +213,29 @@ func AddKey(ctx context.Context, s *Repository, password string, template *crypt
 
 	// fill meta data about key
 	newkey := &Key{
-		Created: time.Now(),
-		KDF:     "scrypt",
-		N:       Params.N,
-		R:       Params.R,
-		P:       Params.P,
+		Created:  time.Now(),
+		Username: username,
+		Hostname: hostname,
+
+		KDF: "scrypt",
+		N:   Params.N,
+		R:   Params.R,
+		P:   Params.P,
 	}
 
-	hn, err := os.Hostname()
-	if err == nil {
-		newkey.Hostname = hn
+	if newkey.Hostname == "" {
+		newkey.Hostname, _ = os.Hostname()
 	}
 
-	usr, err := user.Current()
-	if err == nil {
-		newkey.Username = usr.Username
+	if newkey.Username == "" {
+		usr, err := user.Current()
+		if err == nil {
+			newkey.Username = usr.Username
+		}
 	}
 
 	// generate random salt
+	var err error
 	newkey.Salt, err = crypto.NewSalt()
 	if err != nil {
 		panic("unable to read enough random bytes for salt: " + err.Error())