diff --git a/backend/src/main/scala/industries/sunshine/planningpoker/Auth.scala b/backend/src/main/scala/industries/sunshine/planningpoker/Auth.scala
index 7aae41d..d250e8e 100644
--- a/backend/src/main/scala/industries/sunshine/planningpoker/Auth.scala
+++ b/backend/src/main/scala/industries/sunshine/planningpoker/Auth.scala
@@ -1,7 +1,7 @@
 package industries.sunshine.planningpoker
 
 import cats.effect._
-import cats.syntax._
+import cats.syntax.all._
 import cats.data.Kleisli
 import cats.data.OptionT
 import org.http4s.Request
@@ -9,6 +9,7 @@ import industries.sunshine.planningpoker.common.Models.PlayerID
 import java.util.UUID
 import industries.sunshine.planningpoker.common.Models.{RoomID, Room}
 import org.http4s.ResponseCookie
+import cats.data.EitherT
 
 trait Auth[F[_]] {
 
@@ -26,7 +27,8 @@ trait Auth[F[_]] {
   def joinRoom(
       roomName: String,
       roomPassword: String,
-      nickName: String
+      nickName: String,
+      nickPassword: String
   ): F[Either[Unit, ResponseCookie]]
 
   def deleteSession(
@@ -48,7 +50,8 @@ object Auth {
     override def joinRoom(
         roomName: String,
         roomPassword: String,
-        nickName: String
+        nickName: String,
+        nickPassword: String
     ) =
       IO(
         println(
@@ -61,6 +64,7 @@ object Auth {
   }
 
   type SessionsMap = Map[Long, (RoomID, PlayerID)]
+
   class SimpleAuth[F[_]: Sync](
       sessions: Ref[F, SessionsMap],
       roomService: RoomService[F]
@@ -69,8 +73,12 @@ object Auth {
     override def joinRoom(
         roomName: String,
         roomPassword: String,
-        nickName: String
+        nickName: String,
+        nickPassword: String
     ): F[Either[Unit, ResponseCookie]] = {
+      for {
+        joinedRoom <- roomService.joinRoom(RoomID(roomName), nickName, nickPassword, roomPassword)
+      } yield ()
       ???
     }
 
@@ -86,7 +94,6 @@ object Auth {
     }
   }
 
-  import cats.Monad.ops.toAllMonadOps
   def make[F[_]: Sync](roomsService: RoomService[F]): F[Auth[F]] =
     for {
       sessionsMap <- Ref.of[F, SessionsMap](
diff --git a/backend/src/main/scala/industries/sunshine/planningpoker/MyHttpService.scala b/backend/src/main/scala/industries/sunshine/planningpoker/MyHttpService.scala
index 6322431..53c6691 100644
--- a/backend/src/main/scala/industries/sunshine/planningpoker/MyHttpService.scala
+++ b/backend/src/main/scala/industries/sunshine/planningpoker/MyHttpService.scala
@@ -59,8 +59,8 @@ object MyHttpService {
             authResult <- auth.joinRoom(
               data.roomName,
               data.password,
-              data.nickname
-            )
+              data.nickname,
+              data.nickPassword)
             resp <- authResult match {
               case Left(error) =>
                 Forbidden(error)
diff --git a/backend/src/main/scala/industries/sunshine/planningpoker/RoomService.scala b/backend/src/main/scala/industries/sunshine/planningpoker/RoomService.scala
index 88e425f..1da27c9 100644
--- a/backend/src/main/scala/industries/sunshine/planningpoker/RoomService.scala
+++ b/backend/src/main/scala/industries/sunshine/planningpoker/RoomService.scala
@@ -3,14 +3,24 @@ package industries.sunshine.planningpoker
 import industries.sunshine.planningpoker.common.Models.*
 import cats.effect.{Ref, Sync}
 import cats.syntax.all._
+import cats.data.EitherT
 
 enum RoomError {
   case RoomAlreadyExists(name: String)
+  case RoomMissing(name: String)
+  case RoomPassIncorrect
+  case NickPassIncorrect
 }
 
 trait RoomService[F[_]] {
   def createRoom(newRoom: Room): F[Either[RoomError, Room]]
   def updateRoom(room: Room): F[Unit]
+  def joinRoom(
+      id: RoomID,
+      nickName: String,
+      nickPassword: String,
+      roomPassword: String
+  ): F[Either[RoomError, Room]]
   def deleteRoom(roomID: RoomID): F[Unit]
   def getRoom(roomID: RoomID): F[Option[Room]]
 }
@@ -26,13 +36,36 @@ class InMemoryRoomService[F[_]: Sync](stateRef: Ref[F, Map[RoomID, Room]]) exten
       }
     }
   }
-  def updateRoom(room: Room): F[Unit] = stateRef.update { state =>
+  override def updateRoom(room: Room): F[Unit] = stateRef.update { state =>
     state.get(room.id).fold(state)(oldRoom => state.updated(room.id, room))
   }
 
-  def deleteRoom(roomID: RoomID): F[Unit] = stateRef.update(_.removed(roomID))
+  override def deleteRoom(roomID: RoomID): F[Unit] = stateRef.update(_.removed(roomID))
 
-  def getRoom(roomID: RoomID): F[Option[Room]] = stateRef.get.map(_.get(roomID))
+  override def getRoom(roomID: RoomID): F[Option[Room]] = stateRef.get.map(_.get(roomID))
+
+  override def joinRoom(
+      id: RoomID,
+      nickName: String,
+      nickPassword: String,
+      roomPassword: String
+  ): F[Either[RoomError, Room]] = stateRef.modify[Either[RoomError, Room]] { rooms =>
+    val joinedRoom = for {
+      room <- rooms.get(id).toRight(RoomError.RoomMissing(id.name))
+      _ <- Either.cond(room.password == roomPassword, (), RoomError.RoomPassIncorrect)
+      isNickPassCorrect = room.playersPasswords
+        .get(nickName)
+        .fold(true)(existingPass => existingPass == nickPassword)
+      _ <- Either.cond(
+        isNickPassCorrect,
+        (),
+        RoomError.NickPassIncorrect
+      )
+      updatedRoom = room
+    } yield updatedRoom
+
+    rooms -> joinedRoom
+  }
 
 }
 object RoomService {
diff --git a/common/src/main/scala/industries/sunshine/planningpoker/Models.scala b/common/src/main/scala/industries/sunshine/planningpoker/Models.scala
index d179786..fc87f7a 100644
--- a/common/src/main/scala/industries/sunshine/planningpoker/Models.scala
+++ b/common/src/main/scala/industries/sunshine/planningpoker/Models.scala
@@ -62,7 +62,8 @@ object Models {
       owner: PlayerID,
       password: String,
       allowedCards: List[String],
-      round: RoundState
+      round: RoundState,
+      playersPasswords: Map[String, String] = Map.empty // nickname into password
   ) {
     def toViewFor(playerId: PlayerID): RoomStateView = {
       players
diff --git a/common/src/main/scala/industries/sunshine/planningpoker/Requests.scala b/common/src/main/scala/industries/sunshine/planningpoker/Requests.scala
index 5349a9c..d1fcf0b 100644
--- a/common/src/main/scala/industries/sunshine/planningpoker/Requests.scala
+++ b/common/src/main/scala/industries/sunshine/planningpoker/Requests.scala
@@ -4,6 +4,6 @@ import io.circe.generic.semiauto._
 import io.circe._
 
 object Requests {
-  final case class LogIn(roomName: String, nickname: String, password: String)
+  final case class LogIn(roomName: String, nickname: String, password: String, nickPassword: String)
       derives Codec.AsObject
 }