diff --git a/internal/ca/ca.go b/internal/ca/ca.go
index 638d050..b3c2151 100644
--- a/internal/ca/ca.go
+++ b/internal/ca/ca.go
@@ -148,3 +148,16 @@ func (ca *CA) CACert() *x509.Certificate {
 func (ca *CA) CACertPEM() []byte {
 	return pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE", Bytes: ca.cert.Raw})
 }
+
+func (ca *CA) VerifyCert(cert *x509.Certificate) error {
+	roots := x509.NewCertPool()
+	roots.AddCert(ca.cert)
+	opts := x509.VerifyOptions{
+		Roots:     roots,
+		KeyUsages: []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
+	}
+	if _, err := cert.Verify(opts); err != nil {
+		return fmt.Errorf("cert verify: %w", err)
+	}
+	return nil
+}
diff --git a/internal/identity/manager.go b/internal/identity/manager.go
index 304624e..7588aa1 100644
--- a/internal/identity/manager.go
+++ b/internal/identity/manager.go
@@ -139,6 +139,25 @@ func (m *Manager) Exists(number string) bool {
 	return m.store.Has(numberKeyHash(number))
 }
 
+func (m *Manager) GetByPubKey(pubKey [32]byte) (*Identity, error) {
+	var found *Identity
+	err := m.store.Iterate(func(keyHash uint64, payload []byte) bool {
+		ident := m.unmarshalIdentity(payload)
+		if ident != nil && ident.PubKey == pubKey {
+			found = ident
+			return false
+		}
+		return true
+	}, m.masterKey)
+	if err != nil {
+		return nil, err
+	}
+	if found == nil {
+		return nil, fmt.Errorf("identity not found for pubkey")
+	}
+	return found, nil
+}
+
 func numberKeyHash(number string) uint64 {
 	h, _ := blake2b.New(8, nil)
 	h.Write([]byte(number))
diff --git a/internal/server/server.go b/internal/server/server.go
index e8aa272..31c7e92 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -5,9 +5,11 @@ import (
 	"crypto/ed25519"
 	"crypto/rand"
 	"crypto/tls"
+	"crypto/x509"
 	"encoding/binary"
 	"fmt"
 	"sync"
+	"time"
 
 	"github.com/apernet/quic-go"
 	"github.com/caddyserver/certmagic"
@@ -213,6 +215,8 @@ func (s *Server) handleStream(ctx context.Context, stream *quic.Stream, sess *Se
 			s.handleSolve(sess, frame.Payload)
 		case types.OpReroll:
 			s.handleReroll(sess)
+		case types.OpAuthCert:
+			s.handleAuthCert(sess, frame.Payload)
 		case types.OpDial:
 			if !sess.IsAuthenticated() {
 				sendOpError(st, types.ErrInvalidRequest, "not authenticated")
@@ -286,6 +290,78 @@ func (s *Server) handleSolve(sess *Session, payload []byte) {
 		zap.String("number", ident.Number))
 }
 
+func (s *Server) handleAuthCert(sess *Session, payload []byte) {
+	challenge := sess.GetChallenge()
+	if challenge == nil {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "no challenge issued")
+		return
+	}
+	if time.Since(time.Unix(0, challenge.Timestamp)) > s.cfg.PoW.ChallengeTTL {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "challenge expired")
+		return
+	}
+
+	if len(payload) < 1+64+2 {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "invalid auth payload")
+		return
+	}
+	sigLen := int(payload[0])
+	if sigLen != 64 || len(payload) < 1+sigLen+2 {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "invalid signature length")
+		return
+	}
+	signature := payload[1 : 1+sigLen]
+	certLen := int(binary.BigEndian.Uint16(payload[1+sigLen:]))
+	if len(payload) < 1+sigLen+2+certLen {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "invalid cert length")
+		return
+	}
+	certDER := payload[1+sigLen+2 : 1+sigLen+2+certLen]
+
+	cert, err := x509.ParseCertificate(certDER)
+	if err != nil {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "invalid certificate")
+		return
+	}
+
+	if err := s.ca.VerifyCert(cert); err != nil {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, err.Error())
+		return
+	}
+
+	pubKeyRaw, ok := cert.PublicKey.(ed25519.PublicKey)
+	if !ok || len(pubKeyRaw) != 32 {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "invalid public key")
+		return
+	}
+
+	challengeData := challenge.Marshal()
+	if !ed25519.Verify(pubKeyRaw, challengeData, signature) {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "signature verification failed")
+		return
+	}
+
+	var pubKey [32]byte
+	copy(pubKey[:], pubKeyRaw)
+
+	ident, err := s.identMgr.GetByPubKey(pubKey)
+	if err != nil {
+		sendOpError(sess.Stream(), types.ErrAuthFailed, "identity not found")
+		return
+	}
+
+	sess.Authenticate(ident.Number, pubKey, certDER)
+	s.Register(sess, ident.Number)
+
+	respPayload := marshalIdentityResponse(ident.Number, certDER, s.ca.CACertPEM())
+	if st := sess.Stream(); st != nil {
+		st.WriteFrame(byte(types.OpIdentity), respPayload)
+	}
+
+	s.logger.Info("Client re-authenticated",
+		zap.String("number", ident.Number))
+}
+
 func (s *Server) handleReroll(sess *Session) {
 	if !sess.IsAuthenticated() {
 		sendOpError(sess.Stream(), types.ErrInvalidRequest, "not authenticated")
diff --git a/pkg/types/opcode.go b/pkg/types/opcode.go
index 2cbf14a..144df9f 100644
--- a/pkg/types/opcode.go
+++ b/pkg/types/opcode.go
@@ -8,6 +8,7 @@ const (
 	OpSolve        OpCode = 0x03
 	OpIdentity     OpCode = 0x04
 	OpReroll       OpCode = 0x05
+	OpAuthCert     OpCode = 0x06
 
 	OpDial       OpCode = 0x10
 	OpRing       OpCode = 0x11
diff --git a/pkg/types/types.go b/pkg/types/types.go
index b8ddc8f..353528c 100644
--- a/pkg/types/types.go
+++ b/pkg/types/types.go
@@ -24,6 +24,7 @@ const (
 	ErrBusy           ErrorCode = 0x0A
 	ErrSelfCall       ErrorCode = 0x0B
 	ErrRateLimited    ErrorCode = 0x0C
+	ErrAuthFailed     ErrorCode = 0x0D
 )
 
 type CodecType uint8