feat: pinned master key TLS and per-agent tokens

internal/agenttoken/agenttoken.go

@@ -0,0 +1,31 @@
+// Package agenttoken creates and hashes the per-agent tokens agents present
+// to the master.
+package agenttoken
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/hex"
+	"fmt"
+)
+
+const prefix = "pua_"
+
+// Generate returns a new random agent token. Show it to the operator once;
+// store only Hash(token).
+func Generate() (string, error) {
+	var b [32]byte
+	if _, err := rand.Read(b[:]); err != nil {
+		return "", fmt.Errorf("generate agent token: %w", err)
+	}
+	return prefix + base64.RawURLEncoding.EncodeToString(b[:]), nil
+}
+
+// Hash returns the value stored for token. A plain SHA-256 is sufficient
+// because tokens carry 256 bits of randomness; salting protects low-entropy
+// passwords, which these are not.
+func Hash(token string) string {
+	sum := sha256.Sum256([]byte(token))
+	return hex.EncodeToString(sum[:])
+}

internal/agenttoken/agenttoken_test.go

@@ -0,0 +1,25 @@
+package agenttoken
+
+import (
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGenerate(t *testing.T) {
+	a, err := Generate()
+	require.NoError(t, err)
+	b, err := Generate()
+	require.NoError(t, err)
+
+	assert.Regexp(t, regexp.MustCompile(`^pua_[A-Za-z0-9_-]{43}$`), a)
+	assert.NotEqual(t, a, b)
+}
+
+func TestHash(t *testing.T) {
+	assert.Equal(t, Hash("pua_x"), Hash("pua_x"))
+	assert.NotEqual(t, Hash("pua_x"), Hash("pua_y"))
+	assert.Regexp(t, regexp.MustCompile(`^[0-9a-f]{64}$`), Hash("pua_x"))
+}

internal/pin/pin.go

@@ -0,0 +1,91 @@
+// Package pin lets agents trust the master without a certificate authority.
+// The master serves a self-signed certificate for its own key pair; agents
+// are configured with the fingerprint of that public key and accept the
+// connection only if it matches. The TLS handshake proves the master holds
+// the private key, the same way SSH host keys work.
+package pin
+
+import (
+	"crypto"
+	"crypto/ed25519"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"math/big"
+	"time"
+)
+
+// GenerateKey creates a new master key pair.
+func GenerateKey() (ed25519.PrivateKey, error) {
+	_, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return nil, fmt.Errorf("generate key: %w", err)
+	}
+	return priv, nil
+}
+
+// Certificate returns a self-signed TLS certificate for key. Its validity is
+// effectively unlimited because trust comes from the pinned fingerprint, not
+// from the certificate's dates or issuer.
+func Certificate(key ed25519.PrivateKey) (tls.Certificate, error) {
+	tmpl := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject:      pkix.Name{CommonName: "package-updater-master"},
+		NotBefore:    time.Now().Add(-time.Hour),
+		NotAfter:     time.Now().AddDate(100, 0, 0),
+		KeyUsage:     x509.KeyUsageDigitalSignature,
+		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+	}
+	der, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, key.Public(), key)
+	if err != nil {
+		return tls.Certificate{}, fmt.Errorf("create certificate: %w", err)
+	}
+	return tls.Certificate{Certificate: [][]byte{der}, PrivateKey: key}, nil
+}
+
+// Fingerprint returns "sha256:" + hex SHA-256 of pub's PKIX DER encoding.
+// This is the MASTER_KEY value agents are configured with.
+func Fingerprint(pub crypto.PublicKey) (string, error) {
+	der, err := x509.MarshalPKIXPublicKey(pub)
+	if err != nil {
+		return "", fmt.Errorf("marshal public key: %w", err)
+	}
+	sum := sha256.Sum256(der)
+	return "sha256:" + hex.EncodeToString(sum[:]), nil
+}
+
+// ClientConfig returns a TLS config that accepts only a server whose
+// certificate carries the public key with the given fingerprint.
+func ClientConfig(fingerprint string) *tls.Config {
+	return &tls.Config{
+		MinVersion: tls.VersionTLS13,
+		// Chain and hostname verification are replaced by the fingerprint
+		// check below: the master's certificate is self-signed by design.
+		// The handshake still verifies the server's signature with the
+		// certificate's key, so presenting a copied certificate fails.
+		InsecureSkipVerify: true, //nolint:gosec // see comment above
+		VerifyPeerCertificate: func(raw [][]byte, _ [][]*x509.Certificate) error {
+			if len(raw) == 0 {
+				return errors.New("pin: server sent no certificate")
+			}
+			cert, err := x509.ParseCertificate(raw[0])
+			if err != nil {
+				return fmt.Errorf("pin: parse certificate: %w", err)
+			}
+			got, err := Fingerprint(cert.PublicKey)
+			if err != nil {
+				return err
+			}
+			if subtle.ConstantTimeCompare([]byte(got), []byte(fingerprint)) != 1 {
+				return fmt.Errorf("pin: fingerprint mismatch: server presented %s", got)
+			}
+			return nil
+		},
+	}
+}

internal/pin/pin_test.go

@@ -0,0 +1,61 @@
+package pin
+
+import (
+	"crypto/tls"
+	"net/http"
+	"net/http/httptest"
+	"regexp"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func newPinnedServer(t *testing.T) (*httptest.Server, string) {
+	t.Helper()
+	key, err := GenerateKey()
+	require.NoError(t, err)
+	cert, err := Certificate(key)
+	require.NoError(t, err)
+	fp, err := Fingerprint(key.Public())
+	require.NoError(t, err)
+
+	srv := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write([]byte("ok"))
+	}))
+	srv.TLS = &tls.Config{Certificates: []tls.Certificate{cert}}
+	srv.StartTLS()
+	t.Cleanup(srv.Close)
+	return srv, fp
+}
+
+func clientFor(fp string) *http.Client {
+	return &http.Client{Transport: &http.Transport{TLSClientConfig: ClientConfig(fp)}}
+}
+
+func TestFingerprintFormat(t *testing.T) {
+	key, err := GenerateKey()
+	require.NoError(t, err)
+	fp, err := Fingerprint(key.Public())
+	require.NoError(t, err)
+	assert.Regexp(t, regexp.MustCompile(`^sha256:[0-9a-f]{64}$`), fp)
+}
+
+func TestClientConfigAcceptsPinnedKey(t *testing.T) {
+	srv, fp := newPinnedServer(t)
+	resp, err := clientFor(fp).Get(srv.URL)
+	require.NoError(t, err)
+	_ = resp.Body.Close()
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+}
+
+func TestClientConfigRejectsOtherKey(t *testing.T) {
+	srv, _ := newPinnedServer(t)
+	other, err := GenerateKey()
+	require.NoError(t, err)
+	otherFP, err := Fingerprint(other.Public())
+	require.NoError(t, err)
+
+	_, err = clientFor(otherFP).Get(srv.URL)
+	assert.ErrorContains(t, err, "fingerprint mismatch")
+}