Implement JWT token auth with transparent auto-renewal.

Replace per-call SSH signing with a two-layer auth system:

Server: AuthInterceptor verifies JWT tokens (HMAC-SHA256 signed with
repo-local jwt.key). Authenticate RPC accepts SSH-signed challenges
and issues 30-day JWTs. Expired-but-valid tokens return a
ReauthChallenge in error details (server-provided nonce for fast
re-auth). Authenticate RPC is exempt from token requirement.

Client: TokenCredentials replaces SSHCredentials as the primary
PerRPCCredentials. NewWithAuth creates clients with auto-renewal —
EnsureAuth obtains initial token, retryOnAuth catches Unauthenticated
errors and re-authenticates transparently. Token cached at
$XDG_STATE_HOME/sgard/token (0600).

CLI: dialRemote() helper handles token loading, connection setup,
and initial auth. Push/pull/prune commands simplified to use it.

Proto: Added Authenticate RPC, AuthenticateRequest/Response,
ReauthChallenge messages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -1 +1,3 @@
 /sgard
+.claude/
+result

client/auth.go

@@ -7,59 +7,148 @@ import (
 	"fmt"
 	"net"
 	"os"
-	"strconv"
+	"path/filepath"
 	"strings"
+	"sync"
 	"time"
 
+	"github.com/kisom/sgard/sgardpb"
 	"golang.org/x/crypto/ssh"
 	"golang.org/x/crypto/ssh/agent"
 	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/status"
 )
 
-// SSHCredentials implements grpc.PerRPCCredentials using an SSH signer.
-type SSHCredentials struct {
-	signer ssh.Signer
+// TokenCredentials implements grpc.PerRPCCredentials using a cached JWT token.
+// It is safe for concurrent use.
+type TokenCredentials struct {
+	mu    sync.RWMutex
+	token string
 }
 
-// NewSSHCredentials creates credentials from an SSH signer.
-func NewSSHCredentials(signer ssh.Signer) *SSHCredentials {
-	return &SSHCredentials{signer: signer}
+// NewTokenCredentials creates credentials with an initial token (may be empty).
+func NewTokenCredentials(token string) *TokenCredentials {
+	return &TokenCredentials{token: token}
 }
 
-// GetRequestMetadata signs a nonce+timestamp and returns auth metadata.
-func (c *SSHCredentials) GetRequestMetadata(_ context.Context, _ ...string) (map[string]string, error) {
-	nonce := make([]byte, 32)
-	if _, err := rand.Read(nonce); err != nil {
-		return nil, fmt.Errorf("generating nonce: %w", err)
+// SetToken updates the cached token.
+func (c *TokenCredentials) SetToken(token string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.token = token
+}
+
+// GetRequestMetadata returns the token as gRPC metadata.
+func (c *TokenCredentials) GetRequestMetadata(_ context.Context, _ ...string) (map[string]string, error) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+	if c.token == "" {
+		return nil, nil
 	}
-
-	tsUnix := time.Now().Unix()
-	payload := buildPayload(nonce, tsUnix)
-
-	sig, err := c.signer.Sign(rand.Reader, payload)
-	if err != nil {
-		return nil, fmt.Errorf("signing payload: %w", err)
-	}
-
-	pubkey := c.signer.PublicKey()
-	pubkeyStr := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(pubkey)))
-
-	return map[string]string{
-		"x-sgard-auth-nonce":     base64.StdEncoding.EncodeToString(nonce),
-		"x-sgard-auth-timestamp": strconv.FormatInt(tsUnix, 10),
-		"x-sgard-auth-signature": base64.StdEncoding.EncodeToString(ssh.Marshal(sig)),
-		"x-sgard-auth-pubkey":    pubkeyStr,
-	}, nil
+	return map[string]string{"x-sgard-auth-token": c.token}, nil
 }
 
-// RequireTransportSecurity returns false — auth is via SSH signatures,
-// not TLS. Transport security can be added separately.
-func (c *SSHCredentials) RequireTransportSecurity() bool {
+// RequireTransportSecurity returns false.
+func (c *TokenCredentials) RequireTransportSecurity() bool {
 	return false
 }
 
-// Verify that SSHCredentials implements the interface.
-var _ credentials.PerRPCCredentials = (*SSHCredentials)(nil)
+var _ credentials.PerRPCCredentials = (*TokenCredentials)(nil)
+
+// TokenPath returns the XDG-compliant path for the token cache.
+// Uses $XDG_STATE_HOME/sgard/token, falling back to ~/.local/state/sgard/token.
+func TokenPath() (string, error) {
+	stateHome := os.Getenv("XDG_STATE_HOME")
+	if stateHome == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("determining home directory: %w", err)
+		}
+		stateHome = filepath.Join(home, ".local", "state")
+	}
+	return filepath.Join(stateHome, "sgard", "token"), nil
+}
+
+// LoadCachedToken reads the token from the XDG state path.
+// Returns empty string if the file doesn't exist.
+func LoadCachedToken() string {
+	path, err := TokenPath()
+	if err != nil {
+		return ""
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return ""
+	}
+	return strings.TrimSpace(string(data))
+}
+
+// SaveToken writes the token to the XDG state path with 0600 permissions.
+func SaveToken(token string) error {
+	path, err := TokenPath()
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return fmt.Errorf("creating token directory: %w", err)
+	}
+	return os.WriteFile(path, []byte(token+"\n"), 0o600)
+}
+
+// Authenticate calls the server's Authenticate RPC with an SSH-signed challenge.
+// If challenge is non-nil (reauth fast path), uses the server-provided nonce.
+// Otherwise generates a fresh nonce.
+func Authenticate(ctx context.Context, rpc sgardpb.GardenSyncClient, signer ssh.Signer, challenge *sgardpb.ReauthChallenge) (string, error) {
+	var nonce []byte
+	var tsUnix int64
+
+	if challenge != nil {
+		nonce = challenge.GetNonce()
+		tsUnix = challenge.GetTimestamp()
+	} else {
+		var err error
+		nonce = make([]byte, 32)
+		if _, err = rand.Read(nonce); err != nil {
+			return "", fmt.Errorf("generating nonce: %w", err)
+		}
+		tsUnix = time.Now().Unix()
+	}
+
+	payload := buildPayload(nonce, tsUnix)
+	sig, err := signer.Sign(rand.Reader, payload)
+	if err != nil {
+		return "", fmt.Errorf("signing challenge: %w", err)
+	}
+
+	pubkeyStr := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(signer.PublicKey())))
+
+	resp, err := rpc.Authenticate(ctx, &sgardpb.AuthenticateRequest{
+		Nonce:     nonce,
+		Timestamp: tsUnix,
+		Signature: ssh.Marshal(sig),
+		PublicKey: pubkeyStr,
+	})
+	if err != nil {
+		return "", fmt.Errorf("authenticate RPC: %w", err)
+	}
+
+	return resp.GetToken(), nil
+}
+
+// ExtractReauthChallenge extracts a ReauthChallenge from a gRPC error's
+// details, if present. Returns nil if not found.
+func ExtractReauthChallenge(err error) *sgardpb.ReauthChallenge {
+	st, ok := status.FromError(err)
+	if !ok {
+		return nil
+	}
+	for _, detail := range st.Details() {
+		if challenge, ok := detail.(*sgardpb.ReauthChallenge); ok {
+			return challenge
+		}
+	}
+	return nil
+}
 
 // buildPayload constructs nonce || timestamp (big-endian int64).
 func buildPayload(nonce []byte, tsUnix int64) []byte {
@@ -81,7 +170,6 @@ func LoadSigner(keyPath string) (ssh.Signer, error) {
 		return loadSignerFromFile(keyPath)
 	}
 
-	// Try ssh-agent.
 	if sock := os.Getenv("SSH_AUTH_SOCK"); sock != "" {
 		conn, err := net.Dial("unix", sock)
 		if err == nil {
@@ -94,7 +182,6 @@ func LoadSigner(keyPath string) (ssh.Signer, error) {
 		}
 	}
 
-	// Try default key paths.
 	home, err := os.UserHomeDir()
 	if err != nil {
 		return nil, fmt.Errorf("no SSH key found: %w", err)
@@ -122,3 +209,40 @@ func loadSignerFromFile(path string) (ssh.Signer, error) {
 	}
 	return signer, nil
 }
+
+// SSHCredentials is kept for backward compatibility in tests.
+// It signs every request with SSH (the old approach).
+type SSHCredentials struct {
+	signer ssh.Signer
+}
+
+func NewSSHCredentials(signer ssh.Signer) *SSHCredentials {
+	return &SSHCredentials{signer: signer}
+}
+
+func (c *SSHCredentials) GetRequestMetadata(_ context.Context, _ ...string) (map[string]string, error) {
+	nonce := make([]byte, 32)
+	if _, err := rand.Read(nonce); err != nil {
+		return nil, fmt.Errorf("generating nonce: %w", err)
+	}
+	tsUnix := time.Now().Unix()
+	payload := buildPayload(nonce, tsUnix)
+	sig, err := c.signer.Sign(rand.Reader, payload)
+	if err != nil {
+		return nil, fmt.Errorf("signing: %w", err)
+	}
+	pubkeyStr := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(c.signer.PublicKey())))
+
+	// Send as both token-style metadata (won't work) AND the old SSH fields
+	// for the Authenticate RPC. But this is only used in legacy tests.
+	return map[string]string{
+		"x-sgard-auth-nonce":     base64.StdEncoding.EncodeToString(nonce),
+		"x-sgard-auth-timestamp": fmt.Sprintf("%d", tsUnix),
+		"x-sgard-auth-signature": base64.StdEncoding.EncodeToString(ssh.Marshal(sig)),
+		"x-sgard-auth-pubkey":    pubkeyStr,
+	}, nil
+}
+
+func (c *SSHCredentials) RequireTransportSecurity() bool { return false }
+
+var _ credentials.PerRPCCredentials = (*SSHCredentials)(nil)

client/client.go

@@ -10,28 +10,103 @@ import (
 	"github.com/kisom/sgard/garden"
 	"github.com/kisom/sgard/server"
 	"github.com/kisom/sgard/sgardpb"
+	"golang.org/x/crypto/ssh"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 const chunkSize = 64 * 1024 // 64 KiB
 
 // Client wraps a gRPC connection to a GardenSync server.
 type Client struct {
-	rpc sgardpb.GardenSyncClient
+	rpc    sgardpb.GardenSyncClient
+	creds  *TokenCredentials // may be nil (no auth)
+	signer ssh.Signer        // may be nil (no auth)
 }
 
-// New creates a Client from an existing gRPC connection.
+// New creates a Client from an existing gRPC connection (no auth).
 func New(conn grpc.ClientConnInterface) *Client {
 	return &Client{rpc: sgardpb.NewGardenSyncClient(conn)}
 }
 
+// NewWithAuth creates a Client with token-based auth and auto-renewal.
+// Loads any cached token automatically.
+func NewWithAuth(conn grpc.ClientConnInterface, creds *TokenCredentials, signer ssh.Signer) *Client {
+	return &Client{
+		rpc:    sgardpb.NewGardenSyncClient(conn),
+		creds:  creds,
+		signer: signer,
+	}
+}
+
+// EnsureAuth ensures the client has a valid token. If no token is cached,
+// authenticates with the server using the SSH signer.
+func (c *Client) EnsureAuth(ctx context.Context) error {
+	if c.creds == nil || c.signer == nil {
+		return nil
+	}
+
+	// If we already have a token, assume it's valid until the server says otherwise.
+	md, _ := c.creds.GetRequestMetadata(ctx)
+	if md != nil && md["x-sgard-auth-token"] != "" {
+		return nil
+	}
+
+	// No token — do full auth.
+	return c.authenticate(ctx, nil)
+}
+
+// authenticate calls the Authenticate RPC and caches the resulting token.
+func (c *Client) authenticate(ctx context.Context, challenge *sgardpb.ReauthChallenge) error {
+	token, err := Authenticate(ctx, c.rpc, c.signer, challenge)
+	if err != nil {
+		return err
+	}
+	c.creds.SetToken(token)
+	_ = SaveToken(token)
+	return nil
+}
+
+// retryOnAuth retries a function once after re-authenticating if it fails
+// with Unauthenticated.
+func (c *Client) retryOnAuth(ctx context.Context, fn func() error) error {
+	err := fn()
+	if err == nil || c.signer == nil {
+		return err
+	}
+
+	st, ok := status.FromError(err)
+	if !ok || st.Code() != codes.Unauthenticated {
+		return err
+	}
+
+	// Extract reauth challenge if present (fast path).
+	challenge := ExtractReauthChallenge(err)
+	if authErr := c.authenticate(ctx, challenge); authErr != nil {
+		return fmt.Errorf("re-authentication failed: %w", authErr)
+	}
+
+	// Retry the original call.
+	return fn()
+}
+
 // Push sends the local manifest and any missing blobs to the server.
 // Returns the number of blobs sent, or an error. If the server is newer,
-// returns ErrServerNewer.
+// returns ErrServerNewer. Automatically re-authenticates if the token expires.
 func (c *Client) Push(ctx context.Context, g *garden.Garden) (int, error) {
+	var result int
+	err := c.retryOnAuth(ctx, func() error {
+		n, err := c.doPush(ctx, g)
+		result = n
+		return err
+	})
+	return result, err
+}
+
+func (c *Client) doPush(ctx context.Context, g *garden.Garden) (int, error) {
 	localManifest := g.GetManifest()
 
-	// Step 1: send manifest, get decision.
 	resp, err := c.rpc.PushManifest(ctx, &sgardpb.PushManifestRequest{
 		Manifest: server.ManifestToProto(localManifest),
 	})
@@ -110,9 +185,18 @@ func (c *Client) Push(ctx context.Context, g *garden.Garden) (int, error) {
 
 // Pull downloads the server's manifest and any missing blobs to the local garden.
 // Returns the number of blobs received, or an error. If the local manifest is
-// newer or equal, returns 0 with no error.
+// newer or equal, returns 0 with no error. Automatically re-authenticates if needed.
 func (c *Client) Pull(ctx context.Context, g *garden.Garden) (int, error) {
-	// Step 1: get server manifest.
+	var result int
+	err := c.retryOnAuth(ctx, func() error {
+		n, err := c.doPull(ctx, g)
+		result = n
+		return err
+	})
+	return result, err
+}
+
+func (c *Client) doPull(ctx context.Context, g *garden.Garden) (int, error) {
 	pullResp, err := c.rpc.PullManifest(ctx, &sgardpb.PullManifestRequest{})
 	if err != nil {
 		return 0, fmt.Errorf("pull manifest: %w", err)
@@ -190,12 +274,18 @@ func (c *Client) Pull(ctx context.Context, g *garden.Garden) (int, error) {
 }
 
 // Prune requests the server to remove orphaned blobs. Returns the count removed.
+// Automatically re-authenticates if needed.
 func (c *Client) Prune(ctx context.Context) (int, error) {
-	resp, err := c.rpc.Prune(ctx, &sgardpb.PruneRequest{})
-	if err != nil {
-		return 0, fmt.Errorf("prune: %w", err)
-	}
-	return int(resp.BlobsRemoved), nil
+	var result int
+	err := c.retryOnAuth(ctx, func() error {
+		resp, err := c.rpc.Prune(ctx, &sgardpb.PruneRequest{})
+		if err != nil {
+			return fmt.Errorf("prune: %w", err)
+		}
+		result = int(resp.BlobsRemoved)
+		return nil
+	})
+	return result, err
 }
 
 func writeAndVerify(g *garden.Garden, expectedHash string, data []byte) error {

client/client_test.go

@@ -210,8 +210,9 @@ func TestPrune(t *testing.T) {
 	}
 }
 
-func TestAuthIntegration(t *testing.T) {
-	// Generate an ed25519 key pair.
+var testJWTKey = []byte("test-jwt-secret-key-32-bytes!!")
+
+func TestTokenAuthIntegration(t *testing.T) {
 	_, priv, err := ed25519.GenerateKey(rand.Reader)
 	if err != nil {
 		t.Fatalf("generating key: %v", err)
@@ -227,19 +228,18 @@ func TestAuthIntegration(t *testing.T) {
 		t.Fatalf("init server garden: %v", err)
 	}
 
-	// Set up server with auth interceptor.
-	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()})
+	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()}, testJWTKey)
 	lis := bufconn.Listen(bufSize)
 	srv := grpc.NewServer(
 		grpc.UnaryInterceptor(auth.UnaryInterceptor()),
 		grpc.StreamInterceptor(auth.StreamInterceptor()),
 	)
-	sgardpb.RegisterGardenSyncServer(srv, server.New(serverGarden))
+	sgardpb.RegisterGardenSyncServer(srv, server.NewWithAuth(serverGarden, auth))
 	t.Cleanup(func() { srv.Stop() })
 	go func() { _ = srv.Serve(lis) }()
 
-	// Client with SSH credentials.
-	creds := NewSSHCredentials(signer)
+	// Client with token auth + auto-renewal.
+	creds := NewTokenCredentials("")
 	conn, err := grpc.NewClient("passthrough:///bufconn",
 		grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
 			return lis.Dial()
@@ -252,16 +252,22 @@ func TestAuthIntegration(t *testing.T) {
 	}
 	t.Cleanup(func() { _ = conn.Close() })
 
-	c := New(conn)
+	c := NewWithAuth(conn, creds, signer)
 
-	// Authenticated request should succeed.
-	_, err = c.Pull(context.Background(), serverGarden)
+	// No token yet — EnsureAuth should authenticate via SSH.
+	ctx := context.Background()
+	if err := c.EnsureAuth(ctx); err != nil {
+		t.Fatalf("EnsureAuth: %v", err)
+	}
+
+	// Now requests should work.
+	_, err = c.Pull(ctx, serverGarden)
 	if err != nil {
 		t.Fatalf("authenticated Pull should succeed: %v", err)
 	}
 }
 
-func TestAuthIntegrationRejectsUnauthenticated(t *testing.T) {
+func TestAuthRejectsUnauthenticated(t *testing.T) {
 	_, priv, err := ed25519.GenerateKey(rand.Reader)
 	if err != nil {
 		t.Fatalf("generating key: %v", err)
@@ -277,17 +283,17 @@ func TestAuthIntegrationRejectsUnauthenticated(t *testing.T) {
 		t.Fatalf("init server garden: %v", err)
 	}
 
-	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()})
+	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()}, testJWTKey)
 	lis := bufconn.Listen(bufSize)
 	srv := grpc.NewServer(
 		grpc.UnaryInterceptor(auth.UnaryInterceptor()),
 		grpc.StreamInterceptor(auth.StreamInterceptor()),
 	)
-	sgardpb.RegisterGardenSyncServer(srv, server.New(serverGarden))
+	sgardpb.RegisterGardenSyncServer(srv, server.NewWithAuth(serverGarden, auth))
 	t.Cleanup(func() { srv.Stop() })
 	go func() { _ = srv.Serve(lis) }()
 
-	// Client WITHOUT credentials.
+	// Client WITHOUT credentials — no token, no signer.
 	conn, err := grpc.NewClient("passthrough:///bufconn",
 		grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
 			return lis.Dial()

client/e2e_test.go

@@ -39,19 +39,20 @@ func TestE2EPushPullCycle(t *testing.T) {
 		t.Fatalf("init server: %v", err)
 	}
 
-	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()})
+	jwtKey := []byte("e2e-test-jwt-secret-key-32bytes!")
+	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()}, jwtKey)
 	lis := bufconn.Listen(bufSize)
 	srv := grpc.NewServer(
 		grpc.UnaryInterceptor(auth.UnaryInterceptor()),
 		grpc.StreamInterceptor(auth.StreamInterceptor()),
 	)
-	sgardpb.RegisterGardenSyncServer(srv, server.New(serverGarden))
+	sgardpb.RegisterGardenSyncServer(srv, server.NewWithAuth(serverGarden, auth))
 	t.Cleanup(func() { srv.Stop() })
 	go func() { _ = srv.Serve(lis) }()
 
 	dial := func(t *testing.T) *Client {
 		t.Helper()
-		creds := NewSSHCredentials(signer)
+		creds := NewTokenCredentials("")
 		conn, err := grpc.NewClient("passthrough:///bufconn",
 			grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
 				return lis.Dial()
@@ -63,7 +64,11 @@ func TestE2EPushPullCycle(t *testing.T) {
 			t.Fatalf("dial: %v", err)
 		}
 		t.Cleanup(func() { _ = conn.Close() })
-		return New(conn)
+		c := NewWithAuth(conn, creds, signer)
+		if err := c.EnsureAuth(context.Background()); err != nil {
+			t.Fatalf("EnsureAuth: %v", err)
+		}
+		return c
 	}
 
 	ctx := context.Background()

cmd/sgard/main.go

@@ -1,12 +1,16 @@
 package main
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
 
+	"github.com/kisom/sgard/client"
 	"github.com/spf13/cobra"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
 )
 
 var (
@@ -47,6 +51,41 @@ func resolveRemote() (string, error) {
 	return "", fmt.Errorf("no remote configured; use --remote, SGARD_REMOTE, or create %s/remote", repoFlag)
 }
 
+// dialRemote creates a gRPC client with token-based auth and auto-renewal.
+func dialRemote(ctx context.Context) (*client.Client, func(), error) {
+	addr, err := resolveRemote()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	signer, err := client.LoadSigner(sshKeyFlag)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	cachedToken := client.LoadCachedToken()
+	creds := client.NewTokenCredentials(cachedToken)
+
+	conn, err := grpc.NewClient(addr,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithPerRPCCredentials(creds),
+	)
+	if err != nil {
+		return nil, nil, fmt.Errorf("connecting to %s: %w", addr, err)
+	}
+
+	c := client.NewWithAuth(conn, creds, signer)
+
+	// Ensure we have a valid token before proceeding.
+	if err := c.EnsureAuth(ctx); err != nil {
+		_ = conn.Close()
+		return nil, nil, fmt.Errorf("authentication: %w", err)
+	}
+
+	cleanup := func() { _ = conn.Close() }
+	return c, cleanup, nil
+}
+
 func main() {
 	rootCmd.PersistentFlags().StringVar(&repoFlag, "repo", defaultRepo(), "path to sgard repository")
 	rootCmd.PersistentFlags().StringVar(&remoteFlag, "remote", "", "gRPC server address (host:port)")

cmd/sgard/prune.go

@@ -4,11 +4,8 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/kisom/sgard/client"
 	"github.com/kisom/sgard/garden"
 	"github.com/spf13/cobra"
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/credentials/insecure"
 )
 
 var pruneCmd = &cobra.Command{
@@ -19,7 +16,7 @@ var pruneCmd = &cobra.Command{
 		addr, _ := resolveRemote()
 
 		if addr != "" {
-			return pruneRemote(addr)
+			return pruneRemote()
 		}
 		return pruneLocal()
 	},
@@ -40,24 +37,16 @@ func pruneLocal() error {
 	return nil
 }
 
-func pruneRemote(addr string) error {
-	signer, err := client.LoadSigner(sshKeyFlag)
+func pruneRemote() error {
+	ctx := context.Background()
+
+	c, cleanup, err := dialRemote(ctx)
 	if err != nil {
 		return err
 	}
+	defer cleanup()
 
-	creds := client.NewSSHCredentials(signer)
-	conn, err := grpc.NewClient(addr,
-		grpc.WithTransportCredentials(insecure.NewCredentials()),
-		grpc.WithPerRPCCredentials(creds),
-	)
-	if err != nil {
-		return fmt.Errorf("connecting to %s: %w", addr, err)
-	}
-	defer func() { _ = conn.Close() }()
-
-	c := client.New(conn)
-	removed, err := c.Prune(context.Background())
+	removed, err := c.Prune(ctx)
 	if err != nil {
 		return err
 	}

cmd/sgard/pull.go

@@ -4,44 +4,28 @@ import (
 	"context"
 	"fmt"
 
-	"github.com/kisom/sgard/client"
 	"github.com/kisom/sgard/garden"
 	"github.com/spf13/cobra"
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/credentials/insecure"
 )
 
 var pullCmd = &cobra.Command{
 	Use:   "pull",
 	Short: "Pull checkpoint from remote server",
 	RunE: func(cmd *cobra.Command, args []string) error {
-		addr, err := resolveRemote()
-		if err != nil {
-			return err
-		}
+		ctx := context.Background()
 
 		g, err := garden.Open(repoFlag)
 		if err != nil {
 			return err
 		}
 
-		signer, err := client.LoadSigner(sshKeyFlag)
+		c, cleanup, err := dialRemote(ctx)
 		if err != nil {
 			return err
 		}
+		defer cleanup()
 
-		creds := client.NewSSHCredentials(signer)
-		conn, err := grpc.NewClient(addr,
-			grpc.WithTransportCredentials(insecure.NewCredentials()),
-			grpc.WithPerRPCCredentials(creds),
-		)
-		if err != nil {
-			return fmt.Errorf("connecting to %s: %w", addr, err)
-		}
-		defer func() { _ = conn.Close() }()
-
-		c := client.New(conn)
-		pulled, err := c.Pull(context.Background(), g)
+		pulled, err := c.Pull(ctx, g)
 		if err != nil {
 			return err
 		}

cmd/sgard/push.go

@@ -8,41 +8,26 @@ import (
 	"github.com/kisom/sgard/client"
 	"github.com/kisom/sgard/garden"
 	"github.com/spf13/cobra"
-	"google.golang.org/grpc"
-	"google.golang.org/grpc/credentials/insecure"
 )
 
 var pushCmd = &cobra.Command{
 	Use:   "push",
 	Short: "Push local checkpoint to remote server",
 	RunE: func(cmd *cobra.Command, args []string) error {
-		addr, err := resolveRemote()
-		if err != nil {
-			return err
-		}
+		ctx := context.Background()
 
 		g, err := garden.Open(repoFlag)
 		if err != nil {
 			return err
 		}
 
-		signer, err := client.LoadSigner(sshKeyFlag)
+		c, cleanup, err := dialRemote(ctx)
 		if err != nil {
 			return err
 		}
+		defer cleanup()
 
-		creds := client.NewSSHCredentials(signer)
-		conn, err := grpc.NewClient(addr,
-			grpc.WithTransportCredentials(insecure.NewCredentials()),
-			grpc.WithPerRPCCredentials(creds),
-		)
-		if err != nil {
-			return fmt.Errorf("connecting to %s: %w", addr, err)
-		}
-		defer func() { _ = conn.Close() }()
-
-		c := client.New(conn)
-		pushed, err := c.Push(context.Background(), g)
+		pushed, err := c.Push(ctx, g)
 		if errors.Is(err, client.ErrServerNewer) {
 			fmt.Println("Server is newer; run sgard pull instead.")
 			return nil

cmd/sgardd/main.go

@@ -29,9 +29,10 @@ var rootCmd = &cobra.Command{
 		}
 
 		var opts []grpc.ServerOption
+		var srvInstance *server.Server
 
 		if authKeysPath != "" {
-			auth, err := server.NewAuthInterceptor(authKeysPath)
+			auth, err := server.NewAuthInterceptor(authKeysPath, repoPath)
 			if err != nil {
 				return fmt.Errorf("loading authorized keys: %w", err)
 			}
@@ -39,13 +40,15 @@ var rootCmd = &cobra.Command{
 				grpc.UnaryInterceptor(auth.UnaryInterceptor()),
 				grpc.StreamInterceptor(auth.StreamInterceptor()),
 			)
+			srvInstance = server.NewWithAuth(g, auth)
 			fmt.Printf("Auth enabled: %s\n", authKeysPath)
 		} else {
+			srvInstance = server.New(g)
 			fmt.Println("WARNING: no --authorized-keys specified, running without authentication")
 		}
 
 		srv := grpc.NewServer(opts...)
-		sgardpb.RegisterGardenSyncServer(srv, server.New(g))
+		sgardpb.RegisterGardenSyncServer(srv, srvInstance)
 
 		lis, err := net.Listen("tcp", listenAddr)
 		if err != nil {

go.mod

@@ -3,6 +3,7 @@ module github.com/kisom/sgard
 go 1.25.7
 
 require (
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/jonboulle/clockwork v0.5.0 // indirect
 	github.com/spf13/cobra v1.10.2 // indirect

go.sum

@@ -1,4 +1,6 @@
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
+github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
+github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
 github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
 github.com/jonboulle/clockwork v0.5.0 h1:Hyh9A8u51kptdkR+cqRpT1EebBwTn1oK9YfGYbdFz6I=

proto/sgard/v1/sgard.proto

@@ -82,8 +82,32 @@ message PruneResponse {
   int32 blobs_removed = 1;
 }
 
+// Auth messages.
+
+message AuthenticateRequest {
+  bytes  nonce      = 1; // 32-byte nonce (server-provided or client-generated)
+  int64  timestamp  = 2; // Unix seconds
+  bytes  signature  = 3; // SSH signature over (nonce || timestamp)
+  string public_key = 4; // SSH public key in authorized_keys format
+}
+
+message AuthenticateResponse {
+  string token = 1; // JWT valid for 30 days
+}
+
+// ReauthChallenge is embedded in Unauthenticated error details when a
+// token is expired but was previously valid. The client signs this
+// challenge to obtain a new token without generating its own nonce.
+message ReauthChallenge {
+  bytes nonce     = 1; // server-generated 32-byte nonce
+  int64 timestamp = 2; // server's current Unix timestamp
+}
+
 // GardenSync is the sgard remote sync service.
 service GardenSync {
+  // Authenticate exchanges an SSH-signed challenge for a JWT token.
+  rpc Authenticate(AuthenticateRequest) returns (AuthenticateResponse);
+
   // Push flow: send manifest, then stream missing blobs.
   rpc PushManifest(PushManifestRequest) returns (PushManifestResponse);
   rpc PushBlobs(stream PushBlobsRequest) returns (PushBlobsResponse);

server/auth.go

@@ -3,13 +3,14 @@ package server
 import (
 	"context"
 	"crypto/rand"
-	"encoding/base64"
 	"fmt"
 	"os"
-	"strconv"
+	"path/filepath"
 	"strings"
 	"time"
 
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/kisom/sgard/sgardpb"
 	"golang.org/x/crypto/ssh"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
@@ -18,25 +19,21 @@ import (
 )
 
 const (
-	// Metadata keys for auth.
-	metaNonce     = "x-sgard-auth-nonce"
-	metaTimestamp = "x-sgard-auth-timestamp"
-	metaSignature = "x-sgard-auth-signature"
-	metaPubkey    = "x-sgard-auth-pubkey"
-
-	// authWindow is how far the timestamp can deviate from server time.
+	metaToken  = "x-sgard-auth-token"
 	authWindow = 5 * time.Minute
+	tokenTTL   = 30 * 24 * time.Hour // 30 days
 )
 
-// AuthInterceptor verifies SSH key signatures on gRPC requests.
+// AuthInterceptor verifies JWT tokens or SSH key signatures on gRPC requests.
 type AuthInterceptor struct {
 	authorizedKeys map[string]ssh.PublicKey // keyed by fingerprint
+	jwtKey         []byte                  // HMAC-SHA256 signing key
 }
 
-// NewAuthInterceptor creates an interceptor from an authorized_keys file.
-// The file uses the same format as ~/.ssh/authorized_keys.
-func NewAuthInterceptor(path string) (*AuthInterceptor, error) {
-	data, err := os.ReadFile(path)
+// NewAuthInterceptor creates an interceptor from an authorized_keys file
+// and a repository path (for the JWT secret key).
+func NewAuthInterceptor(authorizedKeysPath, repoPath string) (*AuthInterceptor, error) {
+	data, err := os.ReadFile(authorizedKeysPath)
 	if err != nil {
 		return nil, fmt.Errorf("reading authorized keys: %w", err)
 	}
@@ -54,26 +51,35 @@ func NewAuthInterceptor(path string) (*AuthInterceptor, error) {
 	}
 
 	if len(keys) == 0 {
-		return nil, fmt.Errorf("no valid keys found in %s", path)
+		return nil, fmt.Errorf("no valid keys found in %s", authorizedKeysPath)
 	}
 
-	return &AuthInterceptor{authorizedKeys: keys}, nil
+	jwtKey, err := loadOrGenerateJWTKey(repoPath)
+	if err != nil {
+		return nil, fmt.Errorf("loading JWT key: %w", err)
+	}
+
+	return &AuthInterceptor{authorizedKeys: keys, jwtKey: jwtKey}, nil
 }
 
-// NewAuthInterceptorFromKeys creates an interceptor from pre-parsed keys.
-// Intended for testing.
-func NewAuthInterceptorFromKeys(keys []ssh.PublicKey) *AuthInterceptor {
+// NewAuthInterceptorFromKeys creates an interceptor from pre-parsed keys
+// and a provided JWT key. Intended for testing.
+func NewAuthInterceptorFromKeys(keys []ssh.PublicKey, jwtKey []byte) *AuthInterceptor {
 	m := make(map[string]ssh.PublicKey, len(keys))
 	for _, k := range keys {
 		m[ssh.FingerprintSHA256(k)] = k
 	}
-	return &AuthInterceptor{authorizedKeys: m}
+	return &AuthInterceptor{authorizedKeys: m, jwtKey: jwtKey}
 }
 
 // UnaryInterceptor returns a gRPC unary server interceptor.
 func (a *AuthInterceptor) UnaryInterceptor() grpc.UnaryServerInterceptor {
 	return func(ctx context.Context, req any, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (any, error) {
-		if err := a.verify(ctx); err != nil {
+		// Authenticate RPC is exempt from auth — it's how you get a token.
+		if strings.HasSuffix(info.FullMethod, "/Authenticate") {
+			return handler(ctx, req)
+		}
+		if err := a.verifyToken(ctx); err != nil {
 			return nil, err
 		}
 		return handler(ctx, req)
@@ -83,76 +89,161 @@ func (a *AuthInterceptor) UnaryInterceptor() grpc.UnaryServerInterceptor {
 // StreamInterceptor returns a gRPC stream server interceptor.
 func (a *AuthInterceptor) StreamInterceptor() grpc.StreamServerInterceptor {
 	return func(srv any, ss grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
-		if err := a.verify(ss.Context()); err != nil {
+		if err := a.verifyToken(ss.Context()); err != nil {
 			return err
 		}
 		return handler(srv, ss)
 	}
 }
 
-func (a *AuthInterceptor) verify(ctx context.Context) error {
+// Authenticate verifies an SSH-signed challenge and issues a JWT.
+func (a *AuthInterceptor) Authenticate(_ context.Context, req *sgardpb.AuthenticateRequest) (*sgardpb.AuthenticateResponse, error) {
+	pubkeyStr := req.GetPublicKey()
+	pubkey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(pubkeyStr))
+	if err != nil {
+		return nil, status.Error(codes.Unauthenticated, "invalid public key")
+	}
+
+	fp := ssh.FingerprintSHA256(pubkey)
+	authorized, ok := a.authorizedKeys[fp]
+	if !ok {
+		return nil, status.Errorf(codes.PermissionDenied, "key %s not authorized", fp)
+	}
+
+	// Verify timestamp window.
+	tsUnix := req.GetTimestamp()
+	ts := time.Unix(tsUnix, 0)
+	if time.Since(ts).Abs() > authWindow {
+		return nil, status.Error(codes.Unauthenticated, "timestamp outside allowed window")
+	}
+
+	// Verify signature.
+	payload := buildPayload(req.GetNonce(), tsUnix)
+	sig, err := parseSSHSignature(req.GetSignature())
+	if err != nil {
+		return nil, status.Error(codes.Unauthenticated, "invalid signature format")
+	}
+	if err := authorized.Verify(payload, sig); err != nil {
+		return nil, status.Error(codes.Unauthenticated, "signature verification failed")
+	}
+
+	// Issue JWT.
+	token, err := a.issueToken(fp)
+	if err != nil {
+		return nil, status.Errorf(codes.Internal, "issuing token: %v", err)
+	}
+
+	return &sgardpb.AuthenticateResponse{Token: token}, nil
+}
+
+func (a *AuthInterceptor) verifyToken(ctx context.Context) error {
 	md, ok := metadata.FromIncomingContext(ctx)
 	if !ok {
 		return status.Error(codes.Unauthenticated, "missing metadata")
 	}
 
-	nonceB64 := mdFirst(md, metaNonce)
-	tsStr := mdFirst(md, metaTimestamp)
-	sigB64 := mdFirst(md, metaSignature)
-	pubkeyStr := mdFirst(md, metaPubkey)
-
-	if nonceB64 == "" || tsStr == "" || sigB64 == "" || pubkeyStr == "" {
-		return status.Error(codes.Unauthenticated, "missing auth metadata fields")
+	tokenStr := mdFirst(md, metaToken)
+	if tokenStr == "" {
+		return status.Error(codes.Unauthenticated, "missing auth token")
 	}
 
-	// Parse timestamp and check window.
-	tsUnix, err := strconv.ParseInt(tsStr, 10, 64)
-	if err != nil {
-		return status.Error(codes.Unauthenticated, "invalid timestamp")
-	}
-	ts := time.Unix(tsUnix, 0)
-	if time.Since(ts).Abs() > authWindow {
-		return status.Error(codes.Unauthenticated, "timestamp outside allowed window")
+	claims := &jwt.RegisteredClaims{}
+	token, err := jwt.ParseWithClaims(tokenStr, claims, func(t *jwt.Token) (any, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return a.jwtKey, nil
+	})
+
+	if err != nil || !token.Valid {
+		// Check if the token is expired but otherwise valid.
+		if a.isExpiredButValid(tokenStr, claims) {
+			return a.reauthError()
+		}
+		return status.Error(codes.Unauthenticated, "invalid token")
 	}
 
-	// Parse public key and check authorization.
-	pubkey, _, _, _, err := ssh.ParseAuthorizedKey([]byte(pubkeyStr))
-	if err != nil {
-		return status.Error(codes.Unauthenticated, "invalid public key")
-	}
-	fp := ssh.FingerprintSHA256(pubkey)
-	authorized, ok := a.authorizedKeys[fp]
-	if !ok {
-		return status.Errorf(codes.PermissionDenied, "key %s not authorized", fp)
-	}
-
-	// Decode nonce and signature.
-	nonce, err := base64.StdEncoding.DecodeString(nonceB64)
-	if err != nil {
-		return status.Error(codes.Unauthenticated, "invalid nonce encoding")
-	}
-
-	sigBytes, err := base64.StdEncoding.DecodeString(sigB64)
-	if err != nil {
-		return status.Error(codes.Unauthenticated, "invalid signature encoding")
-	}
-
-	sig, err := parseSSHSignature(sigBytes)
-	if err != nil {
-		return status.Error(codes.Unauthenticated, "invalid signature format")
-	}
-
-	// Build the signed payload: nonce + timestamp bytes.
-	payload := buildPayload(nonce, tsUnix)
-
-	// Verify.
-	if err := authorized.Verify(payload, sig); err != nil {
-		return status.Error(codes.Unauthenticated, "signature verification failed")
+	// Verify the fingerprint is still authorized.
+	fp := claims.Subject
+	if _, ok := a.authorizedKeys[fp]; !ok {
+		return status.Errorf(codes.PermissionDenied, "key %s no longer authorized", fp)
 	}
 
 	return nil
 }
 
+// isExpiredButValid checks if a token has a valid signature and the
+// fingerprint is still in authorized_keys, but the token is expired.
+func (a *AuthInterceptor) isExpiredButValid(tokenStr string, claims *jwt.RegisteredClaims) bool {
+	// Re-parse without time validation.
+	reClaims := &jwt.RegisteredClaims{}
+	_, err := jwt.ParseWithClaims(tokenStr, reClaims, func(t *jwt.Token) (any, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method")
+		}
+		return a.jwtKey, nil
+	}, jwt.WithoutClaimsValidation())
+	if err != nil {
+		return false
+	}
+
+	fp := reClaims.Subject
+	_, authorized := a.authorizedKeys[fp]
+	return authorized
+}
+
+// reauthError returns an Unauthenticated error with a ReauthChallenge
+// embedded in the error details.
+func (a *AuthInterceptor) reauthError() error {
+	nonce := make([]byte, 32)
+	if _, err := rand.Read(nonce); err != nil {
+		return status.Error(codes.Internal, "generating reauth nonce")
+	}
+
+	challenge := &sgardpb.ReauthChallenge{
+		Nonce:     nonce,
+		Timestamp: time.Now().Unix(),
+	}
+
+	st, err := status.New(codes.Unauthenticated, "token expired").
+		WithDetails(challenge)
+	if err != nil {
+		return status.Error(codes.Unauthenticated, "token expired")
+	}
+	return st.Err()
+}
+
+func (a *AuthInterceptor) issueToken(fingerprint string) (string, error) {
+	now := time.Now()
+	claims := &jwt.RegisteredClaims{
+		Subject:   fingerprint,
+		IssuedAt:  jwt.NewNumericDate(now),
+		ExpiresAt: jwt.NewNumericDate(now.Add(tokenTTL)),
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(a.jwtKey)
+}
+
+func loadOrGenerateJWTKey(repoPath string) ([]byte, error) {
+	keyPath := filepath.Join(repoPath, "jwt.key")
+
+	data, err := os.ReadFile(keyPath)
+	if err == nil && len(data) >= 32 {
+		return data[:32], nil
+	}
+
+	key := make([]byte, 32)
+	if _, err := rand.Read(key); err != nil {
+		return nil, fmt.Errorf("generating JWT key: %w", err)
+	}
+
+	if err := os.WriteFile(keyPath, key, 0o600); err != nil {
+		return nil, fmt.Errorf("writing JWT key: %w", err)
+	}
+
+	return key, nil
+}
+
 // buildPayload constructs the message that is signed: nonce || timestamp (big-endian int64).
 func buildPayload(nonce []byte, tsUnix int64) []byte {
 	payload := make([]byte, len(nonce)+8)
@@ -187,7 +278,6 @@ func parseSSHSignature(data []byte) (*ssh.Signature, error) {
 		return nil, fmt.Errorf("signature too short")
 	}
 
-	// SSH signature wire format: string format, string blob
 	formatLen := int(data[0])<<24 | int(data[1])<<16 | int(data[2])<<8 | int(data[3])
 	if 4+formatLen > len(data) {
 		return nil, fmt.Errorf("invalid format length")
@@ -204,8 +294,6 @@ func parseSSHSignature(data []byte) (*ssh.Signature, error) {
 	}
 	blob := rest[4 : 4+blobLen]
 
-	_ = strings.TrimSpace(format) // ensure format is clean
-
 	return &ssh.Signature{
 		Format: format,
 		Blob:   blob,

server/auth_test.go

@@ -4,15 +4,18 @@ import (
 	"context"
 	"crypto/ed25519"
 	"crypto/rand"
-	"encoding/base64"
-	"strconv"
+	"strings"
 	"testing"
 	"time"
 
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/kisom/sgard/sgardpb"
 	"golang.org/x/crypto/ssh"
 	"google.golang.org/grpc/metadata"
 )
 
+var testJWTKey = []byte("test-jwt-secret-key-32-bytes!!")
+
 func generateTestKey(t *testing.T) (ssh.Signer, ssh.PublicKey) {
 	t.Helper()
 	_, priv, err := ed25519.GenerateKey(rand.Reader)
@@ -26,94 +29,118 @@ func generateTestKey(t *testing.T) (ssh.Signer, ssh.PublicKey) {
 	return signer, signer.PublicKey()
 }
 
-func signedContext(t *testing.T, signer ssh.Signer) context.Context {
-	t.Helper()
+func TestAuthenticateAndVerifyToken(t *testing.T) {
+	signer, pubkey := generateTestKey(t)
+	auth := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey}, testJWTKey)
 
-	nonce, err := GenerateNonce()
-	if err != nil {
-		t.Fatalf("generating nonce: %v", err)
-	}
+	// Generate a signed challenge.
+	nonce, _ := GenerateNonce()
 	tsUnix := time.Now().Unix()
 	payload := buildPayload(nonce, tsUnix)
-
 	sig, err := signer.Sign(rand.Reader, payload)
 	if err != nil {
 		t.Fatalf("signing: %v", err)
 	}
 
-	pubkeyStr := string(ssh.MarshalAuthorizedKey(signer.PublicKey()))
+	pubkeyStr := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(signer.PublicKey())))
 
-	md := metadata.New(map[string]string{
-		metaNonce:     base64.StdEncoding.EncodeToString(nonce),
-		metaTimestamp: strconv.FormatInt(tsUnix, 10),
-		metaSignature: base64.StdEncoding.EncodeToString(ssh.Marshal(sig)),
-		metaPubkey:    pubkeyStr,
+	// Call Authenticate.
+	resp, err := auth.Authenticate(context.Background(), &sgardpb.AuthenticateRequest{
+		Nonce:     nonce,
+		Timestamp: tsUnix,
+		Signature: ssh.Marshal(sig),
+		PublicKey: pubkeyStr,
 	})
-	return metadata.NewIncomingContext(context.Background(), md)
-}
+	if err != nil {
+		t.Fatalf("Authenticate: %v", err)
+	}
+	if resp.Token == "" {
+		t.Fatal("expected non-empty token")
+	}
 
-func TestAuthVerifyValid(t *testing.T) {
-	signer, pubkey := generateTestKey(t)
-	interceptor := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey})
-
-	ctx := signedContext(t, signer)
-	if err := interceptor.verify(ctx); err != nil {
-		t.Fatalf("verify should succeed: %v", err)
+	// Use the token in metadata.
+	md := metadata.New(map[string]string{metaToken: resp.Token})
+	ctx := metadata.NewIncomingContext(context.Background(), md)
+	if err := auth.verifyToken(ctx); err != nil {
+		t.Fatalf("verifyToken should accept valid token: %v", err)
 	}
 }
 
-func TestAuthRejectUnauthenticated(t *testing.T) {
+func TestRejectMissingToken(t *testing.T) {
 	_, pubkey := generateTestKey(t)
-	interceptor := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey})
+	auth := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey}, testJWTKey)
 
 	// No metadata at all.
-	ctx := context.Background()
-	if err := interceptor.verify(ctx); err == nil {
-		t.Fatal("verify should reject missing metadata")
+	if err := auth.verifyToken(context.Background()); err == nil {
+		t.Fatal("should reject missing metadata")
+	}
+
+	// Empty metadata.
+	md := metadata.New(nil)
+	ctx := metadata.NewIncomingContext(context.Background(), md)
+	if err := auth.verifyToken(ctx); err == nil {
+		t.Fatal("should reject missing token")
 	}
 }
 
-func TestAuthRejectUnauthorizedKey(t *testing.T) {
+func TestRejectUnauthorizedKey(t *testing.T) {
 	signer1, _ := generateTestKey(t)
 	_, pubkey2 := generateTestKey(t)
 
-	// Interceptor knows key2 but request is signed by key1.
-	interceptor := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey2})
+	// Auth only knows pubkey2, but we authenticate with signer1.
+	auth := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey2}, testJWTKey)
 
-	ctx := signedContext(t, signer1)
-	if err := interceptor.verify(ctx); err == nil {
-		t.Fatal("verify should reject unauthorized key")
-	}
-}
-
-func TestAuthRejectExpiredTimestamp(t *testing.T) {
-	signer, pubkey := generateTestKey(t)
-	interceptor := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey})
-
-	nonce, err := GenerateNonce()
-	if err != nil {
-		t.Fatalf("generating nonce: %v", err)
-	}
-	// Timestamp 10 minutes ago — outside the 5-minute window.
-	tsUnix := time.Now().Add(-10 * time.Minute).Unix()
+	nonce, _ := GenerateNonce()
+	tsUnix := time.Now().Unix()
 	payload := buildPayload(nonce, tsUnix)
+	sig, _ := signer1.Sign(rand.Reader, payload)
+	pubkeyStr := strings.TrimSpace(string(ssh.MarshalAuthorizedKey(signer1.PublicKey())))
 
-	sig, err := signer.Sign(rand.Reader, payload)
-	if err != nil {
-		t.Fatalf("signing: %v", err)
-	}
-
-	pubkeyStr := string(ssh.MarshalAuthorizedKey(signer.PublicKey()))
-
-	md := metadata.New(map[string]string{
-		metaNonce:     base64.StdEncoding.EncodeToString(nonce),
-		metaTimestamp: strconv.FormatInt(tsUnix, 10),
-		metaSignature: base64.StdEncoding.EncodeToString(ssh.Marshal(sig)),
-		metaPubkey:    pubkeyStr,
+	_, err := auth.Authenticate(context.Background(), &sgardpb.AuthenticateRequest{
+		Nonce:     nonce,
+		Timestamp: tsUnix,
+		Signature: ssh.Marshal(sig),
+		PublicKey: pubkeyStr,
 	})
-	ctx := metadata.NewIncomingContext(context.Background(), md)
-
-	if err := interceptor.verify(ctx); err == nil {
-		t.Fatal("verify should reject expired timestamp")
+	if err == nil {
+		t.Fatal("should reject unauthorized key")
 	}
 }
+
+func TestExpiredTokenReturnsChallenge(t *testing.T) {
+	signer, pubkey := generateTestKey(t)
+	auth := NewAuthInterceptorFromKeys([]ssh.PublicKey{pubkey}, testJWTKey)
+
+	// Issue a token, then manually create an expired one.
+	fp := ssh.FingerprintSHA256(signer.PublicKey())
+	expiredToken, err := auth.issueExpiredToken(fp)
+	if err != nil {
+		t.Fatalf("issuing expired token: %v", err)
+	}
+
+	md := metadata.New(map[string]string{metaToken: expiredToken})
+	ctx := metadata.NewIncomingContext(context.Background(), md)
+	err = auth.verifyToken(ctx)
+	if err == nil {
+		t.Fatal("should reject expired token")
+	}
+
+	// The error should contain a ReauthChallenge in its details.
+	// We can't easily extract it here without the client helper,
+	// but verify the error message indicates expiry.
+	if !strings.Contains(err.Error(), "expired") {
+		t.Errorf("error should mention expiry, got: %v", err)
+	}
+}
+
+// issueExpiredToken is a test helper that creates an already-expired JWT.
+func (a *AuthInterceptor) issueExpiredToken(fingerprint string) (string, error) {
+	past := time.Now().Add(-time.Hour)
+	claims := &jwt.RegisteredClaims{
+		Subject:   fingerprint,
+		IssuedAt:  jwt.NewNumericDate(past.Add(-24 * time.Hour)),
+		ExpiresAt: jwt.NewNumericDate(past),
+	}
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	return token.SignedString(a.jwtKey)
+}

server/server.go

@@ -24,6 +24,7 @@ type Server struct {
 	garden          *garden.Garden
 	mu              sync.RWMutex
 	pendingManifest *manifest.Manifest
+	auth            *AuthInterceptor // nil if auth is disabled
 }
 
 // New creates a new Server backed by the given Garden.
@@ -31,6 +32,19 @@ func New(g *garden.Garden) *Server {
 	return &Server{garden: g}
 }
 
+// NewWithAuth creates a new Server with authentication enabled.
+func NewWithAuth(g *garden.Garden, auth *AuthInterceptor) *Server {
+	return &Server{garden: g, auth: auth}
+}
+
+// Authenticate handles the auth RPC by delegating to the AuthInterceptor.
+func (s *Server) Authenticate(ctx context.Context, req *sgardpb.AuthenticateRequest) (*sgardpb.AuthenticateResponse, error) {
+	if s.auth == nil {
+		return nil, status.Error(codes.Unimplemented, "authentication not configured")
+	}
+	return s.auth.Authenticate(ctx, req)
+}
+
 // PushManifest compares the client manifest against the server manifest and
 // decides whether to accept, reject, or report up-to-date.
 func (s *Server) PushManifest(_ context.Context, req *sgardpb.PushManifestRequest) (*sgardpb.PushManifestResponse, error) {

sgardpb/sgard.pb.go

@@ -730,6 +730,173 @@ func (x *PruneResponse) GetBlobsRemoved() int32 {
 	return 0
 }
 
+type AuthenticateRequest struct {
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	Nonce         []byte                 `protobuf:"bytes,1,opt,name=nonce,proto3" json:"nonce,omitempty"`                          // 32-byte nonce (server-provided or client-generated)
+	Timestamp     int64                  `protobuf:"varint,2,opt,name=timestamp,proto3" json:"timestamp,omitempty"`                 // Unix seconds
+	Signature     []byte                 `protobuf:"bytes,3,opt,name=signature,proto3" json:"signature,omitempty"`                  // SSH signature over (nonce || timestamp)
+	PublicKey     string                 `protobuf:"bytes,4,opt,name=public_key,json=publicKey,proto3" json:"public_key,omitempty"` // SSH public key in authorized_keys format
+	unknownFields protoimpl.UnknownFields
+	sizeCache     protoimpl.SizeCache
+}
+
+func (x *AuthenticateRequest) Reset() {
+	*x = AuthenticateRequest{}
+	mi := &file_sgard_v1_sgard_proto_msgTypes[13]
+	ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+	ms.StoreMessageInfo(mi)
+}
+
+func (x *AuthenticateRequest) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*AuthenticateRequest) ProtoMessage() {}
+
+func (x *AuthenticateRequest) ProtoReflect() protoreflect.Message {
+	mi := &file_sgard_v1_sgard_proto_msgTypes[13]
+	if x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use AuthenticateRequest.ProtoReflect.Descriptor instead.
+func (*AuthenticateRequest) Descriptor() ([]byte, []int) {
+	return file_sgard_v1_sgard_proto_rawDescGZIP(), []int{13}
+}
+
+func (x *AuthenticateRequest) GetNonce() []byte {
+	if x != nil {
+		return x.Nonce
+	}
+	return nil
+}
+
+func (x *AuthenticateRequest) GetTimestamp() int64 {
+	if x != nil {
+		return x.Timestamp
+	}
+	return 0
+}
+
+func (x *AuthenticateRequest) GetSignature() []byte {
+	if x != nil {
+		return x.Signature
+	}
+	return nil
+}
+
+func (x *AuthenticateRequest) GetPublicKey() string {
+	if x != nil {
+		return x.PublicKey
+	}
+	return ""
+}
+
+type AuthenticateResponse struct {
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	Token         string                 `protobuf:"bytes,1,opt,name=token,proto3" json:"token,omitempty"` // JWT valid for 30 days
+	unknownFields protoimpl.UnknownFields
+	sizeCache     protoimpl.SizeCache
+}
+
+func (x *AuthenticateResponse) Reset() {
+	*x = AuthenticateResponse{}
+	mi := &file_sgard_v1_sgard_proto_msgTypes[14]
+	ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+	ms.StoreMessageInfo(mi)
+}
+
+func (x *AuthenticateResponse) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*AuthenticateResponse) ProtoMessage() {}
+
+func (x *AuthenticateResponse) ProtoReflect() protoreflect.Message {
+	mi := &file_sgard_v1_sgard_proto_msgTypes[14]
+	if x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use AuthenticateResponse.ProtoReflect.Descriptor instead.
+func (*AuthenticateResponse) Descriptor() ([]byte, []int) {
+	return file_sgard_v1_sgard_proto_rawDescGZIP(), []int{14}
+}
+
+func (x *AuthenticateResponse) GetToken() string {
+	if x != nil {
+		return x.Token
+	}
+	return ""
+}
+
+// ReauthChallenge is embedded in Unauthenticated error details when a
+// token is expired but was previously valid. The client signs this
+// challenge to obtain a new token without generating its own nonce.
+type ReauthChallenge struct {
+	state         protoimpl.MessageState `protogen:"open.v1"`
+	Nonce         []byte                 `protobuf:"bytes,1,opt,name=nonce,proto3" json:"nonce,omitempty"`          // server-generated 32-byte nonce
+	Timestamp     int64                  `protobuf:"varint,2,opt,name=timestamp,proto3" json:"timestamp,omitempty"` // server's current Unix timestamp
+	unknownFields protoimpl.UnknownFields
+	sizeCache     protoimpl.SizeCache
+}
+
+func (x *ReauthChallenge) Reset() {
+	*x = ReauthChallenge{}
+	mi := &file_sgard_v1_sgard_proto_msgTypes[15]
+	ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+	ms.StoreMessageInfo(mi)
+}
+
+func (x *ReauthChallenge) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*ReauthChallenge) ProtoMessage() {}
+
+func (x *ReauthChallenge) ProtoReflect() protoreflect.Message {
+	mi := &file_sgard_v1_sgard_proto_msgTypes[15]
+	if x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use ReauthChallenge.ProtoReflect.Descriptor instead.
+func (*ReauthChallenge) Descriptor() ([]byte, []int) {
+	return file_sgard_v1_sgard_proto_rawDescGZIP(), []int{15}
+}
+
+func (x *ReauthChallenge) GetNonce() []byte {
+	if x != nil {
+		return x.Nonce
+	}
+	return nil
+}
+
+func (x *ReauthChallenge) GetTimestamp() int64 {
+	if x != nil {
+		return x.Timestamp
+	}
+	return 0
+}
+
 var File_sgard_v1_sgard_proto protoreflect.FileDescriptor
 
 const file_sgard_v1_sgard_proto_rawDesc = "" +
@@ -776,9 +943,21 @@ const file_sgard_v1_sgard_proto_rawDesc = "" +
 	"\x05chunk\x18\x01 \x01(\v2\x13.sgard.v1.BlobChunkR\x05chunk\"\x0e\n" +
 	"\fPruneRequest\"4\n" +
 	"\rPruneResponse\x12#\n" +
-	"\rblobs_removed\x18\x01 \x01(\x05R\fblobsRemoved2\xf4\x02\n" +
+	"\rblobs_removed\x18\x01 \x01(\x05R\fblobsRemoved\"\x86\x01\n" +
+	"\x13AuthenticateRequest\x12\x14\n" +
+	"\x05nonce\x18\x01 \x01(\fR\x05nonce\x12\x1c\n" +
+	"\ttimestamp\x18\x02 \x01(\x03R\ttimestamp\x12\x1c\n" +
+	"\tsignature\x18\x03 \x01(\fR\tsignature\x12\x1d\n" +
+	"\n" +
+	"public_key\x18\x04 \x01(\tR\tpublicKey\",\n" +
+	"\x14AuthenticateResponse\x12\x14\n" +
+	"\x05token\x18\x01 \x01(\tR\x05token\"E\n" +
+	"\x0fReauthChallenge\x12\x14\n" +
+	"\x05nonce\x18\x01 \x01(\fR\x05nonce\x12\x1c\n" +
+	"\ttimestamp\x18\x02 \x01(\x03R\ttimestamp2\xc3\x03\n" +
 	"\n" +
 	"GardenSync\x12M\n" +
+	"\fAuthenticate\x12\x1d.sgard.v1.AuthenticateRequest\x1a\x1e.sgard.v1.AuthenticateResponse\x12M\n" +
 	"\fPushManifest\x12\x1d.sgard.v1.PushManifestRequest\x1a\x1e.sgard.v1.PushManifestResponse\x12F\n" +
 	"\tPushBlobs\x12\x1a.sgard.v1.PushBlobsRequest\x1a\x1b.sgard.v1.PushBlobsResponse(\x01\x12M\n" +
 	"\fPullManifest\x12\x1d.sgard.v1.PullManifestRequest\x1a\x1e.sgard.v1.PullManifestResponse\x12F\n" +
@@ -798,7 +977,7 @@ func file_sgard_v1_sgard_proto_rawDescGZIP() []byte {
 }
 
 var file_sgard_v1_sgard_proto_enumTypes = make([]protoimpl.EnumInfo, 1)
-var file_sgard_v1_sgard_proto_msgTypes = make([]protoimpl.MessageInfo, 13)
+var file_sgard_v1_sgard_proto_msgTypes = make([]protoimpl.MessageInfo, 16)
 var file_sgard_v1_sgard_proto_goTypes = []any{
 	(PushManifestResponse_Decision)(0), // 0: sgard.v1.PushManifestResponse.Decision
 	(*ManifestEntry)(nil),              // 1: sgard.v1.ManifestEntry
@@ -814,31 +993,36 @@ var file_sgard_v1_sgard_proto_goTypes = []any{
 	(*PullBlobsResponse)(nil),          // 11: sgard.v1.PullBlobsResponse
 	(*PruneRequest)(nil),               // 12: sgard.v1.PruneRequest
 	(*PruneResponse)(nil),              // 13: sgard.v1.PruneResponse
-	(*timestamppb.Timestamp)(nil),      // 14: google.protobuf.Timestamp
+	(*AuthenticateRequest)(nil),        // 14: sgard.v1.AuthenticateRequest
+	(*AuthenticateResponse)(nil),       // 15: sgard.v1.AuthenticateResponse
+	(*ReauthChallenge)(nil),            // 16: sgard.v1.ReauthChallenge
+	(*timestamppb.Timestamp)(nil),      // 17: google.protobuf.Timestamp
 }
 var file_sgard_v1_sgard_proto_depIdxs = []int32{
-	14, // 0: sgard.v1.ManifestEntry.updated:type_name -> google.protobuf.Timestamp
-	14, // 1: sgard.v1.Manifest.created:type_name -> google.protobuf.Timestamp
-	14, // 2: sgard.v1.Manifest.updated:type_name -> google.protobuf.Timestamp
+	17, // 0: sgard.v1.ManifestEntry.updated:type_name -> google.protobuf.Timestamp
+	17, // 1: sgard.v1.Manifest.created:type_name -> google.protobuf.Timestamp
+	17, // 2: sgard.v1.Manifest.updated:type_name -> google.protobuf.Timestamp
 	1,  // 3: sgard.v1.Manifest.files:type_name -> sgard.v1.ManifestEntry
 	2,  // 4: sgard.v1.PushManifestRequest.manifest:type_name -> sgard.v1.Manifest
 	0,  // 5: sgard.v1.PushManifestResponse.decision:type_name -> sgard.v1.PushManifestResponse.Decision
-	14, // 6: sgard.v1.PushManifestResponse.server_updated:type_name -> google.protobuf.Timestamp
+	17, // 6: sgard.v1.PushManifestResponse.server_updated:type_name -> google.protobuf.Timestamp
 	3,  // 7: sgard.v1.PushBlobsRequest.chunk:type_name -> sgard.v1.BlobChunk
 	2,  // 8: sgard.v1.PullManifestResponse.manifest:type_name -> sgard.v1.Manifest
 	3,  // 9: sgard.v1.PullBlobsResponse.chunk:type_name -> sgard.v1.BlobChunk
-	4,  // 10: sgard.v1.GardenSync.PushManifest:input_type -> sgard.v1.PushManifestRequest
-	6,  // 11: sgard.v1.GardenSync.PushBlobs:input_type -> sgard.v1.PushBlobsRequest
-	8,  // 12: sgard.v1.GardenSync.PullManifest:input_type -> sgard.v1.PullManifestRequest
-	10, // 13: sgard.v1.GardenSync.PullBlobs:input_type -> sgard.v1.PullBlobsRequest
-	12, // 14: sgard.v1.GardenSync.Prune:input_type -> sgard.v1.PruneRequest
-	5,  // 15: sgard.v1.GardenSync.PushManifest:output_type -> sgard.v1.PushManifestResponse
-	7,  // 16: sgard.v1.GardenSync.PushBlobs:output_type -> sgard.v1.PushBlobsResponse
-	9,  // 17: sgard.v1.GardenSync.PullManifest:output_type -> sgard.v1.PullManifestResponse
-	11, // 18: sgard.v1.GardenSync.PullBlobs:output_type -> sgard.v1.PullBlobsResponse
-	13, // 19: sgard.v1.GardenSync.Prune:output_type -> sgard.v1.PruneResponse
-	15, // [15:20] is the sub-list for method output_type
-	10, // [10:15] is the sub-list for method input_type
+	14, // 10: sgard.v1.GardenSync.Authenticate:input_type -> sgard.v1.AuthenticateRequest
+	4,  // 11: sgard.v1.GardenSync.PushManifest:input_type -> sgard.v1.PushManifestRequest
+	6,  // 12: sgard.v1.GardenSync.PushBlobs:input_type -> sgard.v1.PushBlobsRequest
+	8,  // 13: sgard.v1.GardenSync.PullManifest:input_type -> sgard.v1.PullManifestRequest
+	10, // 14: sgard.v1.GardenSync.PullBlobs:input_type -> sgard.v1.PullBlobsRequest
+	12, // 15: sgard.v1.GardenSync.Prune:input_type -> sgard.v1.PruneRequest
+	15, // 16: sgard.v1.GardenSync.Authenticate:output_type -> sgard.v1.AuthenticateResponse
+	5,  // 17: sgard.v1.GardenSync.PushManifest:output_type -> sgard.v1.PushManifestResponse
+	7,  // 18: sgard.v1.GardenSync.PushBlobs:output_type -> sgard.v1.PushBlobsResponse
+	9,  // 19: sgard.v1.GardenSync.PullManifest:output_type -> sgard.v1.PullManifestResponse
+	11, // 20: sgard.v1.GardenSync.PullBlobs:output_type -> sgard.v1.PullBlobsResponse
+	13, // 21: sgard.v1.GardenSync.Prune:output_type -> sgard.v1.PruneResponse
+	16, // [16:22] is the sub-list for method output_type
+	10, // [10:16] is the sub-list for method input_type
 	10, // [10:10] is the sub-list for extension type_name
 	10, // [10:10] is the sub-list for extension extendee
 	0,  // [0:10] is the sub-list for field type_name
@@ -855,7 +1039,7 @@ func file_sgard_v1_sgard_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: unsafe.Slice(unsafe.StringData(file_sgard_v1_sgard_proto_rawDesc), len(file_sgard_v1_sgard_proto_rawDesc)),
 			NumEnums:      1,
-			NumMessages:   13,
+			NumMessages:   16,
 			NumExtensions: 0,
 			NumServices:   1,
 		},

sgardpb/sgard_grpc.pb.go

@@ -19,6 +19,7 @@ import (
 const _ = grpc.SupportPackageIsVersion9
 
 const (
+	GardenSync_Authenticate_FullMethodName = "/sgard.v1.GardenSync/Authenticate"
 	GardenSync_PushManifest_FullMethodName = "/sgard.v1.GardenSync/PushManifest"
 	GardenSync_PushBlobs_FullMethodName    = "/sgard.v1.GardenSync/PushBlobs"
 	GardenSync_PullManifest_FullMethodName = "/sgard.v1.GardenSync/PullManifest"
@@ -32,6 +33,8 @@ const (
 //
 // GardenSync is the sgard remote sync service.
 type GardenSyncClient interface {
+	// Authenticate exchanges an SSH-signed challenge for a JWT token.
+	Authenticate(ctx context.Context, in *AuthenticateRequest, opts ...grpc.CallOption) (*AuthenticateResponse, error)
 	// Push flow: send manifest, then stream missing blobs.
 	PushManifest(ctx context.Context, in *PushManifestRequest, opts ...grpc.CallOption) (*PushManifestResponse, error)
 	PushBlobs(ctx context.Context, opts ...grpc.CallOption) (grpc.ClientStreamingClient[PushBlobsRequest, PushBlobsResponse], error)
@@ -50,6 +53,16 @@ func NewGardenSyncClient(cc grpc.ClientConnInterface) GardenSyncClient {
 	return &gardenSyncClient{cc}
 }
 
+func (c *gardenSyncClient) Authenticate(ctx context.Context, in *AuthenticateRequest, opts ...grpc.CallOption) (*AuthenticateResponse, error) {
+	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
+	out := new(AuthenticateResponse)
+	err := c.cc.Invoke(ctx, GardenSync_Authenticate_FullMethodName, in, out, cOpts...)
+	if err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
 func (c *gardenSyncClient) PushManifest(ctx context.Context, in *PushManifestRequest, opts ...grpc.CallOption) (*PushManifestResponse, error) {
 	cOpts := append([]grpc.CallOption{grpc.StaticMethod()}, opts...)
 	out := new(PushManifestResponse)
@@ -118,6 +131,8 @@ func (c *gardenSyncClient) Prune(ctx context.Context, in *PruneRequest, opts ...
 //
 // GardenSync is the sgard remote sync service.
 type GardenSyncServer interface {
+	// Authenticate exchanges an SSH-signed challenge for a JWT token.
+	Authenticate(context.Context, *AuthenticateRequest) (*AuthenticateResponse, error)
 	// Push flow: send manifest, then stream missing blobs.
 	PushManifest(context.Context, *PushManifestRequest) (*PushManifestResponse, error)
 	PushBlobs(grpc.ClientStreamingServer[PushBlobsRequest, PushBlobsResponse]) error
@@ -136,6 +151,9 @@ type GardenSyncServer interface {
 // pointer dereference when methods are called.
 type UnimplementedGardenSyncServer struct{}
 
+func (UnimplementedGardenSyncServer) Authenticate(context.Context, *AuthenticateRequest) (*AuthenticateResponse, error) {
+	return nil, status.Errorf(codes.Unimplemented, "method Authenticate not implemented")
+}
 func (UnimplementedGardenSyncServer) PushManifest(context.Context, *PushManifestRequest) (*PushManifestResponse, error) {
 	return nil, status.Errorf(codes.Unimplemented, "method PushManifest not implemented")
 }
@@ -172,6 +190,24 @@ func RegisterGardenSyncServer(s grpc.ServiceRegistrar, srv GardenSyncServer) {
 	s.RegisterService(&GardenSync_ServiceDesc, srv)
 }
 
+func _GardenSync_Authenticate_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
+	in := new(AuthenticateRequest)
+	if err := dec(in); err != nil {
+		return nil, err
+	}
+	if interceptor == nil {
+		return srv.(GardenSyncServer).Authenticate(ctx, in)
+	}
+	info := &grpc.UnaryServerInfo{
+		Server:     srv,
+		FullMethod: GardenSync_Authenticate_FullMethodName,
+	}
+	handler := func(ctx context.Context, req interface{}) (interface{}, error) {
+		return srv.(GardenSyncServer).Authenticate(ctx, req.(*AuthenticateRequest))
+	}
+	return interceptor(ctx, in, info, handler)
+}
+
 func _GardenSync_PushManifest_Handler(srv interface{}, ctx context.Context, dec func(interface{}) error, interceptor grpc.UnaryServerInterceptor) (interface{}, error) {
 	in := new(PushManifestRequest)
 	if err := dec(in); err != nil {
@@ -251,6 +287,10 @@ var GardenSync_ServiceDesc = grpc.ServiceDesc{
 	ServiceName: "sgard.v1.GardenSync",
 	HandlerType: (*GardenSyncServer)(nil),
 	Methods: []grpc.MethodDesc{
+		{
+			MethodName: "Authenticate",
+			Handler:    _GardenSync_Authenticate_Handler,
+		},
 		{
 			MethodName: "PushManifest",
 			Handler:    _GardenSync_PushManifest_Handler,