Network Working Group P. Riikonen Internet-Draft draft-riikonen-silc-spec-04.txt 13 November 2001 Expires: 13 May 2002 Secure Internet Live Conferencing (SILC), Protocol Specification <draft-riikonen-silc-spec-04.txt> Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html The distribution of this memo is unlimited. Abstract This memo describes a Secure Internet Live Conferencing (SILC) protocol which provides secure conferencing services over insecure network channel. SILC is IRC [IRC] like protocol, however, it is not equivalent to IRC and does not support IRC. Strong cryptographic methods are used to protect SILC packets inside the SILC network. Three other Internet Drafts relates very closely to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication Protocols [SILC3] and SILC Commands [SILC4]. Riikonen [Page 1]
Internet Draft 13 November 2001 Table of Contents 1 Introduction .................................................. 3 1.1 Requirements Terminology .................................. 4 2 SILC Concepts ................................................. 4 2.1 SILC Network Topology ..................................... 4 2.2 Communication Inside a Cell ............................... 5 2.3 Communication in the Network .............................. 6 2.4 Channel Communication ..................................... 7 2.5 Router Connections ........................................ 7 3 SILC Specification ............................................ 8 3.1 Client .................................................... 8 3.1.1 Client ID ........................................... 9 3.2 Server .................................................... 10 3.2.1 Server's Local ID List .............................. 10 3.2.2 Server ID ........................................... 11 3.2.3 SILC Server Ports ................................... 12 3.3 Router .................................................... 12 3.3.1 Router's Local ID List .............................. 12 3.3.2 Router's Global ID List ............................. 13 3.3.3 Router's Server ID .................................. 14 3.4 Channels .................................................. 14 3.4.1 Channel ID .......................................... 16 3.5 Operators ................................................. 16 3.6 SILC Commands ............................................. 16 3.7 SILC Packets .............................................. 17 3.8 Packet Encryption ......................................... 17 3.8.1 Determination of the Source and the Destination ..... 17 3.8.2 Client To Client .................................... 18 3.8.3 Client To Channel ................................... 19 3.8.4 Server To Server .................................... 20 3.9 Key Exchange And Authentication ........................... 20 3.9.1 Authentication Payload .............................. 20 3.10 Algorithms ............................................... 22 3.10.1 Ciphers ............................................ 22 3.10.2 Public Key Algorithms .............................. 23 3.10.3 Hash Functions ..................................... 24 3.10.4 MAC Algorithms ..................................... 24 3.10.5 Compression Algorithms ............................. 25 3.11 SILC Public Key .......................................... 25 3.12 SILC Version Detection ................................... 27 3.13 Backup Routers ........................................... 28 3.13.1 Switching to Backup Router ......................... 29 3.13.2 Resuming Primary Router ............................ 30 3.13.3 Discussion on Backup Router Scheme ................. 32 4 SILC Procedures ............................................... 33 4.1 Creating Client Connection ................................ 33 4.2 Creating Server Connection ................................ 34 Riikonen [Page 2]
Internet Draft 13 November 2001 4.2.1 Announcing Clients, Channels and Servers ............ 35 4.3 Joining to a Channel ...................................... 36 4.4 Channel Key Generation .................................... 37 4.5 Private Message Sending and Reception ..................... 38 4.6 Private Message Key Generation ............................ 38 4.7 Channel Message Sending and Reception ..................... 39 4.8 Session Key Regeneration .................................. 39 4.9 Command Sending and Reception ............................. 40 4.10 Closing Connection ....................................... 41 5 Security Considerations ....................................... 41 6 References .................................................... 42 7 Author's Address .............................................. 44 List of Figures Figure 1: SILC Network Topology Figure 2: Communication Inside cell Figure 3: Communication Between Cells Figure 4: Router Connections Figure 5: SILC Public Key 1. Introduction This document describes a Secure Internet Live Conferencing (SILC) protocol which provides secure conferencing services over insecure network channel. SILC is IRC [IRC] like protocol, however, it is not equivalent to IRC and does not support IRC. Strong cryptographic methods are used to protect SILC packets inside the SILC network. Three other Internet Drafts relates very closely to this memo; SILC Packet Protocol [SILC2], SILC Key Exchange and Authentication Protocols [SILC3] and SILC Commands [SILC4]. The protocol uses extensively packets as conferencing protocol requires message and command sending. The SILC Packet Protocol is described in [SILC2] and should be read to fully comprehend this document and protocol. [SILC2] also describes the packet encryption and decryption in detail. The security of SILC protocol, and for any security protocol for that matter, is based on strong and secure key exchange protocol. The SILC Key Exchange protocol is described in [SILC3] along with connection authentication protocol and should be read to fully comprehend this document and protocol. Riikonen [Page 3]
Internet Draft 13 November 2001 The SILC protocol has been developed to work on TCP/IP network protocol, although it could be made to work on other network protocols with only minor changes. However, it is recommended that TCP/IP protocol is used under SILC protocol. Typical implementation would be made in client-server model. 1.1 Requirements Terminology The keywords MUST, MUST NOT, REQUIRED, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. 2. SILC Concepts This section describes various SILC protocol concepts that forms the actual protocol, and in the end, the actual SILC network. The mission of the protocol is to deliver messages from clients to other clients through routers and servers in secure manner. The messages may also be delivered from one client to many clients forming a group, also known as a channel. This section does not focus to security issues. Instead, basic network concepts are introduced to make the topology of the SILC network clear. 2.1 SILC Network Topology SILC network is a cellular network as opposed to tree style network topology. The rationale for this is to have servers that can perform specific kind of tasks what other servers cannot perform. This leads to two kinds of servers; normal SILC servers and SILC routers. A difference between normal server and router server is that routers knows everything about everything in the network. They also do the actual routing of the messages to the correct receiver. Normal servers knows only about local information and nothing about global information. This makes the network faster as there are less servers that needs to keep global information up to date at all time. This, on the other hand, leads to cellular like network, where routers are in the center of the cell and servers are connected to the router. Riikonen [Page 4]
Internet Draft 13 November 2001 The following diagram represents SILC network topology. ---- ---- ---- ---- ---- ---- | S8 | S5 | S4 | | S7 | S5 | S6 | ----- ---- ----- ----- ---- ----- | S7 | S/R1 | S2 | --- | S8 | S/R2 | S4 | ---- ------ ---- ---- ------ ---- | S6 | S3 | S1 | | S1 | S3 | S2 | ---- ---- ---- ---- ---- ---- ---- ---- | S3 | S1 | Cell 1. \ Cell 2. | \____ ----- ----- | | | S4 | S/R4 | ---- ---- ---- ---- ---- ---- ---- ------ | S7 | S4 | S2 | | S1 | S3 | S2 | | S2 | S5 | ----- ---- ----- ----- ---- ----- ---- ---- | S6 | S/R3 | S1 | --- | S4 | S/R5 | S5 | ____/ Cell 4. ---- ------ ---- ---- ------ ---- | S8 | S5 | S3 | | S6 | S7 | S8 | ... etc ... ---- ---- ---- ---- ---- ---- Cell 3. Cell 5. Figure 1: SILC Network Topology A cell is formed when a server or servers connect to one router. In SILC network normal server cannot directly connect to other normal server. Normal server may only connect to SILC router which then routes the messages to the other servers in the cell. Router servers on the other hand may connect to other routers to form the actual SILC network, as seen in above figure. However, router is also normal SILC server; clients may connect to it the same way as to normal SILC server. Normal server also cannot have active connections to more than one router. Normal server cannot be connected to two different cells. Router servers, on the other hand, may have as many router to router connections as needed. There are many issues in this network topology that needs to be careful about. Issues like the size of the cells, the number of the routers in the SILC network and the capacity requirements of the routers. These issues should be discussed in the Internet Community and additional documents on the issue may be written. 2.2 Communication Inside a Cell It is always guaranteed that inside a cell message is delivered to the recipient with at most two server hops. A client which is connected to server in the cell and is talking on channel to other client connected to other server in the same cell, will have its messages delivered from Riikonen [Page 5]
Internet Draft 13 November 2001 its local server first to the router of the cell, and from the router to the other server in the cell. The following diagram represents this scenario: 1 --- S1 S4 --- 5 S/R 2 -- S2 S3 / | 4 3 Figure 2: Communication Inside cell Example: Client 1. connected to Server 1. send message to Client 4. connected to Server 2. travels from Server 1. first to Router which routes the message to Server 2. which then sends it to the Client 4. All the other servers in the cell will not see the routed message. If the client is connected directly to the router, as router is also normal SILC server, the messages inside the cell are always delivered only with one server hop. If clients communicating with each other are connected to the same server, no router interaction is needed. This is the optimal situation of message delivery in the SILC network. 2.3 Communication in the Network If the message is destined to server that does not belong to local cell the message is routed to the router server to which the destination server belongs, if the local router is connected to destination router. If there is no direct connection to the destination router, the local router routes the message to its primary route. The following diagram represents message sending between cells. 1 --- S1 S4 --- 5 S2 --- 1 S/R - - - - - - - - S/R 2 -- S2 S3 S1 / | \ 4 3 2 Cell 1. Cell 2. Riikonen [Page 6]
Internet Draft 13 November 2001 Figure 3: Communication Between Cells Example: Client 5. connected to Server 4. in Cell 1. sends message to Client 2. connected to Server 1. in Cell 2. travels from Server 4. to Router which routes the message to Router in Cell 2, which then routes the message to Server 1. All the other servers and routers in the network will not see the routed message. The optimal case of message delivery from the client point of view is when clients are connected directly to the routers and the messages are delivered from one router to the other. 2.4 Channel Communication Messages may be sent to group of clients as well. Sending messages to many clients works the same way as sending messages point to point, from message delivery point of view. Security issues are another matter which are not discussed in this section. Router server handles the message routing to multiple recipients. If any recipient is not in the same cell as the sender the messages are routed further. Server distributes the channel message to its local clients which are joined to the channel. Router also distributes the message to its local clients on the channel. 2.5 Router Connections Router connections play very important role in making the SILC like network topology to work. For example, sending broadcast packets in SILC network require special connections between routers; routers must be connected in a specific way. Every router has their primary route which is a connection to another router in the network. Unless there is only two routers in the network must not routers use each other as their primary routes. The router connections in the network must form a ring. Riikonen [Page 7]
Internet Draft 13 November 2001 Example with three routers in the network: S/R1 - > - > - > - > - > - > - S/R2 \ / ^ v \ - < - < - S/R3 - < - < - / Figure 4: Router Connections Example: Network with three routers. Router 1. uses Router 2. as its primary router. Router 2. uses Router 3. as its primary router, and Router 3. uses Router 1. as its primary router. There may be other direct connections between the routers but they must not be used as primary routes. The above example is applicable to any amount of routers in the network except for two routers. If there are only two routers in the network both routers must be able to handle situation where they use each other as their primary routes. The issue of router connections are very important especially with SILC broadcast packets. Usually all router wide information in the network is distributed by SILC broadcast packets. This sort of ring network, with ability to have other direct routes in the network cause interesting routing problems. The [SILC2] discusses the routing of packets in this sort of network in more detail. 3. SILC Specification This section describes the SILC protocol. However, [SILC2] and [SILC3] describes other important protocols that are part of this SILC specification and must be read. 3.1 Client A client is a piece of software connecting to SILC server. SILC client cannot be SILC server. Purpose of clients is to provide the user interface of the SILC services for end user. Clients are distinguished from other clients by unique Client ID. Client ID is a 128 bit ID that is used in the communication in the SILC network. The client ID is based on the nickname selected by the user. User uses logical nicknames in communication which are then mapped to the corresponding Client ID. Client ID's are low level identifications and must not be seen by the Riikonen [Page 8]
Internet Draft 13 November 2001 end user. Clients provide other information about the end user as well. Information such as the nickname of the user, username and the host name of the end user and user's real name. See section 3.2 Server for information of the requirements of keeping this information. The nickname selected by the user is not unique in the SILC network. There can be 2^8 same nicknames for one IP address. As for comparison to IRC [IRC] where nicknames are unique this is a fundamental difference between SILC and IRC. This causes the server names or client's host names to be used along with the nicknames to identify specific users when sending messages. This feature of SILC makes IRC style nickname-wars obsolete as no one owns their nickname; there can always be someone else with the same nickname. The maximum length of nickname is 128 characters. 3.1.1 Client ID Client ID is used to identify users in the SILC network. The Client ID is unique to the extent that there can be 2^128 different Client ID's, and ID's based on IPv6 addresses extends this to 2^224 different Client ID's. Collisions are not expected to happen. The Client ID is defined as follows. 128 bit Client ID based on IPv4 addresses: 32 bit Server ID IP address (bits 1-32) 8 bit Random number or counter 88 bit Truncated MD5 hash value of the nickname 224 bit Client ID based on IPv6 addresses: 128 bit Server ID IP address (bits 1-128) 8 bit Random number or counter 88 bit Truncated MD5 hash value of the nickname o Server ID IP address - Indicates the server where this client is coming from. The IP address hence equals the server IP address where to the client has connected. o Random number or counter - Random number to further randomize the Client ID. Another choice is to use a counter starting from the zero (0). This makes it possible to have 2^8 same nicknames from the same server IP address. Riikonen [Page 9]
Internet Draft 13 November 2001 o MD5 hash - MD5 hash value of the nickname is truncated taking 88 bits from the start of the hash value. This hash value is used to search the user's Client ID from the ID lists. Collisions could occur when more than 2^8 clients using same nickname from the same server IP address is connected to the SILC network. Server MUST be able to handle this situation by refusing to accept anymore of that nickname. Another possible collision may happen with the truncated hash value of the nickname. It could be possible to have same truncated hash value for two different nicknames. However, this is not expected to happen nor cause any problems if it would occur. Nicknames are usually logical and it is unlikely to have two distinct logical nicknames produce same truncated hash value. 3.2 Server Servers are the most important parts of the SILC network. They form the basis of the SILC, providing a point to which clients may connect to. There are two kinds of servers in SILC; normal servers and router servers. This section focus on the normal server and router server is described in the section 3.3 Router. Normal servers MUST NOT directly connect to other normal server. Normal servers may only directly connect to router server. If the message sent by the client is destined outside the local server it is always sent to the router server for further routing. Server may only have one active connection to router on same port. Normal server MUST NOT connect to other cell's router except in situations where its cell's router is unavailable. 3.2.1 Server's Local ID List Normal server keeps various information about the clients and their end users connected to it. Every normal server MUST keep list of all locally connected clients, Client ID's, nicknames, usernames and host names and user's real name. Normal servers only keeps local information and it does not keep any global information. Hence, normal servers knows only about their locally connected clients. This makes servers efficient as they don't have to worry about global clients. Server is also responsible of creating the Client ID's for their clients. Normal server also keeps information about locally created channels and their Channel ID's. Riikonen [Page 10]
Internet Draft 13 November 2001 Hence, local list for normal server includes: server list - Router connection o Server name o Server IP address o Server ID o Sending key o Receiving key o Public key client list - All clients in server o Nickname o Username@host o Real name o Client ID o Sending key o Receiving key o Public key channel list - All channels in server o Channel name o Channel ID o Client ID's on channel o Client ID modes on channel o Channel key 3.2.2 Server ID Servers are distinguished from other servers by unique 64 bit Server ID (for IPv4) or 160 bit Server ID (for IPv6). The Server ID is used in the SILC to route messages to correct servers. Server ID's also provide information for Client ID's, see section 3.1.1 Client ID. Server ID is defined as follows. 64 bit Server ID based on IPv4 addresses: 32 bit IP address of the server 16 bit Port 16 bit Random number 160 bit Server ID based on IPv6 addresses: 128 bit IP address of the server 16 bit Port 16 bit Random number Riikonen [Page 11]
Internet Draft 13 November 2001 o IP address of the server - This is the real IP address of the server. o Port - This is the port the server is bound to. o Random number - This is used to further randomize the Server ID. Collisions are not expected to happen in any conditions. The Server ID is always created by the server itself and server is responsible of distributing it to the router. 3.2.3 SILC Server Ports The following ports has been assigned by IANA for the SILC protocol: silc 706/tcp SILC silc 706/udp SILC If there are needs to create new SILC networks in the future the port numbers must be officially assigned by the IANA. Server on network above privileged ports (>1023) SHOULD NOT be trusted as they could have been set up by untrusted party. 3.3 Router Router server in SILC network is responsible for keeping the cell together and routing messages to other servers and to other routers. Router server is also a normal server thus clients may connect to it as it would be just normal SILC server. However, router servers has a lot of important tasks that normal servers do not have. Router server knows everything about everything in the SILC. They know all clients currently on SILC, all servers and routers and all channels in SILC. Routers are the only servers in SILC that care about global information and keeping them up to date at all time. And, this is what they must do. 3.3.1 Router's Local ID List Router server as well MUST keep local list of connected clients and locally created channels. However, this list is extended to include all the informations of the entire cell, not just the server itself as for normal servers. Riikonen [Page 12]
Internet Draft 13 November 2001 However, on router this list is a lot smaller since routers do not need to keep information about user's nickname, username and host name and real name since these are not needed by the router. The router keeps only information that it needs. Hence, local list for router includes: server list - All servers in the cell o Server name o Server ID o Router's Server ID o Sending key o Receiving key client list - All clients in the cell o Client ID channel list - All channels in the cell o Channel ID o Client ID's on channel o Client ID modes on channel o Channel key Note that locally connected clients and other information include all the same information as defined in section section 3.2.1 Server's Local ID List. 3.3.2 Router's Global ID List Router server MUST also keep global list. Normal servers do not have global list as they know only about local information. Global list includes all the clients on SILC, their Client ID's, all created channels and their Channel ID's and all servers and routers on SILC and their Server ID's. That is said, global list is for global information and the list must not include the local information already on the router's local list. Note that the global list does not include information like nicknames, usernames and host names or user's real names. Router does not need to keep these informations as they are not needed by the router. This information is available from the client's server which maybe queried when needed. Hence, global list includes: Riikonen [Page 13]
Internet Draft 13 November 2001 server list - All servers in SILC o Server name o Server ID o Router's Server ID client list - All clients in SILC o Client ID channel list - All channels in SILC o Channel ID o Client ID's on channel o Client ID modes on channel 3.3.3 Router's Server ID Router's Server ID's are equivalent to normal Server ID's. As routers are normal servers as well same types of ID's applies for routers as well. Thus, see section 3.2.2 Server ID. 3.4 Channels A channel is a named group of one or more clients which will all receive messages addressed to that channel. The channel is created when first client requests JOIN command to the channel, and the channel ceases to exist when the last client has left it. When channel exists, any client can reference it using the name of the channel. Channel names are unique although the real uniqueness comes from 64 bit Channel ID. However, channel names are still unique and no two global channels with same name may exist. The Channel name is a string of maximum length of 256 characters. Channel names MUST NOT contain any spaces (` '), any non-printable ASCII characters, commas (`,') and wildcard characters. Channels can have operators that can administrate the channel and operate all of its modes. The following operators on channel exist on the SILC network. o Channel founder - When channel is created the joining client becomes channel founder. Channel founder is channel operator with some more privileges. Basically, channel founder can fully operate the channel and all of its modes. The privileges are limited only to the particular channel. There can be only one channel founder per channel. Channel founder supersedes channel operator's privileges. Riikonen [Page 14]
Internet Draft 13 November 2001 Channel founder privileges cannot be removed by any other operator on channel. When channel founder leaves the channel there is no channel founder on the channel. However, it is possible to set a mode for the channel which allows the original channel founder to regain the founder privileges even after leaving the channel. Channel founder also cannot be removed by force from the channel. o Channel operator - When client joins to channel that has not existed previously it will become automatically channel operator (and channel founder discussed above). Channel operator is able administrate the channel, set some modes on channel, remove a badly behaving client from the channel and promote other clients to become channel operator. The privileges are limited only to the particular channel. Normal channel user may be promoted (opped) to channel operator gaining channel operator privileges. Channel founder or other channel operator may also demote (deop) channel operator to normal channel user. 3.4.1 Channel ID Channels are distinguished from other channels by unique Channel ID. The Channel ID is a 64 bit ID (for IPv4) or 160 bit ID (for IPv6), and collisions are not expected to happen in any conditions. Channel names are just for logical use of channels. The Channel ID is created by the server where the channel is created. The Channel ID is defined as follows. 64 bit Channel ID based on IPv4 addresses: 32 bit Router's Server ID IP address (bits 1-32) 16 bit Router's Server ID port (bits 33-48) 16 bit Random number 160 bit Channel ID based on IPv6 addresses: 128 bit Router's Server ID IP address (bits 1-128) 16 bit Router's Server ID port (bits 129-144) 16 bit Random number o Router's Server ID IP address - Indicates the IP address of the router of the cell where this channel is created. This is taken from the router's Server ID. This way SILC router knows where this channel resides in the SILC network. o Router's Server ID port - Indicates the port of the channel on the server. This is taken from the router's Server ID. Riikonen [Page 15]
Internet Draft 13 November 2001 o Random number - To further randomize the Channel ID. This makes sure that there are no collisions. This also means that in a cell there can be 2^16 channels. 3.5 Operators Operators are normal users with extra privileges to their server or router. Usually these people are SILC server and router administrators that take care of their own server and clients on them. The purpose of operators is to administrate the SILC server or router. However, even an operator with highest privileges is not able to enter invite-only channel, to gain access to the contents of a encrypted and authenticated packets traveling in the SILC network or to gain channel operator privileges on public channels without being promoted. They have the same privileges as everyone else except they are able to administrate their server or router. 3.6 SILC Commands Commands are very important part on SILC network especially for client which uses commands to operate on the SILC network. Commands are used to set nickname, join to channel, change modes and many other things. Client usually sends the commands and server replies by sending a reply packet to the command. Server MAY also send commands usually to serve the original client's request. However, server MUST NOT send commands to client and there are some commands that server must not send. Note that the command reply is usually sent only after client has sent the command request but server is allowed to send command reply packet to client even if client has not requested the command. Client MAY, choose to ignore the command reply. It is expected that some of the commands may be miss-used by clients resulting various problems on the server side. Every implementation SHOULD assure that commands may not be executed more than once, say, in two (2) seconds. However, to keep response rate up, allowing for example five (5) commands before limiting is allowed. It is RECOMMENDED that commands such as SILC_COMMAND_NICK, SILC_COMMAND_JOIN, SILC_COMMAND_LEAVE and SILC_COMMAND_KILL SHOULD be limited in all cases as they require heavy operations. This should be sufficient to prevent the miss-use of commands. SILC commands are described in [SILC4]. Riikonen [Page 16]
Internet Draft 13 November 2001 3.7 SILC Packets Packets are naturally the most important part of the protocol and the packets are what actually makes the protocol. Packets in SILC network are always encrypted using, usually the shared secret session key or some other key, for example, channel key, when encrypting channel messages. It is not possible to send packet in SILC network without encryption. The SILC Packet Protocol is a wide protocol and is described in [SILC2]. This document does not define or describe details of SILC packets. 3.8 Packet Encryption All packets passed in SILC network MUST be encrypted. This section defines how packets must be encrypted in the SILC network. The detailed description of the actual encryption process of the packets are described in [SILC2]. Client and its server shares secret symmetric session key which is established by the SILC Key Exchange Protocol, described in [SILC3]. Every packet sent from client to server, with exception of packets for channels, are encrypted with this session key. Channels has their own key that are shared by every client on the channel. However, the channel keys are cell specific thus one cell does not know the channel key of the other cell, even if that key is for same channel. Channel key is also known by the routers and all servers that has clients on the channel. However, channels MAY have channel private keys that are entirely local setting for the client. All clients on the channel MUST know the channel private key before hand to be able to talk on the channel. In this case, no server or router know the key for channel. Server shares secret symmetric session key with router which is established by the SILC Key Exchange Protocol. Every packet passed from server to router, with exception of packets for channels, are encrypted with the shared session key. Same way, router server shares secret symmetric key with its primary route. However, every packet passed from router to other router, including packets for channels, are encrypted with the shared session key. Every router connection has their own session keys. 3.8.1 Determination of the Source and the Destination The source and the destination of the packet needs to be determined to be able to route the packets to correct receiver. This information is available in the SILC Packet Header which is included in all packets Riikonen [Page 17]
Internet Draft 13 November 2001 sent in SILC network. The SILC Packet Header is described in [SILC2]. The header MUST be encrypted with the session key who is next receiver of the packet along the route. The receiver of the packet, for example a router along the route, is able to determine the sender and the destination of the packet by decrypting the SILC Packet Header and checking the ID's attached to the header. The ID's in the header will tell to where the packet needs to be sent and where it is coming from. The header in the packet MUST NOT change during the routing of the packet. The original sender, for example client, assembles the packet and the packet header and server or router between the sender and the receiver MUST NOT change the packet header. Note that the packet and the packet header may be encrypted with different keys. For example, packets to channels are encrypted with the channel key, however, the header is encrypted with the session key as described above. However, the header and the packet may be encrypted with same key. This is the case, for example, with command packets. 3.8.2 Client To Client The process of message delivery and encryption from client to another client is as follows. Example: Private message from client to another client on different servers. Clients do not share private message delivery keys; normal session keys are used. o Client 1. sends encrypted packet to its server. The packet is encrypted with the session key shared between client and its server. o Server determines the destination of the packet and decrypts the packet. Server encrypts the packet with session key shared between the server and its router, and sends the packet to the router. o Router determines the destination of the packet and decrypts the packet. Router encrypts the packet with session key shared between the router and the destination server, and sends the packet to the server. o Server determines the client to which the packet is destined to and decrypts the packet. Server encrypts the packet with session key shared between the server and the destination client, and sends the packet to the client. Riikonen [Page 18]
Internet Draft 13 November 2001 o Client 2. decrypts the packet. Example: Private message from client to another client on different servers. Clients has established secret shared private message delivery key with each other and that is used in the message encryption. o Client 1. sends encrypted packet to its server. The packet is encrypted with the private message delivery key shared between clients. o Server determines the destination of the packet and sends the packet to the router. o Router determines the destination of the packet and sends the packet to the server. o Server determines the client to which the packet is destined to and sends the packet to the client. o Client 2. decrypts the packet with the secret shared key. If clients share secret key with each other the private message delivery is much simpler since servers and routers between the clients do not need to decrypt and re-encrypt the packet. The process for clients on same server is much simpler as there are no need to send the packet to the router. The process for clients on different cells is same as above except that the packet is routed outside the cell. The router of the destination cell routes the packet to the destination same way as described above. 3.8.3 Client To Channel Process of message delivery from client on channel to all the clients on the channel. Example: Channel of four users; two on same server, other two on different cells. Client sends message to the channel. o Client 1. encrypts the packet with channel key and sends the packet to its server. o Server determines local clients on the channel and sends the packet to the Client on the same server. Server then sends Riikonen [Page 19]
Internet Draft 13 November 2001 the packet to its router for further routing. o Router determines local clients on the channel, if found sends packet to the local clients. Router determines global clients on the channel and sends the packet to its primary router or fastest route. o (Other router(s) do the same thing and sends the packet to the server(s)) o Server determines local clients on the channel and sends the packet to the client. o All clients receiving the packet decrypts the packet. 3.8.4 Server To Server Server to server packet delivery and encryption is described in above examples. Router to router packet delivery is analogous to server to server. However, some packets, such as channel packets, are processed differently. These cases are described later in this document and more in detail in [SILC2]. 3.9 Key Exchange And Authentication Key exchange is done always when for example client connects to server but also when server and router, and router and router connects to each other. The purpose of key exchange protocol is to provide secure key material to be used in the communication. The key material is used to derive various security parameters used to secure SILC packets. The SILC Key Exchange protocol is described in detail in [SILC3]. Authentication is done after key exchange protocol has been successfully completed. The purpose of authentication is to authenticate for example client connecting to the server. However, usually clients are accepted to connect to server without explicit authentication. Servers are required use authentication protocol when connecting. The authentication may be based on passphrase (pre-shared-secret) or public key. The connection authentication protocol is described in detail in [SILC3]. 3.9.1 Authentication Payload Authentication payload is used separately from the SKE and the Connection Authentication protocol. It is used during the session to authenticate with the remote. For example, the client can authenticate itself to the Riikonen [Page 20]
Internet Draft 13 November 2001 server to become server operator. In this case, Authentication Payload is used. The format of the Authentication Payload is as follows: 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Payload Length | Authentication Method | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Data Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Public Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication Data Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Authentication Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Authentication Payload o Payload Length (2 bytes) - Length of the entire payload. o Authentication Method (2) - The method of the authentication. The authentication methods are defined in [SILC2] in the Connection Auth Request Payload. The NONE authentication method SHOULD NOT be used. o Public Data Length (2 bytes) - Indicates the length of the Public Data field. o Public Data (variable length) - This is defined only if the authentication method is public key. If it is any other this field MAY include a random data for padding purposes. However, in this case the field MUST be ignored by the receiver. When the authentication method is public key this includes 128 to 4096 bytes of non-zero random data that is used in the signature process, described subsequently. o Authentication Data Length (2 bytes) - Indicates the Riikonen [Page 21]
Internet Draft 13 November 2001 length of the Authentication Data field. o Authentication Data (variable length) - Authentication method dependent authentication data. If the authentication method is password based, the Authentication Data field includes the plaintext password. It is safe to send plaintext password since the entire payload is encrypted. In this case the Public Data Length is set to zero (0), but MAY also include random data for padding purposes. It is also RECOMMENDED that maximum amount of padding is applied to SILC packet when using password based authentication. This way it is not possible to approximate the length of the password from the encrypted packet. If the authentication method is public key based (or certificate) the Authentication Data is computed as follows: HASH = hash(random bytes | ID | public key (or certificate)); Authentication Data = sign(HASH); The hash() and the sign() are the hash function and the public key cryptography function selected in the SKE protocol. The public key is SILC style public key unless certificates are used. The ID is the entity's ID (Client or Server ID) which is authenticating itself. The ID is raw ID data. The random bytes are non-zero random bytes of length between 128 and 4096 bytes, and will be included into the Public Data field as is. The receiver will compute the signature using the random data received in the payload, the ID associated to the connection and the public key (or certificate) received in the SKE protocol. After computing the receiver MUST verify the signature. In this case also, the entire payload is encrypted. 3.10 Algorithms This section defines all the allowed algorithms that can be used in the SILC protocol. This includes mandatory cipher, mandatory public key algorithm and MAC algorithms. 3.10.1 Ciphers Cipher is the encryption algorithm that is used to protect the data in the SILC packets. See [SILC2] of the actual encryption process and definition of how it must be done. SILC has a mandatory algorithm that Riikonen [Page 22]
Internet Draft 13 November 2001 must be supported in order to be compliant with this protocol. The following ciphers are defined in SILC protocol: aes-256-cbc AES in CBC mode, 256 bit key (REQUIRED) aes-192-cbc AES in CBC mode, 192 bit key (OPTIONAL) aes-128-cbc AES in CBC mode, 128 bit key (OPTIONAL) twofish-256-cbc Twofish in CBC mode, 256 bit key (OPTIONAL) twofish-192-cbc Twofish in CBC mode, 192 bit key (OPTIONAL) twofish-128-cbc Twofish in CBC mode, 128 bit key (OPTIONAL) blowfish-128-cbc Blowfish in CBC mode, 128 bit key (OPTIONAL) cast-256-cbc CAST-256 in CBC mode, 256 bit key (OPTIONAL) cast-192-cbc CAST-256 in CBC mode, 192 bit key (OPTIONAL) cast-128-cbc CAST-256 in CBC mode, 128 bit key (OPTIONAL) rc6-256-cbc RC6 in CBC mode, 256 bit key (OPTIONAL) rc6-192-cbc RC6 in CBC mode, 192 bit key (OPTIONAL) rc6-128-cbc RC6 in CBC mode, 128 bit key (OPTIONAL) mars-256-cbc Mars in CBC mode, 256 bit key (OPTIONAL) mars-192-cbc Mars in CBC mode, 192 bit key (OPTIONAL) mars-128-cbc Mars in CBC mode, 128 bit key (OPTIONAL) none No encryption (OPTIONAL) Algorithm none does not perform any encryption process at all and thus is not recommended to be used. It is recommended that no client or server implementation would accept none algorithms except in special debugging mode. Additional ciphers MAY be defined to be used in SILC by using the same name format as above. 3.10.2 Public Key Algorithms Public keys are used in SILC to authenticate entities in SILC network and to perform other tasks related to public key cryptography. The public keys are also used in the SILC Key Exchange protocol [SILC3]. The following public key algorithms are defined in SILC protocol: rsa RSA (REQUIRED) dss DSS (OPTIONAL) DSS is described in [Menezes]. The RSA MUST be implemented according PKCS #1 [PKCS1]. The mandatory PKCS #1 implementation in SILC MUST be compliant to either PKCS #1 version 1.5 or newer with the following notes: The signature encoding is always in same format as the encryption encoding regardless of the PKCS #1 version. The signature with appendix Riikonen [Page 23]
Internet Draft 13 November 2001 (with hash algorithm OID in the data) MUST NOT be used in the SILC. The rationale for this is that there is no binding between the PKCS #1 OIDs and the hash algorithms used in the SILC protocol. Hence, the encoding is always in PKCS #1 version 1.5 format. Additional public key algorithms MAY be defined to be used in SILC. 3.10.3 Hash Functions Hash functions are used as part of MAC algorithms defined in the next section. They are also used in the SILC Key Exchange protocol defined in the [SILC3]. The following Hash algorithm are defined in SILC protocol: sha1 SHA-1, length = 20 (REQUIRED) md5 MD5, length = 16 (OPTIONAL) 3.10.4 MAC Algorithms Data integrity is protected by computing a message authentication code (MAC) of the packet data. See [SILC2] for details how to compute the MAC. The following MAC algorithms are defined in SILC protocol: hmac-sha1-96 HMAC-SHA1, length = 12 (REQUIRED) hmac-md5-96 HMAC-MD5, length = 12 (OPTIONAL) hmac-sha1 HMAC-SHA1, length = 20 (OPTIONAL) hmac-md5 HMAC-MD5, length = 16 (OPTIONAL) none No MAC (OPTIONAL) The none MAC is not recommended to be used as the packet is not authenticated when MAC is not computed. It is recommended that no client or server would accept none MAC except in special debugging mode. The HMAC algorithm is described in [HMAC] and hash algorithms that are used as part of the HMACs are described in [Scheneir] and in [Menezes] Additional MAC algorithms MAY be defined to be used in SILC. Riikonen [Page 24]
Internet Draft 13 November 2001 3.10.5 Compression Algorithms SILC protocol supports compression that may be applied to unencrypted data. It is recommended to use compression on slow links as it may significantly speed up the data transmission. By default, SILC does not use compression which is the mode that must be supported by all SILC implementations. The following compression algorithms are defined: none No compression (REQUIRED) zlib GNU ZLIB (LZ77) compression (OPTIONAL) Additional compression algorithms MAY be defined to be used in SILC. 3.11 SILC Public Key This section defines the type and format of the SILC public key. All implementations MUST support this public key type. See [SILC3] for other optional public key and certificate types allowed in the SILC protocol. Public keys in SILC may be used to authenticate entities and to perform other tasks related to public key cryptography. The format of the SILC Public Key is as follows: 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Public Key Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm Name Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Algorithm Name ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier Length | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ Identifier ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ Public Data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Riikonen [Page 25]
Internet Draft 13 November 2001 Figure 5: SILC Public Key o Public Key Length (4 bytes) - Indicates the full length of the public key, not including this field. o Algorithm Name Length (2 bytes) - Indicates the length of the Algorithm Length field, not including this field. o Algorithm name (variable length) - Indicates the name of the public key algorithm that the key is. See the section 3.10.2 Public Key Algorithms for defined names. o Identifier Length (2 bytes) - Indicates the length of the Identifier field, not including this field. o Identifier (variable length) - Indicates the identifier of the public key. This data can be used to identify the owner of the key. The identifier is of the following format: UN User name HN Host name or IP address RN Real name E EMail address O Organization C Country Examples of an identifier: `UN=priikone, HN=poseidon.pspt.fi, E=priikone@poseidon.pspt.fi' `UN=sam, HN=dummy.fi, RN=Sammy Sam, O=Company XYZ, C=Finland' At least user name (UN) and host name (HN) MUST be provided as identifier. The fields are separated by commas (`,'). If comma is in the identifier string it must be written as `\,', for example, `O=Company XYZ\, Inc.'. o Public Data (variable length) - Includes the actual public data of the public key. The format of this field for RSA algorithm is as follows: 4 bytes Length of e variable length e Riikonen [Page 26]
Internet Draft 13 November 2001 4 bytes Length of n variable length n The format of this field for DSS algorithm is as follows: 4 bytes Length of p variable length p 4 bytes Length of q variable length q 4 bytes Length of g variable length g 4 bytes Length of y variable length y The variable length fields are multiple precession integers encoded as strings in both examples. Other algorithms must define their own type of this field if they are used. All fields in the public key are in MSB (most significant byte first) order. 3.12 SILC Version Detection The version detection of both client and server is performed at the connection phase while executing the SILC Key Exchange protocol. The version identifier is exchanged between initiator and responder. The version identifier is of the following format: SILC-<protocol version>-<software version> The version strings are of the following format: protocol version = <major>.<minor> software version = <major>[.<minor>[.<build>]] Protocol version MAY provide both major and minor version. Currently implementations MUST set the protocol version and accept the protocol version as SILC-1.0-<software version>. If new protocol version causes in compatibilities with older version the the <minor> versio number MUST be incremented. The <major> is incremented if new protocol version is fully incompatible. Software version MAY provide major, minor and build version. The Riikonen [Page 27]
Internet Draft 13 November 2001 software version MAY be freely set and accepted. Thus, the version string could be, for example: SILC-1.0-1.2 3.13 Backup Routers Backup routers may exist in the cell in addition of the primary router. However, they must not be active routers and act as routers in the cell. Only one router may be acting as primary router in the cell. In the case of failure of the primary router may one of the backup routers become active. The purpose of backup routers are in case of failure of the primary router to maintain working connections inside the cell and outside the cell and to avoid netsplits. Backup routers are normal servers in the cell that are prepared to take over the tasks of the primary router if needed. They need to have at least one direct and active connection to the primary router of the cell. This communication channel is used to send the router information to the backup router. When the backup router connects to the primary router of the cell it MUST present itself as router server in the Connection Authentication protocol, even though it is normal server as long as the primary router is available. Reason for this is that the configuration needed in the responder end requires usually router connection level configuration. The responder, however must understand and treat the connection as normal server (except when feeding router level data to the backup router). Backup router must know everything that the primary router knows to be able to take over the tasks of the primary router. It is the primary router's responsibility to feed the data to the backup router. If the backup router does not know all the data in the case of failure some connections may be lost. The primary router of the cell must consider the backup router being actual router server when it feeds the data to it. In addition of having direct connection to the primary router of the cell, the backup router must also have connection to the same router the primary router of the cell is connected. However, it must not be active router connection meaning that the backup router must not use that channel as its primary route and it must not notify the router about having connected servers, channels and clients behind it. It merely connects to the router. This sort of connection is later referred as being passive connection. Some keepalive actions may be needed by the router to keep the connection alive. Riikonen [Page 28]
Internet Draft 13 November 2001 It is required that other normal servers have passive connections to the backup router(s) in the cell. Some keepalive actions may be needed by the server to keep the connection alive. After they notice the failure of the primary router they must start using the connection to the first backup router as their primary route. Also, if any other router in the network is using the cell's primary router as its own primary router, it must also have passive connection to the cell's backup router. It too is prepared to switch to use the backup router as its new primary router as soon as the orignal primary router becomes unresponsive. All of the parties of this protocol knows which one is the backup router of the cell from their local configuration. Each of the entity must be configured accordingly and care must be taken when configuring the backup routers, servers and other routers in the network. It must be noted that some of the channel messages and private messages may be lost during the switch to the backup router. The announcements assures that the state of the network is not lost during the switch. It is RECOMMENDED that there would be at least one backup router in the cell. It is NOT RECOMMENDED to have all servers in the cell acting as backup routers as it requires establishing several connections to several servers in the cell. Large cells can easily have several backup routers in the cell. The order of the backup routers are decided at the configuration phase. All the parties of this protocol must be configured accordingly to understand the order of the backup routers. It is not required that the backup server is actually active server in the cell. Backup router may be a spare server in the cell that does not accept normal client connections at all. It may be reserved purely for the backup purposes. These, however, are cell management issues. If also the first backup router is down as well and there is another backup router in the cell then it will start acting as the primary router as described above. 3.13.1 Switching to Backup Router When the primary router of the cell becomes unresponsive, for example by sending EOF to the connection, all the parties of this protocol MUST replace the old connection to the primary router with first configured backup router. The backup router usually needs to do local modifications to its database in order to update all the information needed to maintain working routes. The backup router must understand that clients that Riikonen [Page 29]
Internet Draft 13 November 2001 were orignated from the primary router are now originated from some of the existing server connections and must update them accordingly. It must also remove those clients that were owned by the primary router since those connections were lost when the primary router became unresponsive. All the other parties of the protocol must also update their local database to understand that the route to the primary router will now go to the backup router. The servers connected to the backup router must announce their clients, channels, channel users, channel user modes and channel modes to the backup router. This is to assure that none of the important notify packets were lost during the switch to the backup router. The backup router must check which of these announced entities it already have and distribute the new ones to the primary route. The backup router too must announce its servers, clients, channels and other information to the new primary router. The primary router of the backup router too must announce its informations to the backup router. Both must process only the ones they do not know about. If any of the announced modes does not match then they are enforced in normal manner defined later in this specification. 3.13.2 Resuming Primary Router Usually the primary router is unresponsive only a short period of time and it is intended that the original router of the cell will reassume its position as primary router when it comes back online. The backup router that is now acting as primary router of the cell must constantly try to connect to the original primary router of the cell. It is RECOMMENDED that it would try to reconnect in 30 second intervals to the primary router. When the connection is established to the primary router the backup resuming protocol is executed. The protocol is advanced as follows: 1. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type value 1 the primary router that came back online. The packet will indicate the primary router has been replaced by the backup router. After sending the packet the backup router will announce all of its channels, channel users, modes etc. to the primary router. 2. Backup router sends SILC_PACKET_RESUME_ROUTER packet with type value 2 to its current primary router to indicate that it will resign as being primary router. Then, backup router sends the Riikonen [Page 30]
Internet Draft 13 November 2001 SILC_PACKET_RESUME_ROUTER packet with type value 1 to all connected servers to also indicate that it will resign as being primary router. 3. Backup router also send SILC_PACKET_RESUME_ROUTER packet with type value 2 to the router that is using the backup router currently as its primary router. 4. Any server and router that receives the SILC_PACKET_RESUME_ROUTER with type value 1 or 2 must reconnect immediately to the primary router of the cell that came back online. After they have created the connection they MUST NOT use that connection as active primary route but still route all packets to the backup router. After the connection is created they MUST send SILC_PACKET_RESUME_ROUTER with type value 3 back to the backup router. The session ID value found in the first packet MUST be set in this packet. 5. Backup router MUST wait for all packets with type value 3 before it continues with the protocol. It knows from the session ID values set in the packet when it have received all packets. The session value should be different in all packets it have send earlier. After the packets is received the backup router sends the SILC_PACKET_RESUME_ROUTER packet with type value 4 to the primary router that came back online. This packet will indicate that the backup router is now ready to resign as being primary router. The session ID value in this packet MUST be the same as in first packet sent to the primary router. During this time the backup router should still route all packets it is receiving from server connections. 6. The primary router receives the packet and send the SILC_PACKET_RESUME_ROUTER with type value 5 to all connected servers including the backup router. It also sends the packet with type value 6 to its primary router, and to the router that is using it as its primary router. The Session ID value in this packet SHOULD be zero (0). 7. Any server and router that receives the SILC_PACKET_RESUME_ROUTER with type value 5 or 6 must switch their primary route to the new primary router and remove the route for the backup router, since it is not anymore the primary router of the cell. They must also update their local database to understand that the clients are not originated from the backup router but from the locally connected servers. After that they MUST announce their channels, channel users, modes etc. to the primary router. They must not use the backup router connection after this and the connection is considered to be passive connection. The implementations SHOULD be able Riikonen [Page 31]
Internet Draft 13 November 2001 to disable the connection without closing the actual link. After this protocol is executed the backup router is now again normal server in the cell that has the backup link to the primary router. The primary router feeds the router specific data again to the backup router. All server connections in the backup router are considered passive connections. When the primary router of the cell comes back online and connects to its primary router, the remote primary router must send the SILC_PACKET_RESUME_ROUTER with type value 20 indicating that the connection is not allowed since the router has been replaced by an backup router. The session ID value in this packet SHOULD be zero (0). When the router receives this packet it must not use the connection as active connection but to understand that it cannot act as primary router in the cell. It must wait that the backup router connects to it, and the backup resuming protocol is executed. The following type values has been defined for SILC_PACKET_RESUME_ROUTER packet: 1 SILC_SERVER_BACKUP_START 2 SILC_SERVER_BACKUP_START_GLOBAL 3 SILC_SERVER_BACKUP_START_CONNECTED 4 SILC_SERVER_BACKUP_START_ENDING 5 SILC_SERVER_BACKUP_START_RESUMED 6 SILC_SERVER_BACKUP_START_GLOBAL 20 SILC_SERVER_BACKUP_START_REPLACED If any other value is found in the type field the packet must be discarded. The SILC_PACKET_RESUME_ROUTER packet and its payload is defined in [SILC2]. 3.13.3 Discussion on Backup Router Scheme It is clear that this backup router support is not able to handle all possible situations arrising in unreliable network environment. This scheme for example does not handle situation when the router actually does not go offline but the network link goes down temporarily. It would require some intelligence to figure out when it is best time to switch to the backup router. To make it even more complicated it is possible that the backup router may have not lost the network link to the primary router. Other possible situation is when the network link is lost temporarily between two primary routers in the SILC network. Unless the routers notice the link going down they cannot perhaps find alternative routes. Riikonen [Page 32]
Internet Draft 13 November 2001 Worst situation is when the link goes down only for a short period of time, thus causing lag. Should the routers or servers find alternative routes if they cannot get response from the router during the lag? When alternative routes are being found it must be careful not to mess up existing primary routes between routers in the network. It is suggested that the current backup router scheme is only temporary solution and existing backup router protocols are studied further. It is also suggested that the backup router specification will be separated from this SILC specification Internet-Draft and additional specification is written on the subject. 4 SILC Procedures This section describes various SILC procedures such as how the connections are created and registered, how channels are created and so on. The section describes the procedures only generally as details are described in [SILC2] and [SILC3]. 4.1 Creating Client Connection This section describes the procedure when client connects to SILC server. When client connects to server the server MUST perform IP address lookup and reverse IP address lookup to assure that the origin host really is who it claims to be. Client, host, connecting to server SHOULD have both valid IP address and fully qualified domain name (FQDN). After that the client and server performs SILC Key Exchange protocol which will provide the key material used later in the communication. The key exchange protocol MUST be completed successfully before the connection registration may continue. The SILC Key Exchange protocol is described in [SILC3]. Typical server implementation would keep a list of connections that it allows to connect to the server. The implementation would check, for example, the connecting client's IP address from the connection list before the SILC Key Exchange protocol has been started. Reason for this is that if the host is not allowed to connect to the server there is no reason to perform the key exchange protocol. After successful key exchange protocol the client and server performs connection authentication protocol. The purpose of the protocol is to authenticate the client connecting to the server. Flexible implementation could also accept the client to connect to the server without explicit authentication. However, if authentication is desired for a specific client it may be based on passphrase or Riikonen [Page 33]
Internet Draft 13 November 2001 public key authentication. If authentication fails the connection MUST be terminated. The connection authentication protocol is described in [SILC3]. After successful key exchange and authentication protocol the client registers itself by sending SILC_PACKET_NEW_CLIENT packet to the server. This packet includes various information about the client that the server uses to create the client. Server creates the client and sends SILC_PACKET_NEW_ID to the client which includes the created Client ID that the client MUST start using after that. After that all SILC packets from the client MUST have the Client ID as the Source ID in the SILC Packet Header, described in [SILC2]. Client MUST also get the server's Server ID that is to be used as Destination ID in the SILC Packet Header when communicating with the server (for example when sending commands to the server). The ID may be resolved in two ways. Client can take the ID from an previously received packet from server that MUST include the ID, or to send SILC_COMMAND_INFO command and receive the Server ID as command reply. Server MAY choose not to use the information received in the SILC_PACKET_NEW_CLIENT packet. For example, if public key or certificate were used in the authentication, server MAY use those informations rather than what it received from client. This is suitable way to get the true information about client if it is available. The nickname of client is initially set to the username sent in the SILC_PACKET_NEW_CLIENT packet. User should set the nickname to more suitable by sending SILC_COMMAND_NICK command. However, this is not required as part of registration process. Server MUST also distribute the information about newly registered client to its router (or if the server is router, to all routers in the SILC network). More information about this in [SILC2]. 4.2 Creating Server Connection This section describes the procedure when server connects to its router (or when router connects to other router, the cases are equivalent). The procedure is very much alike when client connects to the server thus it is not repeated here. One difference is that server MUST perform connection authentication protocol with proper authentication. A proper authentication is based on passphrase or public key authentication. Riikonen [Page 34]
Internet Draft 13 November 2001 After server and router has successfully performed the key exchange and connection authentication protocol, the server register itself to the router by sending SILC_PACKET_NEW_SERVER packet. This packet includes the server's Server ID that it has created by itself and other relevant information about the server. After router has received the SILC_PACKET_NEW_SERVER packet it distributes the information about newly registered server to all routers in the SILC network. More information about this in [SILC2]. As client needed to resolve the destination ID this MUST be done by the server that connected to the router, as well. The way to resolve it is to get the ID from previously received packet. The server MAY also use SILC_COMMAND_INFO command to resolve the ID. Server MUST also start using its own Server ID as Source ID in SILC Packet Header and the router's Server ID as Destination when communicating with the router. 4.2.1 Announcing Clients, Channels and Servers After server or router has connected to the remote router, and it already has connected clients and channels it MUST announce them to the router. If the server is router server, also all the local servers in the cell MUST be announced. All clients are announced by compiling a list of ID Payloads into the SILC_PACKET_NEW_ID packet. All channels are announced by compiling a list of Channel Payloads into the SILC_PACKET_NEW_CHANNEL packet. Also, the channel users on the channels must be announced by compiling a list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the SILC_PACKET_NOTIFY packet. The users' modes on the channel must also be announced by compiling list of Notify Payloads with the SILC_NOTIFY_TYPE_CUMODE_CHANGE notify type into the SILC_PACKET_NOTIFY packet. The router MUST also announce the local servers by compiling list of ID Payloads into the SILC_PACKET_NEW_ID packet. Also, clients' modes (user modes in SILC) MUST be announced. This is done by compiling a list of Notify Payloads with the SILC_NOTIFY_UMODE_CHANGE nofity type into the SILC_PACKET_NOTIFY packet. Also, channel's topics MUST be announced by compiling a list of Notify Payloads with the SILC_NOTIFY_TOPIC_SET notify type into the SILC_PACKET_NOTIFY packet. The router which receives these lists MUST process them and broadcast the packets to its primary route. Riikonen [Page 35]
Internet Draft 13 November 2001 When processing the announced channels and channel users the router MUST check whether a channel exists already with the same name. If channel exists with the same name it MUST check whether the Channel ID is different. If the Channel ID is different the router MUST send the notify type SILC_NOTIFY_TYPE_CHANNEL_CHANGE to the server to force the channel ID change to the ID the router has. If the mode of the channel is different the router MUST send the notify type SILC_NOTIFY_TYPE_CMODE_CHANGE to the server to force the mode change to the mode that the router has. The router MUST also generate new channel key and distribute it to the channel. The key MUST NOT be generated if the SILC_CMODE_PRIVKEY mode is set. If the channel has channel founder on the router the router MUST send the notify type SILC_NOTIFY_TYPE_CUMODE_CHANGE to the server to force the mode change for the channel founder on the server. The channel founder privileges MUST be removed. The router processing the channels MUST also compile a list of Notify Payloads with the SILC_NOTIFY_TYPE_JOIN notify type into the SILC_PACKET_NOTIFY and send the packet to the server. This way the server (or router) will receive the clients on the channel that the router has. 4.3 Joining to a Channel This section describes the procedure when client joins to a channel. Client joins to channel by sending command SILC_COMMAND_JOIN to the server. If the receiver receiving join command is normal server the server MUST check its local list whether this channel already exists locally. This would indicate that some client connected to the server has already joined to the channel. If this is case the client is joined to the channel, new channel key is created and information about newly joined channel is sent to the router. The router is informed by sending SILC_NOTIFY_TYPE_JOIN notify type. The notify type MUST also be sent to the local clients on the channel. The new channel key is also sent to the router and to local clients on the channel. If the channel does not exist in the local list the client's command MUST be sent to the router which will then perform the actual joining procedure. When server receives the reply to the command from the router it MUST be sent to the client which sent the command originally. Server will also receive the channel key from the server that it MUST send to the client which originally requested the join command. The server MUST also save the channel key. If the receiver of the join command is router it MUST first check its Riikonen [Page 36]
Internet Draft 13 November 2001 local list whether anyone in the cell has already joined to the channel. If this is the case the client is joined to the channel and reply is sent to the client. If the command was sent by server the command reply is sent to the server which sent it. Then the router MUST also create new channel key and distribute it to all clients on the channel and all servers that has clients on the channel. Router MUST also send the SILC_NOTIFY_TYPE_JOIN notify type to local clients on the channel and to local servers that has clients on the channel. If the channel does not exist on the router's local list it MUST check the global list whether the channel exists at all. If it does the client is joined to the channel as described previously. If the channel does not exist the channel is created and the client is joined to the channel. The channel key is also created and distributed as previously described. The client joining to the created channel is made automatically channel founder and both channel founder and channel operator privileges is set for the client. If the router created the channel in the process, information about the new channel MUST be broadcasted to all routers. This is done by broadcasting SILC_PACKET_NEW_CHANNEL packet to the router's primary route. When the router joins the client to the channel it MUST also send information about newly joined client to all routers in the SILC network. This is done by broadcasting the SILC_NOTIFY_TYPE_JOIN notify type to the router's primary route. It is important to note that new channel key is created always when new client joins to channel, whether the channel has existed previously or not. This way the new client on the channel is not able to decrypt any of the old traffic on the channel. Client which receives the reply to the join command MUST start using the received Channel ID in the channel message communication thereafter. Client also receives the key for the channel in the command reply. Note that the channel key is never generated if the SILC_CMODE_PRIVKEY mode is set. 4.4 Channel Key Generation Channel keys are created by router which creates the channel by taking enough randomness from cryptographically strong random number generator. The key is generated always when channel is created, when new client joins a channel and after the key has expired. Key could expire for example in an hour. The key MUST also be re-generated whenever some client leaves a channel. In this case the key is created from scratch by taking enough randomness from the random number generator. After that the key is distributed to all clients on the channel. However, channel keys are cell specific thus Riikonen [Page 37]
Internet Draft 13 November 2001 the key is created only on the cell where the client, which left the channel, exists. While the server or router is creating the new channel key, no other client may join to the channel. Messages that are sent while creating the new key are still processed with the old key. After server has sent the SILC_PACKET_CHANNEL_KEY packet MUST client start using the new key. If server creates the new key the server MUST also send the new key to its router. See [SILC2] on more information about how channel messages must be encrypted and decrypted when router is processing them. When client receives the SILC_PACKET_CHANNEL_KEY packet with the Channel Key Payload it MUST process the key data to create encryption and decryption key, and to create the HMAC key that is used to compute the MACs of the channel messages. The processing is as follows: channel_key = raw key data HMAC key = hash(raw key data) The raw key data is the key data received in the Channel Key Payload. The hash() function is the hash function used in the HMAC of the channel. Note that the server MUST also save the channel key. 4.5 Private Message Sending and Reception Private messages are sent point to point. Client explicitly destines a private message to specific client that is delivered to only to that client. No other client may receive the private message. The receiver of the private message is destined in the SILC Packet Header as any other packet as well. If the sender of a private message does not know the receiver's Client ID, it MUST resolve it from server. There are two ways to resolve the client ID from server; it is RECOMMENDED that client implementations send SILC_COMMAND_IDENTIFY command to receive the Client ID. Client MAY also send SILC_COMMAND_WHOIS command to receive the Client ID. If the sender has received earlier a private message from the receiver it should have cached the Client ID from the SILC Packet Header. See [SILC2] for description of private message encryption and decryption process. 4.6 Private Message Key Generation Private message MAY be protected by the key generated by the client. The key may be generated and sent to the other client by sending packet SILC_PACKET_PRIVATE_MESSAGE_KEY which travels through the network Riikonen [Page 38]
Internet Draft 13 November 2001 and is secured by session keys. After that the private message key is used in the private message communication between those clients. Other choice is to entirely use keys that are not sent through the SILC network at all. This significantly adds security. This key would be pre-shared-key that is known by both of the clients. Both agree about using the key and starts sending packets that indicate that the private message is secured using private message key. The key material used as private message key is implementation issue. However, SILC_PACKET_KEY_AGREEMENT packet MAY be used to negotiate the key material. If the key is normal pre-shared-key or randomly generated key, and the SILC_PACKET_KEY_AGREEMENT was not used, then the key material SHOULD be processed as defined in the [SILC3]. In the processing, however, the HASH, as defined in [SILC3] MUST be ignored. After processing the key material it is employed as defined in [SILC3], however, the HMAC key material MUST be discarded. If the key is pre-shared-key or randomly generated the implementations should use the SILC protocol's mandatory cipher as the cipher. If the SKE was used to negotiate key material the cipher was negotiated as well. 4.7 Channel Message Sending and Reception Channel messages are delivered to group of users. The group forms a channel and all clients on the channel receives messages sent to the channel. Channel messages are destined to channel by specifying the Channel ID as Destination ID in the SILC Packet Header. The server MUST then distribute the message to all clients on the channel by sending the channel message destined explicitly to a client on the channel. See the [SILC2] for description of channel messege routing for router servers. See [SILC2] for description of channel message encryption and decryption process. 4.8 Session Key Regeneration Session keys MUST be regenerated periodically, say, once in an hour. The re-key process is started by sending SILC_PACKET_REKEY packet to other end, to indicate that re-key must be performed. The initiator of the connection SHOULD initiate the re-key. If perfect forward secrecy (PFS) flag was selected in the SILC Key Riikonen [Page 39]
Internet Draft 13 November 2001 Exchange protocol [SILC3] the re-key MUST cause new key exchange with SKE protocol. In this case the protocol is secured with the old key and the protocol results to new key material. See [SILC3] for more information. After the SILC_PACKET_REKEY packet is sent the sender will perform the SKE protocol. If PFS flag was set the resulted key material is processed as described in the section Processing the Key Material in [SILC3]. The difference with re-key in the processing is that the initial data for the hash function is just the resulted key material and not the HASH as it is not computed at all with re-key. Other than that, the key processing it equivalent to normal SKE negotiation. If PFS flag was not set, which is the default case, then re-key is done without executing SKE protocol. In this case, the new key is created by providing the current sending encryption key to the SKE protocol's key processing function. The process is described in the section Processing the Key Material in [SILC3]. The difference in the processing is that the initial data for the hash function is the current sending encryption key and not the SKE's KEY and HASH values. Other than that, the key processing is equivalent to normal SKE negotiation. After both parties has regenerated the session key, both MUST send SILC_PACKET_REKEY_DONE packet to each other. These packets are still secured with the old key. After these packets, the subsequent packets MUST be protected with the new key. 4.9 Command Sending and Reception Client usually sends the commands in the SILC network. In this case the client simply sends the command packet to server and the server processes it and replies with command reply packet. However, if the server is not able to process the command, it is sent to the server's router. This is case for example with commands such as, SILC_COMMAND_JOIN and SILC_COMMAND_WHOIS commands. However, there are other commands as well. For example, if client sends the WHOIS command requesting specific information about some client the server must send the WHOIS command to router so that all clients in SILC network are searched. The router, on the other hand, sends the WHOIS command further to receive the exact information about the requested client. The WHOIS command travels all the way to the server which owns the client and it replies with command reply packet. Finally, the server which sent the command receives the command reply and it must be able to determine which client sent the original command. The server then sends command reply to the client. Implementations should have some kind of cache to handle, for example, WHOIS information. Servers Riikonen [Page 40]
Internet Draft 13 November 2001 and routers along the route could all cache the information for faster referencing in the future. The commands sent by server may be sent hop by hop until someone is able to process the command. However, it is preferred to destine the command as precisely as it is possible. In this case, other routers en route MUST route the command packet by checking the true sender and true destination of the packet. However, servers and routers MUST NOT route command reply packets to clients coming from other server. Client MUST NOT accept command reply packet originated from anyone else but from its own server. 4.10 Closing Connection When remote client connection is closed the server MUST send the notify type SILC_NOTIFY_TYPE_SIGNOFF to its primary router and to all channels the client was joined. The server MUST also save the client's information for a period of time for history purposes. When remote server or router connection is closed the server or router MUST also remove all the clients that was behind the server or router from the SILC Network. The server or router MUST also send the notify type SILC_NOTIFY_TYPE_SERVER_SIGNOFF to its primary router and to all local clients that are joined on the same channels with the remote server's or router's clients. 5 Security Considerations Security is central to the design of this protocol, and these security considerations permeate the specification. Common security considerations such as keeping private keys truly private and using adequate lengths for symmetric and asymmetric keys must be followed in order to maintain the security of this protocol. Special attention must also be paid on the servers and routers that are running the SILC service. The SILC protocol's security depends greatly on the security and the integrity of the servers and administrators that are running the service. It is recommended that some form of registration is required by the server and router administrator prior acceptance to the SILC Network. Even though, the SILC protocol is secure in a network of mutual distrust between clients, servers, routers and adminstrators of the servers, the client should be able to trust the servers they are using if they whish to do so. It however must be noted that if the client requires absolute security by not trusting any of the servers or routers in the SILC Network, it can Riikonen [Page 41]
Internet Draft 13 November 2001 be accomplished by negotiating private keys outside the SILC Network, either using SKE or some other key exchange protocol, or to use some other external means for distributing the keys. This applies for all messages, private messages and channel messages. It is important to note that SILC, like any other security protocol is not full proof system and cannot secure from insecure environment; the SILC servers and routers could very well be compromised. However, to provide acceptable level of security and usability for end user the protocol use many times session keys or other keys generated by the servers to secure the messages. This is intentional design feature to allow ease of use for end user. This way the network is still usable, and remains encrypted even if the external means of distributing the keys is not working. The implementation, however, may like to not follow this design feature, and always negotiate the keys outside SILC network. This is acceptable solution and many times recommended. The implementation still must be able to work with the server generated keys. If this is unacceptable for the client or end user, the private keys negotiatied outside the SILC Network should always be used. In the end it is always implementor's choice whether to negotiate private keys by default or whether to use the keys generated by the servers. It is also recommended that router operators in the SILC Network would form a joint forum to discuss the router and SILC Network management issues. Also, router operators along with the cell's server operators should have a forum to discuss the cell management issues. 6 References [SILC2] Riikonen, P., "SILC Packet Protocol", Internet Draft, April 2001. [SILC3] Riikonen, P., "SILC Key Exchange and Authentication Protocols", Internet Draft, April 2001. [SILC4] Riikonen, P., "SILC Commands", Internet Draft, April 2001. [IRC] Oikarinen, J., and Reed D., "Internet Relay Chat Protocol", RFC 1459, May 1993. [IRC-ARCH] Kalt, C., "Internet Relay Chat: Architecture", RFC 2810, April 2000. [IRC-CHAN] Kalt, C., "Internet Relay Chat: Channel Management", RFC 2811, April 2000. Riikonen [Page 42]
Internet Draft 13 November 2001 [IRC-CLIENT] Kalt, C., "Internet Relay Chat: Client Protocol", RFC 2812, April 2000. [IRC-SERVER] Kalt, C., "Internet Relay Chat: Server Protocol", RFC 2813, April 2000. [SSH-TRANS] Ylonen, T., et al, "SSH Transport Layer Protocol", Internet Draft. [PGP] Callas, J., et al, "OpenPGP Message Format", RFC 2440, November 1998. [SPKI] Ellison C., et al, "SPKI Certificate Theory", RFC 2693, September 1999. [PKIX-Part1] Housley, R., et al, "Internet X.509 Public Key Infrastructure, Certificate and CRL Profile", RFC 2459, January 1999. [Schneier] Schneier, B., "Applied Cryptography Second Edition", John Wiley & Sons, New York, NY, 1996. [Menezes] Menezes, A., et al, "Handbook of Applied Cryptography", CRC Press 1997. [OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", RFC 2412, November 1998. [ISAKMP] Maughan D., et al, "Internet Security Association and Key Management Protocol (ISAKMP)", RFC 2408, November 1998. [IKE] Harkins D., and Carrel D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [HMAC] Krawczyk, H., "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [PKCS1] Kalinski, B., and Staddon, J., "PKCS #1 RSA Cryptography Specifications, Version 2.0", RFC 2437, October 1998. [RFC2119] Bradner, S., "Key Words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Riikonen [Page 43]
Internet Draft 13 November 2001 7 Author's Address Pekka Riikonen Snellmanninkatu 34 A 15 70100 Kuopio Finland EMail: priikone@silcnet.org This Internet-Draft expires 13 Nay 2002 Riikonen [Page 44]
