server: fix expiry detection for verification keys

server/rotation.go

@@ -22,8 +22,9 @@ type rotationStrategy struct {
 	// Time between rotations.
 	rotationFrequency time.Duration
 
-	// After being rotated how long can a key validate signatues?
-	verifyFor time.Duration
+	// After being rotated how long should the key be kept around for validating
+	// signatues?
+	idTokenValidFor time.Duration
 
 	// Keys are always RSA keys. Though cryptopasta recommends ECDSA keys, not every
 	// client may support these (e.g. github.com/coreos/go-oidc/oidc).
@@ -35,17 +36,17 @@ func staticRotationStrategy(key *rsa.PrivateKey) rotationStrategy {
 	return rotationStrategy{
 		// Setting these values to 100 years is easier than having a flag indicating no rotation.
 		rotationFrequency: time.Hour * 8760 * 100,
-		verifyFor:         time.Hour * 8760 * 100,
+		idTokenValidFor:   time.Hour * 8760 * 100,
 		key:               func() (*rsa.PrivateKey, error) { return key, nil },
 	}
 }
 
 // defaultRotationStrategy returns a strategy which rotates keys every provided period,
 // holding onto the public parts for some specified amount of time.
-func defaultRotationStrategy(rotationFrequency, verifyFor time.Duration) rotationStrategy {
+func defaultRotationStrategy(rotationFrequency, idTokenValidFor time.Duration) rotationStrategy {
 	return rotationStrategy{
 		rotationFrequency: rotationFrequency,
-		verifyFor:         verifyFor,
+		idTokenValidFor:   idTokenValidFor,
 		key: func() (*rsa.PrivateKey, error) {
 			return rsa.GenerateKey(rand.Reader, 2048)
 		},
@@ -128,11 +129,14 @@ func (k keyRotater) rotate() error {
 			return storage.Keys{}, errors.New("keys already rotated")
 		}
 
-		// Remove expired verification keys.
-		i := 0
+		expired := func(key storage.VerificationKey) bool {
+			return tNow.After(key.Expiry)
+		}
 
+		// Remove any verification keys that have expired.
+		i := 0
 		for _, key := range keys.VerificationKeys {
-			if !key.Expiry.After(tNow) {
+			if !expired(key) {
 				keys.VerificationKeys[i] = key
 				i++
 			}
@@ -140,10 +144,15 @@ func (k keyRotater) rotate() error {
 		keys.VerificationKeys = keys.VerificationKeys[:i]
 
 		if keys.SigningKeyPub != nil {
-			// Move current signing key to a verification only key.
+			// Move current signing key to a verification only key, throwing
+			// away the private part.
 			verificationKey := storage.VerificationKey{
 				PublicKey: keys.SigningKeyPub,
-				Expiry:    tNow.Add(k.strategy.verifyFor),
+				// After demoting the signing key, keep the token around for at least
+				// the amount of time an ID Token is valid for. This ensures the
+				// verification key won't expire until all ID Tokens it's signed
+				// expired as well.
+				Expiry: tNow.Add(k.strategy.idTokenValidFor),
 			}
 			keys.VerificationKeys = append(keys.VerificationKeys, verificationKey)
 		}

server/rotation_test.go

 package server
+
+import (
+	"os"
+	"sort"
+	"testing"
+	"time"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/coreos/dex/storage"
+	"github.com/coreos/dex/storage/memory"
+)
+
+func signingKeyID(t *testing.T, s storage.Storage) string {
+	keys, err := s.GetKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return keys.SigningKey.KeyID
+}
+
+func verificationKeyIDs(t *testing.T, s storage.Storage) (ids []string) {
+	keys, err := s.GetKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, key := range keys.VerificationKeys {
+		ids = append(ids, key.PublicKey.KeyID)
+	}
+	return ids
+}
+
+// slicesEq compare two string slices without modifying the ordering
+// of the slices.
+func slicesEq(s1, s2 []string) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+
+	cp := func(s []string) []string {
+		c := make([]string, len(s))
+		copy(c, s)
+		return c
+	}
+
+	cp1 := cp(s1)
+	cp2 := cp(s2)
+	sort.Strings(cp1)
+	sort.Strings(cp2)
+
+	for i, el := range cp1 {
+		if el != cp2[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestKeyRotater(t *testing.T) {
+	now := time.Now()
+
+	delta := time.Millisecond
+	rotationFrequency := time.Second * 5
+	validFor := time.Second * 21
+
+	// Only the last 5 verification keys are expected to be kept around.
+	maxVerificationKeys := 5
+
+	l := &logrus.Logger{
+		Out:       os.Stderr,
+		Formatter: &logrus.TextFormatter{DisableColors: true},
+		Level:     logrus.DebugLevel,
+	}
+
+	r := &keyRotater{
+		Storage:  memory.New(l),
+		strategy: defaultRotationStrategy(rotationFrequency, validFor),
+		now:      func() time.Time { return now },
+		logger:   l,
+	}
+
+	var expVerificationKeys []string
+
+	for i := 0; i < 10; i++ {
+		now = now.Add(rotationFrequency + delta)
+		if err := r.rotate(); err != nil {
+			t.Fatal(err)
+		}
+
+		got := verificationKeyIDs(t, r.Storage)
+
+		if !slicesEq(expVerificationKeys, got) {
+			t.Errorf("after %d rotation, expected varification keys %q, got %q", i+1, expVerificationKeys, got)
+		}
+
+		expVerificationKeys = append(expVerificationKeys, signingKeyID(t, r.Storage))
+		if n := len(expVerificationKeys); n > maxVerificationKeys {
+			expVerificationKeys = expVerificationKeys[n-maxVerificationKeys:]
+		}
+	}
+}