SES-3053 Send only the first 32 bytes of admin key as promotion message (#860)

app/src/main/java/org/thoughtcrime/securesms/groups/GroupManagerV2Impl.kt

@@ -501,7 +501,7 @@ class GroupManagerV2Impl @Inject constructor(
             GroupUpdateMessage.newBuilder()
                 .setPromoteMessage(
                     DataMessage.GroupUpdatePromoteMessage.newBuilder()
-                        .setGroupIdentitySeed(ByteString.copyFrom(adminKey))
+                        .setGroupIdentitySeed(ByteString.copyFrom(adminKey).substring(0, 32))
                         .setName(groupName)
                 )
                 .build()
@@ -693,7 +693,7 @@ class GroupManagerV2Impl @Inject constructor(
         handleInvitation(
             groupId = groupId,
             groupName = groupName,
-            authDataOrAdminKey = authData,
+            authDataOrAdminSeed = authData,
             fromPromotion = false,
             inviter = inviter,
             inviterName = inviterName,
@@ -721,7 +721,7 @@ class GroupManagerV2Impl @Inject constructor(
             handleInvitation(
                 groupId = groupId,
                 groupName = groupName,
-                authDataOrAdminKey = adminKey,
+                authDataOrAdminSeed = adminKey,
                 fromPromotion = true,
                 inviter = promoter,
                 inviterName = promoterName,
@@ -759,7 +759,7 @@ class GroupManagerV2Impl @Inject constructor(
      *
      * @param groupId the group ID
      * @param groupName the group name
-     * @param authDataOrAdminKey the auth data or admin key. If this is an invitation, this is the auth data, if this is a promotion, this is the admin key.
+     * @param authDataOrAdminSeed the auth data or admin key. If this is an invitation, this is the auth data, if this is a promotion, this is the admin key.
      * @param fromPromotion true if this is a promotion, false if this is an invitation
      * @param inviter the invite message sender
      * @return The newly created group info if the invitation is processed, null otherwise.
@@ -767,7 +767,7 @@ class GroupManagerV2Impl @Inject constructor(
     private suspend fun handleInvitation(
         groupId: AccountId,
         groupName: String,
-        authDataOrAdminKey: ByteArray,
+        authDataOrAdminSeed: ByteArray,
         fromPromotion: Boolean,
         inviter: AccountId,
         inviterName: String?,
@@ -781,8 +781,8 @@ class GroupManagerV2Impl @Inject constructor(
             storage.getRecipientApproved(Address.fromSerialized(inviter.hexString))
         val closedGroupInfo = GroupInfo.ClosedGroupInfo(
             groupAccountId = groupId,
-            adminKey = authDataOrAdminKey.takeIf { fromPromotion },
+            adminKey = authDataOrAdminSeed.takeIf { fromPromotion }?.let { GroupInfo.ClosedGroupInfo.adminKeyFromSeed(it) },
-            authData = authDataOrAdminKey.takeIf { !fromPromotion },
+            authData = authDataOrAdminSeed.takeIf { !fromPromotion },
             priority = PRIORITY_VISIBLE,
             invited = !shouldAutoApprove,
             name = groupName,

libsession-util/src/main/cpp/user_groups.cpp

@@ -1,6 +1,8 @@
 #include "user_groups.h"
 #include "oxenc/hex.h"
 
+#include "session/ed25519.hpp"
+
 extern "C"
 JNIEXPORT jint JNICALL
 Java_network_loki_messenger_libsession_1util_util_GroupInfo_00024LegacyGroupInfo_00024Companion_NAME_1MAX_1LENGTH(
@@ -324,3 +326,21 @@ Java_network_loki_messenger_libsession_1util_UserGroupsConfig_eraseClosedGroup(J
     env->ReleaseStringUTFChars(session_id, session_id_bytes);
     return return_value;
 }
+
+extern "C"
+JNIEXPORT jbyteArray JNICALL
+Java_network_loki_messenger_libsession_1util_util_GroupInfo_00024ClosedGroupInfo_adminKeyFromSeed(
+        JNIEnv *env, jclass clazz, jbyteArray seed) {
+    auto len = env->GetArrayLength(seed);
+    if (len != 32) {
+        env->ThrowNew(env->FindClass("java/lang/IllegalArgumentException"), "Seed must be 32 bytes");
+        return nullptr;
+    }
+
+    auto seed_bytes = env->GetByteArrayElements(seed, nullptr);
+    auto admin_key = session::ed25519::ed25519_key_pair(
+            session::ustring_view(reinterpret_cast<unsigned char *>(seed_bytes), 32)).second;
+    env->ReleaseByteArrayElements(seed, seed_bytes, 0);
+
+    return util::bytes_from_ustring(env, session::ustring_view(admin_key.data(), admin_key.size()));
+}

libsession-util/src/main/java/network/loki/messenger/libsession_util/util/GroupInfo.kt

@@ -17,7 +17,6 @@ sealed class GroupInfo {
         val destroyed: Boolean,
         val joinedAtSecs: Long
     ): GroupInfo() {
-
         init {
             require(adminKey == null || adminKey.isNotEmpty()) {
                 "Admin key must be non-empty if present"
@@ -29,6 +28,17 @@ sealed class GroupInfo {
         }
 
         fun hasAdminKey() = adminKey != null
+
+        companion object {
+            /**
+             * Generate the group's admin key(64 bytes) from seed (32 bytes, normally used
+             * in group promotions).
+             *
+             * Use of JvmStatic makes the JNI signature less esoteric.
+             */
+            @JvmStatic
+            external fun adminKeyFromSeed(seed: ByteArray): ByteArray
+        }
     }
 
     data class LegacyGroupInfo(