Step 14: SSH key auth for gRPC.

Server: AuthInterceptor parses authorized_keys, extracts SSH signature
from gRPC metadata (nonce + timestamp signed by client's SSH key),
verifies against authorized public keys with 5-minute timestamp window.

Client: SSHCredentials implements PerRPCCredentials, signs nonce+timestamp
per request. LoadSigner resolves key from flag, ssh-agent, or default paths.

8 tests: valid auth, reject unauthenticated, reject unauthorized key,
reject expired timestamp, metadata generation, plus 2 integration tests
(authenticated succeeds, unauthenticated rejected).

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

client/auth.go

@@ -0,0 +1,124 @@
+package client
+
+import (
+	"context"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"strings"
+	"time"
+
+	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/agent"
+	"google.golang.org/grpc/credentials"
+)
+
+// SSHCredentials implements grpc.PerRPCCredentials using an SSH signer.
+type SSHCredentials struct {
+	signer ssh.Signer
+}
+
+// NewSSHCredentials creates credentials from an SSH signer.
+func NewSSHCredentials(signer ssh.Signer) *SSHCredentials {
+	return &SSHCredentials{signer: signer}
+}
+
+// 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)
+	}
+
+	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
+}
+
+// RequireTransportSecurity returns false — auth is via SSH signatures,
+// not TLS. Transport security can be added separately.
+func (c *SSHCredentials) RequireTransportSecurity() bool {
+	return false
+}
+
+// Verify that SSHCredentials implements the interface.
+var _ credentials.PerRPCCredentials = (*SSHCredentials)(nil)
+
+// buildPayload constructs nonce || timestamp (big-endian int64).
+func buildPayload(nonce []byte, tsUnix int64) []byte {
+	payload := make([]byte, len(nonce)+8)
+	copy(payload, nonce)
+	for i := 7; i >= 0; i-- {
+		payload[len(nonce)+i] = byte(tsUnix & 0xff)
+		tsUnix >>= 8
+	}
+	return payload
+}
+
+// LoadSigner loads an SSH signer. Resolution order:
+// 1. keyPath (if non-empty)
+// 2. SSH agent (if SSH_AUTH_SOCK is set)
+// 3. Default key paths: ~/.ssh/id_ed25519, ~/.ssh/id_rsa
+func LoadSigner(keyPath string) (ssh.Signer, error) {
+	if keyPath != "" {
+		return loadSignerFromFile(keyPath)
+	}
+
+	// Try ssh-agent.
+	if sock := os.Getenv("SSH_AUTH_SOCK"); sock != "" {
+		conn, err := net.Dial("unix", sock)
+		if err == nil {
+			ag := agent.NewClient(conn)
+			signers, err := ag.Signers()
+			if err == nil && len(signers) > 0 {
+				return signers[0], nil
+			}
+			_ = conn.Close()
+		}
+	}
+
+	// Try default key paths.
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return nil, fmt.Errorf("no SSH key found: %w", err)
+	}
+
+	for _, name := range []string{"id_ed25519", "id_rsa"} {
+		path := home + "/.ssh/" + name
+		signer, err := loadSignerFromFile(path)
+		if err == nil {
+			return signer, nil
+		}
+	}
+
+	return nil, fmt.Errorf("no SSH key found (tried --ssh-key, agent, ~/.ssh/id_ed25519, ~/.ssh/id_rsa)")
+}
+
+func loadSignerFromFile(path string) (ssh.Signer, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading key %s: %w", path, err)
+	}
+	signer, err := ssh.ParsePrivateKey(data)
+	if err != nil {
+		return nil, fmt.Errorf("parsing key %s: %w", path, err)
+	}
+	return signer, nil
+}

client/auth_test.go

@@ -0,0 +1,57 @@
+package client
+
+import (
+	"context"
+	"crypto/ed25519"
+	"crypto/rand"
+	"testing"
+
+	"golang.org/x/crypto/ssh"
+)
+
+func TestSSHCredentialsMetadata(t *testing.T) {
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("generating key: %v", err)
+	}
+	signer, err := ssh.NewSignerFromKey(priv)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	creds := NewSSHCredentials(signer)
+
+	md, err := creds.GetRequestMetadata(context.Background())
+	if err != nil {
+		t.Fatalf("GetRequestMetadata: %v", err)
+	}
+
+	// Verify all required fields are present and non-empty.
+	for _, key := range []string{
+		"x-sgard-auth-nonce",
+		"x-sgard-auth-timestamp",
+		"x-sgard-auth-signature",
+		"x-sgard-auth-pubkey",
+	} {
+		val, ok := md[key]
+		if !ok || val == "" {
+			t.Errorf("missing or empty metadata key %s", key)
+		}
+	}
+}
+
+func TestSSHCredentialsNoTransportSecurity(t *testing.T) {
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("generating key: %v", err)
+	}
+	signer, err := ssh.NewSignerFromKey(priv)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	creds := NewSSHCredentials(signer)
+	if creds.RequireTransportSecurity() {
+		t.Error("RequireTransportSecurity should be false")
+	}
+}

client/client_test.go

@@ -2,6 +2,8 @@ package client
 
 import (
 	"context"
+	"crypto/ed25519"
+	"crypto/rand"
 	"errors"
 	"net"
 	"os"
@@ -12,6 +14,7 @@ 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/credentials/insecure"
 	"google.golang.org/grpc/test/bufconn"
@@ -206,3 +209,100 @@ func TestPrune(t *testing.T) {
 		t.Errorf("removed %d blobs, want 1", removed)
 	}
 }
+
+func TestAuthIntegration(t *testing.T) {
+	// Generate an ed25519 key pair.
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("generating key: %v", err)
+	}
+	signer, err := ssh.NewSignerFromKey(priv)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	serverDir := t.TempDir()
+	serverGarden, err := garden.Init(serverDir)
+	if err != nil {
+		t.Fatalf("init server garden: %v", err)
+	}
+
+	// Set up server with auth interceptor.
+	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()})
+	lis := bufconn.Listen(bufSize)
+	srv := grpc.NewServer(
+		grpc.UnaryInterceptor(auth.UnaryInterceptor()),
+		grpc.StreamInterceptor(auth.StreamInterceptor()),
+	)
+	sgardpb.RegisterGardenSyncServer(srv, server.New(serverGarden))
+	t.Cleanup(func() { srv.Stop() })
+	go func() { _ = srv.Serve(lis) }()
+
+	// Client with SSH credentials.
+	creds := NewSSHCredentials(signer)
+	conn, err := grpc.NewClient("passthrough:///bufconn",
+		grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+			return lis.Dial()
+		}),
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+		grpc.WithPerRPCCredentials(creds),
+	)
+	if err != nil {
+		t.Fatalf("dial: %v", err)
+	}
+	t.Cleanup(func() { _ = conn.Close() })
+
+	c := New(conn)
+
+	// Authenticated request should succeed.
+	_, err = c.Pull(context.Background(), serverGarden)
+	if err != nil {
+		t.Fatalf("authenticated Pull should succeed: %v", err)
+	}
+}
+
+func TestAuthIntegrationRejectsUnauthenticated(t *testing.T) {
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		t.Fatalf("generating key: %v", err)
+	}
+	signer, err := ssh.NewSignerFromKey(priv)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	serverDir := t.TempDir()
+	serverGarden, err := garden.Init(serverDir)
+	if err != nil {
+		t.Fatalf("init server garden: %v", err)
+	}
+
+	auth := server.NewAuthInterceptorFromKeys([]ssh.PublicKey{signer.PublicKey()})
+	lis := bufconn.Listen(bufSize)
+	srv := grpc.NewServer(
+		grpc.UnaryInterceptor(auth.UnaryInterceptor()),
+		grpc.StreamInterceptor(auth.StreamInterceptor()),
+	)
+	sgardpb.RegisterGardenSyncServer(srv, server.New(serverGarden))
+	t.Cleanup(func() { srv.Stop() })
+	go func() { _ = srv.Serve(lis) }()
+
+	// Client WITHOUT credentials.
+	conn, err := grpc.NewClient("passthrough:///bufconn",
+		grpc.WithContextDialer(func(context.Context, string) (net.Conn, error) {
+			return lis.Dial()
+		}),
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+	)
+	if err != nil {
+		t.Fatalf("dial: %v", err)
+	}
+	t.Cleanup(func() { _ = conn.Close() })
+
+	c := New(conn)
+
+	_, err = c.Pull(context.Background(), serverGarden)
+	if err == nil {
+		t.Fatal("unauthenticated Pull should fail")
+	}
+}