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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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101
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"
@@ -21,14 +24,16 @@ type Peer struct {
RealAddr *net.UDPAddr
InternalIP string
HexPublicKey string
AuthSecret []byte
}
var (
peers = shared.NewTMap[string, *Peer]()
peersByInternal = shared.NewTMap[string, *Peer]()
peersByPubKey = shared.NewTMap[string, *Peer]()
conn *net.UDPConn
multicastKey = make([]byte, 32)
peers = shared.NewTMap[string, *Peer]()
peersByInternal = shared.NewTMap[string, *Peer]()
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")