implement source address token generation and validation

ref #121

crypto/source_address_token.go

@@ -0,0 +1,119 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"time"
+
+	"github.com/lucas-clemente/quic-go/protocol"
+
+	"golang.org/x/crypto/hkdf"
+)
+
+type sourceAddressToken struct {
+	ip net.IP
+	// unix timestamp in seconds
+	timestamp uint64
+}
+
+func (t *sourceAddressToken) serialize() []byte {
+	res := make([]byte, 8+len(t.ip))
+	binary.LittleEndian.PutUint64(res, t.timestamp)
+	copy(res[8:], t.ip)
+	return res
+}
+
+func parseToken(data []byte) (*sourceAddressToken, error) {
+	if len(data) != 8+4 && len(data) != 8+16 {
+		return nil, fmt.Errorf("invalid STK length %d", len(data))
+	}
+	return &sourceAddressToken{
+		ip:        data[8:],
+		timestamp: binary.LittleEndian.Uint64(data),
+	}, nil
+}
+
+type stkSource struct {
+	aead cipher.AEAD
+}
+
+const stkKeySize = 16
+
+// Chrome currently sets this to 12, but discusses changing it to 16. We start
+// at 16 :)
+const stkNonceSize = 16
+
+func newStkSource(secret []byte) (*stkSource, error) {
+	key, err := deriveKey(secret)
+	if err != nil {
+		return nil, err
+	}
+	c, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, err
+	}
+	aead, err := cipher.NewGCMWithNonceSize(c, stkNonceSize)
+	if err != nil {
+		return nil, err
+	}
+	return &stkSource{aead: aead}, nil
+}
+
+func (s *stkSource) NewToken(ip net.IP) ([]byte, error) {
+	return encryptToken(s.aead, &sourceAddressToken{
+		ip:        ip,
+		timestamp: uint64(time.Now().Unix()),
+	})
+}
+
+func (s *stkSource) VerifyToken(ip net.IP, data []byte) error {
+	if len(data) < stkNonceSize {
+		return errors.New("STK too short")
+	}
+	nonce := data[:stkNonceSize]
+
+	res, err := s.aead.Open(nil, nonce, data[stkNonceSize:], nil)
+	if err != nil {
+		return err
+	}
+
+	token, err := parseToken(res)
+	if err != nil {
+		return err
+	}
+
+	if subtle.ConstantTimeCompare(token.ip, ip) != 1 {
+		return errors.New("invalid ip in STK")
+	}
+
+	if time.Now().Unix() > int64(token.timestamp)+protocol.STKExpiryTimeSec {
+		return errors.New("STK expired")
+	}
+
+	return nil
+}
+
+func deriveKey(secret []byte) ([]byte, error) {
+	r := hkdf.New(sha256.New, secret, nil, []byte("QUIC source address token key"))
+	key := make([]byte, stkKeySize)
+	if _, err := io.ReadFull(r, key); err != nil {
+		return nil, err
+	}
+	return key, nil
+}
+
+func encryptToken(aead cipher.AEAD, token *sourceAddressToken) ([]byte, error) {
+	nonce := make([]byte, stkNonceSize)
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+	return aead.Seal(nonce, nonce, token.serialize(), nil), nil
+}