feat: Implement PoWeb handshake messages

build.gradle

@@ -3,6 +3,8 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 buildscript {
     ext {
         kotlinVersion = '1.3.72'
+        protobufVersion = '3.12.2'
+        protobufGradleVersion = '0.8.12'
     }
 }
 
@@ -13,6 +15,7 @@ plugins {
     id('maven-publish')
     id("com.diffplug.gradle.spotless") version "3.27.1"
     id('jacoco')
+    id("com.google.protobuf") version "$protobufGradleVersion"
 }
 
 group = "tech.relaycorp"
@@ -26,6 +29,11 @@ dependencies {
 
     implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlinVersion")
 
+    // Protobuf
+    implementation "com.google.protobuf:protobuf-gradle-plugin:$protobufGradleVersion"
+    implementation "com.google.protobuf:protobuf-java:$protobufVersion"
+    implementation "com.google.protobuf:protobuf-java-util:$protobufVersion"
+
     testImplementation("org.jetbrains.kotlin:kotlin-test")
 
     // Use the Kotlin JUnit5 integration.
@@ -43,6 +51,17 @@ tasks.withType(KotlinCompile).configureEach {
     kotlinOptions.jvmTarget = "1.8"
 }
 
+protobuf {
+    protoc { artifact = "com.google.protobuf:protoc:$protobufVersion" }
+}
+sourceSets {
+    main {
+        kotlin {
+            srcDirs "build/generated/source/proto/main/java"
+        }
+    }
+}
+
 jacoco {
     toolVersion = "0.8.5"
 }

src/main/proto/poweb-handshake.proto

@@ -0,0 +1,19 @@
+syntax = "proto3";
+
+option java_multiple_files = true;
+option java_package = "tech.relaycorp.poweb.messages.handshake";
+
+package relaynet.poweb.handshake;
+
+message Challenge {
+    // Sent by the gateway at the start of the connection
+
+    string gateway_nonce = 1;
+}
+
+message Response {
+    // Sent by the endpoint in response to Challenge
+
+    // The gateway's nonce signed by each endpoint certificate.
+    repeated bytes gateway_nonce_signatures = 1;
+}