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actually authenticate messages with challenges and HMACs

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This commit is contained in:
Antoni Piasecki
2026-07-23 12:39:19 +02:00
parent 4d710fb72b
commit 3f11053834
5 changed files with 165 additions and 52 deletions
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1
.gitignore vendored
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@@ -1 +1,2 @@
Justfile
/test

4
README.md
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@@ -5,5 +5,9 @@ A hub-and-spoke VPN for IPv6
## Features
* **Post-quantum security:** Uses [XWingMLKEM768X25519](https://datatracker.ietf.org/doc/html/draft-connolly-cfrg-xwing-kem/) + [XChaCha20-Poly1305](https://en.wikipedia.org/wiki/ChaCha20-Poly1305#XChaCha20-Poly1305_%E2%80%93_extended_nonce_variant) + [HKDF-SHA-256](https://en.wikipedia.org/wiki/HKDF) for end-to-end encryption
## Protocol
See [proto.go](https://git.ton1.dev/toni/baalvpn/src/branch/main/shared/proto.go)
## Third-party dependencies
* [`github.com/songgao/water`](https://github.com/songgao/water) - cross-platform wrapper around TUN interfaces

63
client/main.go
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@@ -1,7 +1,9 @@
package main
import (
"crypto/hmac"
"crypto/hpke"
"crypto/sha256"
"encoding/binary"
"encoding/hex"
"log"
@@ -26,9 +28,11 @@ var (
iface *water.Interface
conn *net.UDPConn
serverAddr *net.UDPAddr
internalIP net.IP = nil
internalIP net.IP
privKey hpke.PrivateKey
multicastKey []byte
challengeCh = make(chan []byte, 1)
authSecret []byte
establishMutex sync.Mutex
peerPubKeys = shared.NewTMap[string, hpke.PublicKey]()
@@ -70,8 +74,23 @@ func register() {
}
pubKey := privKey.PublicKey()
// TODO: this probably should be repeated until we get RESP_REGISTER
send(shared.BuildPkt(shared.REQ_REGISTER, pubKey.Bytes()))
// TODO: all this probably should be retried until we get RESP_REGISTER
log.Println("requesting register challenge")
send(shared.BuildPkt(shared.REQ_GET_CHALLENGE, pubKey.Bytes()))
challenge := <-challengeCh
log.Println("got register challenge")
authSecret, err = hpke.Open(privKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-challenge"), challenge)
if err != nil {
panic(err)
}
log.Println("registering")
mac := hmac.New(sha256.New, authSecret)
mac.Write(pubKey.Bytes())
send(shared.BuildPkt(shared.REQ_REGISTER, mac.Sum(nil), pubKey.Bytes()))
}
func receivePackets() {
@@ -116,11 +135,13 @@ func handleIncomingPkt(pkt []byte) {
pktType := binary.LittleEndian.Uint16(pkt[2:])
switch pktType {
case shared.RESP_GET_CHALLENGE:
challengeCh <- pkt[4:]
case shared.RESP_REGISTER:
internalIP = net.IP(pkt[4:20])
var err error
multicastKey, err = hpke.Open(privKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn"), pkt[20:])
multicastKey, err = hpke.Open(privKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-register"), pkt[20:])
if err != nil {
panic(err)
}
@@ -129,11 +150,11 @@ func handleIncomingPkt(pkt []byte) {
setupInterface()
go relayPackets()
case shared.ENC_PKT:
srcIP := net.IP(pkt[20:36]).String()
srcIP := net.IP(pkt[52:68]).String()
sessionKey := getOrEstablishSessionKey(srcIP)
decryptedPkt, err := DecryptSym(sessionKey, pkt[36:])
decryptedPkt, err := DecryptSym(sessionKey, pkt[68:])
if err != nil {
panic(err)
}
@@ -143,7 +164,7 @@ func handleIncomingPkt(pkt []byte) {
log.Println(err)
}
case shared.BROADCAST_PKT:
decryptedPkt, err := DecryptSym(multicastKey, pkt[4:])
decryptedPkt, err := DecryptSym(multicastKey, pkt[36:])
if err != nil {
panic(err)
}
@@ -159,14 +180,14 @@ func handleIncomingPkt(pkt []byte) {
}
peerPubKeys.Set(respIP, pubKey)
case shared.REQ_ESTABLISH:
srcIP := net.IP(pkt[20:36]).String()
ciphertext := pkt[36:]
srcIP := net.IP(pkt[52:68]).String()
ciphertext := pkt[68:]
r, err := hpke.NewRecipient(ciphertext, privKey, hpke.HKDFSHA256(), hpke.ExportOnly(), nil)
if err != nil {
panic(err)
}
sessionKey, err := r.Export("baalvpn", 32)
sessionKey, err := r.Export("baalvpn-establish", 32)
if err != nil {
panic(err)
}
@@ -174,10 +195,9 @@ func handleIncomingPkt(pkt []byte) {
log.Println("received session key from", srcIP)
peerSessionKeys.Set(srcIP, sessionKey)
reqData := append(net.ParseIP(srcIP), internalIP...)
send(shared.BuildPkt(shared.RESP_ESTABLISH, reqData))
send(buildAuthPkt(shared.RESP_ESTABLISH, net.ParseIP(srcIP), internalIP))
case shared.RESP_ESTABLISH:
srcIP := net.IP(pkt[20:36]).String()
srcIP := net.IP(pkt[52:68]).String()
log.Println("established session key with", srcIP)
peerEstablishAcks.Set(srcIP, true)
default:
@@ -215,13 +235,12 @@ func getOrEstablishSessionKey(ip string) []byte {
if err != nil {
panic(err)
}
sessionKey, err := sender.Export("baalvpn", 32)
sessionKey, err := sender.Export("baalvpn-establish", 32)
if err != nil {
panic(err)
}
reqData := append(net.ParseIP(ip), append(internalIP, ciphertext...)...)
send(shared.BuildPkt(shared.REQ_ESTABLISH, reqData))
send(buildAuthPkt(shared.REQ_ESTABLISH, net.ParseIP(ip), internalIP, ciphertext))
for {
// TODO: eww
@@ -279,7 +298,7 @@ func relayPackets() {
panic(err)
}
send(shared.BuildPkt(shared.BROADCAST_PKT, encryptedPkt))
send(buildAuthPkt(shared.BROADCAST_PKT, encryptedPkt))
} else {
destSessionKey := getOrEstablishSessionKey(destIP)
@@ -288,12 +307,18 @@ func relayPackets() {
panic(err)
}
reqBody := append(net.ParseIP(destIP), append(internalIP, encryptedPkt...)...)
send(shared.BuildPkt(shared.ENC_PKT, reqBody))
send(buildAuthPkt(shared.ENC_PKT, net.ParseIP(destIP), internalIP, encryptedPkt))
}
}
}
func buildAuthPkt(pktType uint16, parts ...[]byte) []byte {
pkt := shared.BuildPkt(pktType, parts...)
mac := hmac.New(sha256.New, authSecret)
mac.Write(pkt[4:])
return append(pkt[:4], append(mac.Sum(nil), pkt[4:]...)...)
}
func send(req []byte) {
if _, err := conn.WriteToUDP(req, serverAddr); err != nil {
panic(err)

91
server/main.go
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@@ -3,11 +3,14 @@
// TODO: somehow persist IPs
// TODO: key rotation
// TODO: dont trust claimed IP at all
// TODO: replay attacks are still a thing
package main
import (
"crypto/hmac"
"crypto/hpke"
"crypto/rand"
"crypto/sha256"
"encoding/binary"
"encoding/hex"
"log"
@@ -21,6 +24,7 @@ type Peer struct {
RealAddr *net.UDPAddr
InternalIP string
HexPublicKey string
AuthSecret []byte
}
var (
@@ -29,6 +33,7 @@ var (
peersByPubKey = shared.NewTMap[string, *Peer]()
conn *net.UDPConn
multicastKey = make([]byte, 32)
authSecretsByPubKey = shared.NewTMap[string, []byte]()
)
func main() {
@@ -93,7 +98,7 @@ func handleReq(req []byte, addr *net.UDPAddr) {
pktType := binary.LittleEndian.Uint16(req[2:])
switch pktType {
case shared.REQ_REGISTER:
case shared.REQ_GET_CHALLENGE:
pubKey, err := hpke.MLKEM768X25519().NewPublicKey(req[4:])
if err != nil {
log.Println("invalid pubkey")
@@ -102,10 +107,48 @@ func handleReq(req []byte, addr *net.UDPAddr) {
hexPubKey := hex.EncodeToString(pubKey.Bytes())
authSecret := make([]byte, 32)
if _, err = rand.Read(authSecret); err != nil {
panic(err)
}
authSecretsByPubKey.Set(hexPubKey, authSecret)
ciphertext, err := hpke.Seal(pubKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-challenge"), authSecret)
if err != nil {
panic(err)
}
sendTo(addr, shared.BuildPkt(shared.RESP_GET_CHALLENGE, ciphertext))
case shared.REQ_REGISTER:
auth := req[4:36]
pubKey, err := hpke.MLKEM768X25519().NewPublicKey(req[36:])
if err != nil {
log.Println("invalid pubkey")
return
}
hexPubKey := hex.EncodeToString(pubKey.Bytes())
authSecret, ok := authSecretsByPubKey.GetOK(hexPubKey)
if !ok {
log.Println("unknown pubkey tried to REGISTER")
return
}
defer authSecretsByPubKey.Delete(hexPubKey)
mac := hmac.New(sha256.New, authSecret)
mac.Write(pubKey.Bytes())
if !hmac.Equal(mac.Sum(nil), auth) {
log.Println("failed to authenticate REGISTER")
return
}
var internalIP string
// TODO: authenticating only by PUBLIC key is obviously a bad idea
if peer, ok := peersByPubKey.GetOK(hexPubKey); ok {
internalIP = peer.InternalIP
peer.AuthSecret = authSecret
peers.Delete(peer.RealAddr.String())
peer.RealAddr = addr
peers.Set(addr.String(), peer)
@@ -115,28 +158,35 @@ func handleReq(req []byte, addr *net.UDPAddr) {
RealAddr: addr,
InternalIP: internalIP,
HexPublicKey: hexPubKey,
AuthSecret: authSecret,
}
peers.Set(addr.String(), peer)
peersByInternal.Set(internalIP, peer)
peersByPubKey.Set(hexPubKey, peer)
}
ciphertext, err := hpke.Seal(pubKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn"), multicastKey)
ciphertext, err := hpke.Seal(pubKey, hpke.HKDFSHA256(), hpke.ChaCha20Poly1305(), []byte("baalvpn-register"), multicastKey)
if err != nil {
panic(err)
}
resp := shared.BuildPkt(shared.RESP_REGISTER, append(net.ParseIP(internalIP), ciphertext...))
sendTo(addr, resp)
sendTo(addr, shared.BuildPkt(shared.RESP_REGISTER, net.ParseIP(internalIP), ciphertext))
case shared.ENC_PKT:
peer := peers.Get(addr.String())
if peer == nil {
log.Println("data from unregistered peer:", addr.String())
log.Println("ENC_PKT from unregistered peer:", addr.String())
return
}
destIP := net.IP(req[4:20]).String()
srcIP := net.IP(req[20:36]).String()
mac := hmac.New(sha256.New, peer.AuthSecret)
mac.Write(req[36:])
if !hmac.Equal(mac.Sum(nil), req[4:36]) {
log.Println("failed to authenticate ENC_PKT")
return
}
destIP := net.IP(req[36:52]).String()
srcIP := net.IP(req[52:68]).String()
if srcIP != peer.InternalIP {
log.Println("rejected spoofed srcIP in ENC_PKT")
return
@@ -151,7 +201,14 @@ func handleReq(req []byte, addr *net.UDPAddr) {
case shared.BROADCAST_PKT:
peer := peers.Get(addr.String())
if peer == nil {
log.Println("data from unregistered peer:", addr.String())
log.Println("BROADCAST_PKT from unregistered peer:", addr.String())
return
}
mac := hmac.New(sha256.New, peer.AuthSecret)
mac.Write(req[36:])
if !hmac.Equal(mac.Sum(nil), req[4:36]) {
log.Println("failed to authenticate BROADCAST_PKT")
return
}
@@ -165,23 +222,29 @@ func handleReq(req []byte, addr *net.UDPAddr) {
if err != nil {
panic(err)
}
resp := shared.BuildPkt(shared.RESP_GET_PUBKEY, append(net.ParseIP(peerIP), pubKey...))
sendTo(addr, resp)
sendTo(addr, shared.BuildPkt(shared.RESP_GET_PUBKEY, net.ParseIP(peerIP), pubKey))
}
case shared.REQ_ESTABLISH, shared.RESP_ESTABLISH:
peer := peers.Get(addr.String())
if peer == nil {
log.Println("unregistered peer tried to ESTABLISH")
log.Println("ESTABLISH from unregistered peer:", addr.String())
return
}
srcIP := net.IP(req[20:36]).String()
mac := hmac.New(sha256.New, peer.AuthSecret)
mac.Write(req[36:])
if !hmac.Equal(mac.Sum(nil), req[4:36]) {
log.Println("failed to authenticate ESTABLISH")
return
}
srcIP := net.IP(req[52:68]).String()
if srcIP != peer.InternalIP {
log.Println("rejected spoofed srcIP in ESTABLISH")
return
}
dstIP := net.IP(req[4:20]).String()
dstIP := net.IP(req[36:52]).String()
dstPeer := peersByInternal.Get(dstIP)
if dstPeer == nil {
log.Println("tried to ESTABLISH with an unknown peer")

48
shared/proto.go
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@@ -4,38 +4,58 @@ import "encoding/binary"
const PROTO_VERSION uint16 = 1
// pubKey - public key generated by the client
// privKey - private key generated by the client
// sessionKey - key established between two peers
// multicastKey - key shared between everyone for encrypting multicast pkts
// authSecret - secret given during registration for authenticating subsequent requests
const (
_ uint16 = iota
// (client -> server) requests a challenge to prove the ownership of the privKey
// [ pubKey - 1216 bytes ]
REQ_GET_CHALLENGE
// (server -> client) provides a pubKey-encrypted authSecret
// [ ciphertext - 1168 bytes ]
RESP_GET_CHALLENGE
// (client -> server) requests an IP
// [ pubkey - 1216 bytes ]
// [ HMAC(authSecret, pubKey) - 32 bytes ] [ pubKey - 1216 bytes ]
REQ_REGISTER
// (server -> client) returns the assigned IP and encapsulated multicast key
// (server -> client) returns the assigned IP and pubKey-encrypted multicastKey
// [ ip - 16 bytes ] [ ciphertext - 1168 bytes ]
RESP_REGISTER
// (client -> server -> client2) relays an encrypted packet to a specified peer
// [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ encrypted pkt ]
// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ encryptedPkt ]
ENC_PKT
// (client -> server -> *) broadcasts an encrypted packet
// [ encrypted pkt ]
// [ HMAC(authSecret, rest) - 32 bytes ] [ encryptedPkt ]
BROADCAST_PKT
// (client -> server) requests peer's pubkey from the server for encapsulation
// (client -> server) requests peer's pubKey from the server for encapsulation
// [ ip - 16 bytes ]
REQ_GET_PUBKEY
// (server -> client) provides requested pubkey
// [ ip - 16 bytes ] [ pubkey - 1216 bytes ]
// (server -> client) provides requested pubKey
// [ ip - 16 bytes ] [ pubKey - 1216 bytes ]
RESP_GET_PUBKEY
// (client -> server -> client2) establishes a session key with another peer
// [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ ciphertext - 1120 bytes ]
// (client -> server -> client2) establishes a sessionKey with another peer
// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ] [ ciphertext - 1120 bytes ]
REQ_ESTABLISH
// (client2 -> server -> client) acknowledges the session key was established
// [ destIP - 16 bytes ] [ srcIP - 16 bytes ]
// (client2 -> server -> client) acknowledges the sessionKey was established
// [ HMAC(authSecret, rest) - 32 bytes ] [ destIP - 16 bytes ] [ srcIP - 16 bytes ]
RESP_ESTABLISH
)
func BuildPkt(pktType uint16, data []byte) []byte {
out := make([]byte, 4+len(data))
func BuildPkt(pktType uint16, parts ...[]byte) []byte {
var total int
for _, p := range parts {
total += len(p)
}
out := make([]byte, 4, 4+total)
binary.LittleEndian.PutUint16(out[0:], PROTO_VERSION)
binary.LittleEndian.PutUint16(out[2:], pktType)
copy(out[4:], data)
for _, p := range parts {
out = append(out, p...)
}
return out
}