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package applications
import (
torProvider ""
// AuthMessage is exchanged between peers to obtain the Auth Capability
type AuthMessage struct {
LongTermPublicKey ed25519.PublicKey
EphemeralPublicKey ed25519.PublicKey
// AuthCapability defines the Authentication Capability granted by AuthApp
const AuthCapability = tapir.Capability("AuthenticationCapability")
// AuthApp is the concrete Application type that handles Authentication
type AuthApp struct {
// NewInstance creates a new instance of the AuthApp
func (ea AuthApp) NewInstance() tapir.Application {
return new(AuthApp)
// Init runs the entire AuthApp protocol, at the end of the protocol either the connection is granted AUTH capability
// or the connection is closed.
func (ea *AuthApp) Init(connection tapir.Connection) {
longTermPubKey := ed25519.PublicKey(connection.ID().PublicKeyBytes())
ephemeralIdentity, _ := primitives.InitializeEphemeralIdentity()
authMessage := AuthMessage{LongTermPublicKey: longTermPubKey, EphemeralPublicKey: ephemeralIdentity.PublicKey()}
serialized, _ := json.Marshal(authMessage)
message := connection.Expect()
var remoteAuthMessage AuthMessage
err := json.Unmarshal(message, &remoteAuthMessage)
if err != nil {
// If we are an outbound connection we can perform an additional check to ensure that the server sent us back the correct long term
// public key
if connection.IsOutbound() && torProvider.GetTorV3Hostname(remoteAuthMessage.LongTermPublicKey) != connection.Hostname() {
log.Errorf("The remote server (%v) has attempted to authenticate with a different public key %v", connection.Hostname(), torProvider.GetTorV3Hostname(remoteAuthMessage.LongTermPublicKey))
// Perform the triple-diffie-hellman exchange.
key, err := primitives.Perform3DH(connection.ID(), &ephemeralIdentity, remoteAuthMessage.LongTermPublicKey, remoteAuthMessage.EphemeralPublicKey, connection.IsOutbound())
if err != nil {
log.Errorf("Failed Auth Challenge %v", err)
// We just successfully unmarshaled both of these, so we can safely ignore the err return from these functions.
challengeRemote, _ := json.Marshal(remoteAuthMessage)
challengeLocal, _ := json.Marshal(authMessage)
// Define canonical labels so both sides of the connection can generate the same key
var outboundAuthMessage []byte
var outboundHostname string
var inboundAuthMessage []byte
var inboundHostname string
if connection.IsOutbound() {
outboundHostname = connection.ID().Hostname()
inboundHostname = torProvider.GetTorV3Hostname(remoteAuthMessage.LongTermPublicKey)
outboundAuthMessage = challengeLocal
inboundAuthMessage = challengeRemote
} else {
outboundHostname = torProvider.GetTorV3Hostname(remoteAuthMessage.LongTermPublicKey)
inboundHostname = connection.ID().Hostname()
outboundAuthMessage = challengeRemote
inboundAuthMessage = challengeLocal
// Derive a challenge from the transcript of the public parameters of this authentication protocol
transcript := ea.Transcript()
transcript.AddToTranscript("outbound-hostname", []byte(outboundHostname))
transcript.AddToTranscript("inbound-hostname", []byte(inboundHostname))
transcript.AddToTranscript("outbound-challenge", outboundAuthMessage)
transcript.AddToTranscript("inbound-challenge", inboundAuthMessage)
challengeBytes := transcript.CommitToTranscript("3dh-auth-challenge")
// If debug is turned on we will dump the transcript to log.
// There is nothing sensitive in this transcript
log.Debugf("Transcript: %s", transcript.OutputTranscriptToAudit())
// Since we have set the encryption key on the connection the connection will encrypt any messages we send with that key
// To test that the remote peer has done the same we calculate a challenge hash based on the transcript so far and send it to them
// along with our hostname
// We expect the remote to do the same, and compare the two.
// If successful we extend our auth capability to the connection and reassert the hostname.
// We note that the only successful scenario here requires that the remote peer have successfully derived the same
// encryption key and the same transcript challenge.
connection.Send(append(challengeBytes, []byte(connection.ID().Hostname())...))
remoteChallenge := connection.Expect()
assertedHostname := torProvider.GetTorV3Hostname(remoteAuthMessage.LongTermPublicKey)
if subtle.ConstantTimeCompare(append(challengeBytes, []byte(assertedHostname)...), remoteChallenge) == 1 {
} else {
log.Debugf("Failed Decrypt Challenge: [%x] [%x]\n", remoteChallenge, challengeBytes)