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diff --git a/src/messenger/gnunet-service-messenger_member_session.h b/src/messenger/gnunet-service-messenger_member_session.h
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+/*
+   This file is part of GNUnet.
+   Copyright (C) 2021 GNUnet e.V.
+
+   GNUnet is free software: you can redistribute it and/or modify it
+   under the terms of the GNU Affero General Public License as published
+   by the Free Software Foundation, either version 3 of the License,
+   or (at your option) any later version.
+
+   GNUnet is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+   SPDX-License-Identifier: AGPL3.0-or-later
+ */
+/**
+ * @author Tobias Frisch
+ * @file src/messenger/gnunet-service-messenger_member_session.h
+ * @brief GNUnet MESSENGER service
+ */
+
+#ifndef GNUNET_SERVICE_MESSENGER_MEMBER_SESSION_H
+#define GNUNET_SERVICE_MESSENGER_MEMBER_SESSION_H
+
+#include "platform.h"
+#include "gnunet_crypto_lib.h"
+#include "gnunet_container_lib.h"
+#include "gnunet_identity_service.h"
+
+#include "gnunet-service-messenger_member.h"
+
+#include "messenger_api_contact.h"
+
+struct GNUNET_MESSENGER_MemberSession {
+  struct GNUNET_MESSENGER_Member *member;
+
+  struct GNUNET_IDENTITY_PublicKey public_key;
+  struct GNUNET_HashCode context;
+
+  struct GNUNET_MESSENGER_Contact *contact;
+
+  struct GNUNET_CONTAINER_MultiHashMap *history;
+  struct GNUNET_MESSENGER_ListMessages messages;
+
+  struct GNUNET_MESSENGER_MemberSession* next;
+
+  int closed;
+  int completed;
+};
+
+/**
+ * Creates and allocates a new member session of a <i>member</i> with a given
+ * public key.
+ *
+ * If the creation fails, NULL gets returned.
+ *
+ * @param[in/out] member Member
+ * @param[in] pubkey Public key of EGO
+ * @return New member session
+ */
+struct GNUNET_MESSENGER_MemberSession*
+create_member_session (struct GNUNET_MESSENGER_Member *member,
+                       const struct GNUNET_IDENTITY_PublicKey *pubkey);
+
+/**
+ * Creates and allocates a new member session closing and replacing a given
+ * other <i>session</i> of the same member. The new session could have significant
+ * changes to the members public key or its member id depending on the used
+ * <i>message</i> to switch session. The new session will be linked to the old
+ * one.
+ *
+ * @param[in/out] session Old member session
+ * @param[in] message Message
+ * @param[in] hash Hash of message
+ * @return New member session
+ */
+struct GNUNET_MESSENGER_MemberSession*
+switch_member_session (struct GNUNET_MESSENGER_MemberSession *session,
+                       const struct GNUNET_MESSENGER_Message *message,
+                       const struct GNUNET_HashCode *hash);
+
+/**
+ * Destroys a member session and frees its memory fully.
+ *
+ * @param[in/out] session Member session
+ */
+void
+destroy_member_session(struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Resets a given member <i>session</i> which re-opens a member
+ * session for new usage. Every connection to other sessions will be
+ * be dropped. The member sessions messages will be cleared but old
+ * history from uncompleted sessions however can be reused!
+ *
+ * @param[in/out] session Member session
+ * @param[in] hash Hash of initial message (JOIN message!)
+ * @return #GNUNET_OK on success, #GNUNET_SYSERR otherwise
+ */
+int
+reset_member_session (struct GNUNET_MESSENGER_MemberSession* session,
+                      const struct GNUNET_HashCode *hash);
+
+/**
+ * Closes a given member <i>session</i> which opens the request
+ * for completion of the given member session.
+ *
+ * Closing a session may complete a session and can't be used without
+ * a reset! ( @see #reset_member_session() )
+ *
+ * @param[in/out] session Member session
+ */
+void
+close_member_session (struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns if the given member <i>session</i> has been closed.
+ *
+ * @param[in] session Member session
+ * @return #GNUNET_YES or #GNUNET_NO
+ */
+int
+is_member_session_closed (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns if the given member <i>session</i> has been completed.
+ *
+ * A completed member session can't verify any message as its own and
+ * it won't add any message to its history.
+ *
+ * @param[in] session Member session
+ * @return #GNUNET_YES or #GNUNET_NO
+ */
+int
+is_member_session_completed (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns the key of the room a given member <i>session</i> belongs to.
+ *
+ * @param[in] session Member session
+ * @return Key of room
+ */
+const struct GNUNET_HashCode*
+get_member_session_key (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns the member id of a given member <i>session</i>.
+ *
+ * @param[in] session Member session
+ * @return Member id
+ */
+const struct GNUNET_ShortHashCode*
+get_member_session_id (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns the public key from an EGO of a given member <i>session</i>.
+ *
+ * @param[in] session Member session
+ * @return Public key of EGO
+ */
+const struct GNUNET_IDENTITY_PublicKey*
+get_member_session_public_key (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Returns the member context of a given member <i>session</i>.
+ *
+ * @param[in] session Member session
+ * @return Member context as hash
+ */
+const struct GNUNET_HashCode*
+get_member_session_context (const struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Retruns the contact which is connected to a given member <i>session</i>.
+ *
+ * @param[in] session Member session
+ * @return Contact
+ */
+struct GNUNET_MESSENGER_Contact*
+get_member_session_contact (struct GNUNET_MESSENGER_MemberSession* session);
+
+/**
+ * Verifies a given member <i>session</i> as sender of a selected <i>message</i> and
+ * its <i>hash</i>. The function returns #GNUNET_OK if the message session is verified
+ * as sender, otherwise #GNUNET_SYSERR.
+ *
+ * @see #is_member_session_completed() for verification.
+ *
+ * @param[in] session Member session
+ * @param[in] message Message
+ * @param[in] hash Hash of message
+ * @return #GNUNET_OK on success, otherwise #GNUNET_SYSERR
+ */
+int
+verify_member_session_as_sender (const struct GNUNET_MESSENGER_MemberSession *session,
+                                 const struct GNUNET_MESSENGER_Message *message,
+                                 const struct GNUNET_HashCode *hash);
+
+/**
+ * Checks the history of a <i>session</i> for a specific message which is identified
+ * by its <i>hash</i> and if the <i>ownership</i> flag is set, if the message is
+ * owned by the sessions contact.
+ *
+ * @param[in] session Member session
+ * @param[in] hash Hash of message
+ * @param[in] ownership Ownership flag
+ * @return #GNUNET_YES if found, otherwise #GNUNET_NO
+ */
+int
+check_member_session_history (const struct GNUNET_MESSENGER_MemberSession *session,
+                              const struct GNUNET_HashCode *hash, int ownership);
+
+/**
+ * Adds a given <i>message</i> to the history of a <i>session</i> using the messages
+ * <i>hash</i>. The ownership will be set automatically.
+ *
+ * @see #is_member_session_completed() for updating a history.
+ *
+ * @param[in/out] session Member session
+ * @param[in] message Message
+ * @param[in] hash Hash of message
+ */
+void
+update_member_session_history (struct GNUNET_MESSENGER_MemberSession *session,
+                               const struct GNUNET_MESSENGER_Message *message,
+                               const struct GNUNET_HashCode *hash);
+
+/**
+ * Removes a message from the history of a <i>session</i> using the messages
+ * <i>hash</i>.
+ *
+ * @param[in/out] session Member session
+ * @param[in] hash Hash of message
+ */
+void
+clear_member_session_history (struct GNUNET_MESSENGER_MemberSession *session,
+                              const struct GNUNET_HashCode *hash);
+
+/**
+ * Loads data from a <i>directory</i> into a new allocated and created member
+ * session of a <i>member</i> if the required information can be read from the
+ * content of the given directory.
+ *
+ * @param[out] member Member
+ * @param[in] directory Path to a directory
+ */
+void
+load_member_session (struct GNUNET_MESSENGER_Member *member, const char *directory);
+
+/**
+ * Loads the connection from one <i>session</i> to another through the
+ * next attribute. Necessary information will be loaded from a configuration
+ * file inside of a given <i>directory</i>.
+ *
+ * @param[in/out] session Member session
+ * @param[in] directory Path to a directory
+ */
+void
+load_member_session_next (struct GNUNET_MESSENGER_MemberSession *session, const char *directory);
+
+/**
+ * Saves data from a member <i>session</i> into a <i>directory</i> which can be
+ * load to restore the member session completely.
+ *
+ * @param[in] session Member session
+ * @param[in] directory Path to a directory
+ */
+void
+save_member_session (struct GNUNET_MESSENGER_MemberSession *session, const char *directory);
+
+#endif //GNUNET_SERVICE_MESSENGER_MEMBER_SESSION_H