Add authentication to webhook calls (#34)

A new `X-Phoenix-Signature` header is added to webhook calls, which contains the HMAC-SHA256 signature of the whole json body, encoded in utf-8, using the `webhook-secret` configuration parameter, also encoded in utf-8. 

For example:
- webhook request body:
```
{
    "type": "payment_received",
    "timestamp": 1712785550079,
    "amountSat": 8,
    "paymentHash": "e628f8a516e9d3ee5e212a675f8d0c9dc5e7a5d500c5f4f91c62e9e921492653",
    "externalId": null
}
```
- `webhook-secret` in `phoenix.conf`:`ef72d3b96324106dfbf83f2a4efeff7dddb4ce923e9664cb56baf34cc52936b6`

Will produce the header `X-Phoenix-Signature: 77ffc40401024fb417e45fdd002de06bdbf3b48b90d09d05cccd06462920aed7`

A `timestamp` has been added to the events, to provide protection against replay attacks. Users should check that the timestamp is not too old. Stripe uses a [5 min default tolerance](https://docs.stripe.com/webhooks#replay-attacks).

Suggested by @danielcharrua in #33.

src/commonMain/kotlin/fr/acinq/lightning/bin/Api.kt

@@ -26,6 +26,7 @@ import fr.acinq.lightning.utils.*
 import io.ktor.client.*
 import io.ktor.client.request.*
 import io.ktor.http.*
+import io.ktor.http.content.*
 import io.ktor.serialization.kotlinx.*
 import io.ktor.serialization.kotlinx.json.*
 import io.ktor.server.application.*
@@ -41,8 +42,9 @@ import io.ktor.server.websocket.*
 import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.launch
 import kotlinx.serialization.json.Json
+import okio.ByteString.Companion.encodeUtf8
 
-class Api(private val nodeParams: NodeParams, private val peer: Peer, private val eventsFlow: SharedFlow<ApiEvent>, private val password: String, private val webhookUrl: Url?) {
+class Api(private val nodeParams: NodeParams, private val peer: Peer, private val eventsFlow: SharedFlow<ApiEvent>, private val password: String, private val webhookUrl: Url?, private val webhookSecret: String) {
 
     fun Application.module() {
 
@@ -199,6 +201,16 @@ class Api(private val nodeParams: NodeParams, private val peer: Peer, private va
                     })
                 }
             }
+            client.sendPipeline.intercept(HttpSendPipeline.State) {
+                when (val body = context.body) {
+                    is TextContent -> {
+                        val bodyBytes = body.text.encodeUtf8()
+                        val secretBytes = webhookSecret.encodeUtf8()
+                        val sig = bodyBytes.hmacSha256(secretBytes)
+                        context.headers.append("X-Phoenix-Signature", sig.hex())
+                    }
+                }
+            }
             launch {
                 eventsFlow.collect { event ->
                     client.post(url) {
@@ -231,5 +243,3 @@ class Api(private val nodeParams: NodeParams, private val peer: Peer, private va
     private fun Parameters.getOptionalLong(argName: String): Long? = this[argName]?.let { it.toLongOrNull() ?: invalidType(argName, "integer") }
 
 }
-
-

src/commonMain/kotlin/fr/acinq/lightning/bin/Main.kt

@@ -102,16 +102,26 @@ class Phoenixd : CliktCommand() {
         .default(10.minutes)
     private val httpBindIp by option("--http-bind-ip", help = "Bind ip for the http api").default("127.0.0.1")
     private val httpBindPort by option("--http-bind-port", help = "Bind port for the http api").int().default(9740)
-    private val httpPassword by option("--http-password", help = "Password for the http api").defaultLazy {
-        // the additionalValues map already contains values in phoenix.conf, so if we are here then there are no existing password
-        terminal.print(yellow("Generating default api password..."))
-        val value = randomBytes32().toHex()
-        FileSystem.SYSTEM.appendingSink(confFile, mustExist = false).buffer().use { it.writeUtf8("\nhttp-password=$value\n") }
-        terminal.println(white("done"))
-        value
-    }
+    private val httpPassword by option("--http-password", help = "Password for the http api")
+        .defaultLazy {
+            // if we are here then no value is defined in phoenix.conf
+            terminal.print(yellow("Generating default api password..."))
+            val value = randomBytes32().toHex()
+            FileSystem.SYSTEM.appendingSink(confFile, mustExist = false).buffer().use { it.writeUtf8("\nhttp-password=$value") }
+            terminal.println(white("done"))
+            value
+        }
     private val webHookUrl by option("--webhook", help = "Webhook http endpoint for push notifications (alternative to websocket)")
         .convert { Url(it) }
+    private val webHookSecret by option("--webhook-secret", help = "Secret used to authenticate webhook calls")
+        .defaultLazy {
+            // if we are here then no value is defined in phoenix.conf
+            terminal.print(yellow("Generating webhook secret..."))
+            val value = randomBytes32().toHex()
+            FileSystem.SYSTEM.appendingSink(confFile, mustExist = false).buffer().use { it.writeUtf8("\nwebhook-secret=$value") }
+            terminal.println(white("done"))
+            value
+        }
 
     class LiquidityOptions : OptionGroup(name = "Liquidity Options") {
         val autoLiquidity by option("--auto-liquidity", help = "Amount automatically requested when inbound liquidity is needed").choice(
@@ -353,7 +363,7 @@ class Phoenixd : CliktCommand() {
                 reuseAddress = true
             },
             module = {
-                Api(nodeParams, peer, eventsFlow, httpPassword, webHookUrl).run { module() }
+                Api(nodeParams, peer, eventsFlow, httpPassword, webHookUrl, webHookSecret).run { module() }
             }
         )
         val serverJob = scope.launch {

src/commonMain/kotlin/fr/acinq/lightning/bin/json/JsonSerializers.kt

@@ -25,6 +25,7 @@ import fr.acinq.lightning.channel.states.ChannelStateWithCommitments
 import fr.acinq.lightning.db.LightningOutgoingPayment
 import fr.acinq.lightning.json.JsonSerializers
 import fr.acinq.lightning.utils.UUID
+import kotlinx.datetime.Clock
 import kotlinx.serialization.SerialName
 import kotlinx.serialization.Serializable
 import kotlinx.serialization.UseSerializers
@@ -70,7 +71,9 @@ sealed class ApiType {
     data class GeneratedInvoice(@SerialName("amountSat") val amount: Satoshi?, val paymentHash: ByteVector32, val serialized: String) : ApiType()
 
     @Serializable
-    sealed class ApiEvent : ApiType()
+    sealed class ApiEvent : ApiType() {
+        val timestamp: Long = Clock.System.now().toEpochMilliseconds()
+    }
 
     @Serializable
     @SerialName("payment_received")