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<meta name="Author" content="Pekka Riikonen - SILC Project">
<meta name="Description"
content="SILC - Secure Internet Live Conferencing Protocol">
-<meta name="Created" content="Version 1.0 / 03 Aug 2001">
+<meta name="Created" content="Version 1.2 / 22 October 2003">
</head>
<body bgcolor="#ffffff">
<font face="Helvetica">
<font size="6"><b>SILC Protocol White Paper</b></font><br>
-<font size="2">Version 1.0 / 03 Aug 2001</font>
+<font size="2">Version 1.2 / 22 October 2003</font>
<p>
<h1>Introduction</h1>
have something in common; they are all insecure.
<p>
-The security is important feature in applications and protocols in
+The security is important feature in applications and protocols in
contemporary network environment. The older chat protocols, however have
failed to meet the growing security requirements on the Internet.
It is not anymore enough to just provide services, like for example
The SILC Protocol have been developed as Open Source project. The
protocol specifications are freely available and they have been submitted to
-the IETF. The very first implementations of the protocol are also already
-available.
+the IETF. The protocol is currently stabilizing and has reached a version
+1.2.
<p><br>
<h1>About This White Paper</h1>
<p>
<p>
-(c) Copyright 2001 Pekka Riikonen
+(c) Copyright 2001 - 2003 Pekka Riikonen
(<a href="mailto:priikone at silcnet.org">priikone at silcnet.org</a>)
<p>
This document is free document; you can redistribute it and/or modify
<h1>SILC Protocol</h1>
<p>
-The Secure Internet Live Conferencing (SILC) protocol provides secure
-conferencing services over insecure network channel. The SILC is IRC
-like protocol, however it does not support IRC. Strong cryptographic
-methods are used to protect SILC packets inside the SILC network. SILC
-provides all the common conferencing services like channels, channel
-messages, private messages, nicknames and various commands. Difference
-to other chat protocol is in the design of the protocol. The SILC
-protocol has been designed from the day one security in mind and it
-shows in the protocol design.
-<p>
-
-Generally it is assumed that the SILC Network is trusted. This means
-that clients can fully trust the servers and routers in the SILC Network.
-In real life this is not always possible. In the Internet it is possible
-that some server or router would get compromised by a malicious
-cracker. However, if the SILC Network is closed network, for example
-inside a orgranization the assumption generally is true. The SILC
-protocol is secure even if the end users consider the network
-untrusted, and provides several ways to still have secure conversation
-on the SILC Network.
-<p>
-
-The packets in the SILC network are always encrypted. It is not possible
-to send unencrypted messages in SILC. This assures that end user cannot
-even accidently send unencrypted messages while thinking that it is
-encrypted. This is the problem of most other chat protocols that provide
-so called plugin encryption. They are not secure by default but try
-to provide security by applying external security protocol such as PGP
-or SSL. In these cases the security is achieved usually by encrypting the
-data while key management and other security issues may be left out, leaving
-the implementation vulnerable to various security problems. The other
-problem is also that the external protocols tend to leave the network
-only partly secured; usually only two points in the network are secured
-with for example SSL. While SSL does provide provable security it is not
-enough to provide security for a chat network as a whole.
-<p>
-
-The network topology is also different to various other chat protocol,
-like for example IRC. IRC has tree style network where SILC has so
-called cellular network. A cell consists of a router, servers and clients.
-The cell can also have backup routers in case the private router becomes
-unresponsive.
+Secure Internet Live Conferencing, or SILC in short, is a modern
+conferencing protocol which provides rich conferencing features with
+high security. One of the main design principles of the protocol was
+security. Many of the SILC features are found in traditional chat
+protocols such as IRC but many of the SILC features can also be found
+in Instant Message (IM) style protocols.
+<p>
+
+SILC combines features from both of these chat protocol styles, and
+can be implemented as either IRC-like system or IM-like system. In
+fact, SILC removes the need to make such distinction between these
+two protocol styles. Some of the more advanced and security features
+of the protocol are new to all conferencing protocols. SILC also
+supports multimedia messages and can also be implemented as a
+video and audio conferencing system. The protocol is also compact
+and robust and suites well for mobile environments where the low
+bandwidth sets special requirements for protocols. All packet sizes
+in SILC can be even further reduced by utlizing compression.
+<p>
+
+The packets and messages in the SILC network are always encrypted and
+authenticated. It is not possible to send unencrypted messages in SILC
+at all. This assures that end user cannot even accidently send unencrypted
+messages while thinking that it is encrypted. This is one of the problems
+of most of the other chat protocols that provide so called plugin encryption.
+They are not secure by default but try to provide security by applying
+external security protocol such as PGP or SSL over the insecure
+protocol. In these cases the security is achieved usually by encrypting the
+data while key management, message authentication and other security issues
+may be left out, leaving the implementation vulnerable to various security
+problems. The other problem is also that the external protocols tend to
+leave the network only partly secured; usually only two points in the
+network are secured with for example SSL. While SSL does provide provable
+security it is not enough to provide security for a chat network as a whole.
+<p>
+
+SILC is secure in environment of mutual distrust between entities
+in the network. It is possible to encrypt messages end to end, so that only
+the sender and the receiver is able to encrypt and decrypt messages. It
+is also possible to send messages to group of users, so that only the
+specified group of users is able to encrypt and decrypt messages. Many
+times the protocol use keys that are generated by the servers, so that
+if other external key exchange methods fail the network still remains
+encrypted. However, it is always possible to negotiate and use locally
+generated keys to secure messages, so that the servers do not know the
+key.
+<p>
+
+Like so many other contemporary chat protocols, SILC too provides
+file transfer. It is possible to transfer files securely between users
+in the SILC Network. The actual file transfer stream is always sent
+outside the network peer to peer. Before the file transfer is started
+a key exchange protocol is executed to negotiate file transfer session
+key.
+<p>
+
+The SILC protocol also supports so called detaching, a novel idea where
+it is possible to detach from the server without actually quitting the
+network. It is then later possible to resume the connection back to some
+server in the network, and be like you were never gone.
+<p>
+
+The SILC protocol also allows distribution and exchange of public keys
+and certificates through the SILC network. It is also possible to fetch
+detailed user information from other users through the SILC network. It
+is possible to fetch for example users's business card, pictures,
+certificates, etc.
+<p>
+
+SILC protocol also supports services, which are extensions to the core
+protocol. They can be used to augment the features of the protocol or
+to add entirely new features without breaking backwards compatibility.
+Services can be negotiated online and authenticated with passphrases or
+with digital signatures.
+<p>
+
+The network topology is also different from traditional conferencing and
+chat protocols. The SILC network forms so called hybrid ring-mesh network
+at the router level, and star network at the server level. This sort of
+network topology allows better scalability and faster delivery of packets
+than traditional spanning tree style network. The router servers and normal
+servers also has the distinction that only router's know global information
+and keep the global network state up to date, and normal servers keep only
+local information up to date. This significantly increases the scalability
+of the network. The network also supports backup routers which can be
+used to protect the network against netsplits.
<p><br>
<object data="silc_network.jpg" type="application/postscript">
-<img src="silc_network.png" alt="SILC Network" align="center" border"0">
+<a href="silc_network.png">
+<img src="s_silc_network.png" alt="SILC Network" align="center"
+border="0"></a>
</object>
<p><br>
The diagram above illustrates a portion of the SILC network. It shows
-two cells that both has several servers, and backup routers and several
+two cells that both have several servers, and backup routers and several
clients. Clients can connect to server and routers if they want to.
The following sections will describe the entities of the SILC Network
in greater detail.
The clients are distinquished from other clients by unique Client ID.
There cannot be multiple same Client IDs in the SILC Network at the same time.
The end user, however does not use Client IDs. The end users usually selects
-a perferred nickname they want to use, and identifies themself with that
+a preferred nickname they want to use, and identifies themself with that
nickname to other users on the network. The nicknames are not unique in
the SILC Network. There can be multiple same nicknames at the same time
-on the network. The maximum length for the nickname is 128 characters.
+on the network. The maximum length for the nickname is 128 bytes.
<p>
Most of the other chat protocols have unique nicknames. This is where SILC
differs from most of the other chat protocols. The purpose of this
feature is to make IRC style nickname wars obsolete, as no one owns their
-nickname; there can always be somene else with the same nickname.
+nickname; there can always be somene else with the same nickname. This
+feature also makes nickname registering services obsolete.
<p>
When client connects to the server the SILC Key Exchange (SKE) protocol and
Perfect Forward Secrecy (PFS). The connection authentication protocol is
used to authenticate the client to the server. The server may allow the
client to connect without authentication, or it may require a passphrase or
-public key based authentication.
+public key based (or certificates) authentication.
<p><br>
when client connects to server. The SKE results in to the session key
that is used to secure the communication between the server and the
router. The connection authentication protocol is used to authenticate
-the server to the router. The authentication is always based in either
-passphrase or public key.
+the server to the router. The authentication is always based in either
+passphrase or public key (or certificates).
<p><br>
<p><br>
<object data="silc_routers.jpg" type="application/postscript">
-<img src="silc_routers.png" alt="SILC Routers" align="center" border"0">
+<a href="silc_routers.png">
+<img src="s_silc_routers.png" alt="SILC Routers" align="center"
+border="0"></a>
</object>
<p><br>
<p>
The basis of SILC protocol relies in the SILC packets and they are with
-out a doubt the most important part of the protocol. The SILC Packet
-protocol is a binary packet protocol. The protocol provides secure
+out a doubt the most important part of the protocol. The SILC Packet
+protocol is a secure binary packet protocol. The protocol provides secure
binary packets and assures that the contents of the packets are secured
and authenticated.
<p>
assured by computed Message Authentication Codes (MAC). The protocol
defines several packet types and packet payloads. Each packet type
usually has a specific packet payload that actually defines the contents
-of the packet. Hence, the actual data in the packet is the packet payload
+of the packet. Hence, the actual data in the packet is the packet payload
defined in the protocol.
<p><br>
<object data="silc_packet.jpg" type="application/postscript">
-<img src="silc_packet.png" alt="Typical SILC Packet" align="center" border"0">
+<a href="silc_packet_png">
+<img src="s_silc_packet.png" alt="Typical SILC Packet" align="center"
+border="0"></a>
</object>
<p><br>
with the session key and some are encrypted with other keys, for example
with channel message keys. The SILC Packet Header is always encrypted with
the session key. The MAC is computed from the SILC Packet Header and the
-data area before encrypting the packet.
+data area after encryption. This is so called Encrypt-Then-MAC order.
<p><br>
between two clients too, if two clients would need to create secret key.
The purpose of the SKE protocol is to create session keys to be used
in current SILC session. The SKE is based on the Diffie-Hellman key
-exchange algorithm, and is immune to man-in-the-middle attack.
+exchange algorithm, and is immune to for example man-in-the-middle attacks
+by using digital signatures.
<p>
This is the first protocol that is executed when creating connection to,
After the security properties are selected the protocol continues
by performing the Diffie-Hellman key exchange algorithm. At the same
time the intiator and responder also sends their public keys or
-certificates to each other. The responder also computes a signature
-that the initiator will verify. It is also possible to perform a
-mutual authentication where both of the parties computes a signature
-which are verified by each other independently. If any of the phases
-of the protocol are to fail the connection is closed immeadiately.
+certificates to each other. The initiator and responder also computes a
+signature that the other party will verify. By default the protocol is
+executed in so called mutual authentication mode, where both of the
+parties computes a signature which are verified by each other
+independently. This way both of the parties will have prove the
+posession of the private key to the public key they are providing
+in the protocol. If any of the phases of the protocol are to fail the
+connection is closed immeadiately.
<p>
The public key or certificate that is received during the SKE protocol
-must be verified. If it is not verified it would be possible to
+must be verified. If it is not verified it would be possible to
execute a man-in-the-middle attack against the SKE protocol. If
certificates are used they can be verified by a third party Certification
Authority (CA). Verifying a public key requires either confirming
a fingerprint of the public key over phone or email, or the server
-can for example publish the fingerprint (and the public key) on some
+can for example publish the fingerprint (and the public key) on some
website. In real life systems accepting the public key without
verification, however is often desired. In many security protocols,
such as in SSH2, the public key is accepted without verification
Since the SILC Connection Authentication protocol is always executed after
the SKE protocol, session keys has been established already. This means
-that all packets sent in the connection authentication protocol are encrypted
+that all packets sent in the connection authentication protocol are encrypted
and authenticated.
<p>
encrypted it is safe to send the passphrase inside the packet.
<p>
-If the authentication is based to public key then, for example the client,
+If the authentication is based to public key then, for example the client,
signs data with its private key and sends it to the server. The server
then verifies this signature by using the client's public key. The
packet is also encrypted in the case of public key authentication.
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 joins to it, and the channel ceases to exist when the last client
-leaves it. When channel exists, any client can reference it using the
+leaves it. When channel exists, any client can reference it using the
name of the channel. Channel is a place where group of people can engage
conversation.
<p>
<p><br>
<object data="silc_channel.jpg" type="application/postscript">
-<img src="silc_channel.png" alt="Channel Message Delivery" align="center" border"0">
+<a href="silc_channel.png">
+<img src="s_silc_channel.png" alt="Channel Message Delivery"
+align="center" border="0"></a>
</object>
<p><br>
<p><br>
<h1>Private Messages</h1>
<p>
-Private messages are messages that are sent from one client to another
+Private messages are messages that are sent from one client to another
through the SILC Network. They are private because they are not sent to
anyone else except to the true receiver of the message. Private messages
can be used to engage private conversation with another client if channels
<p>
Sending private messages are by default secured with session keys established
in the SKE protocol. This means that the private message is always encrypted
-with the session key of the next receiver of the message enroute to the
+with the session key of the next receiver of the message enroute to the
receiving client. This also means that the message is decrypted and
re-encrypted everytime it is sent further to the receiving client.
<p><br>
<object data="silc_priv1.jpg" type="application/postscript">
-<img src="silc_priv1.png" alt="Basic Private Message Delivery" align="center" border"0">
+<a href="silc_priv1.png">
+<img src="s_silc_priv1.png" alt="Basic Private Message Delivery"
+align="center" border="0"></a>
</object>
<p><br>
to be untrusted.
<p>
-If the clients on the other hand trust the servers and routers in their
+If the clients on the other hand trust the servers and routers in their
SILC Network, or they do not care that servers can decrypt their messages,
sending private messages in this way is very simple from client's point
of view. For servers and routers this of course means that they need
<p><br>
<object data="silc_priv2.jpg" type="application/postscript">
-<img src="silc_priv2.png" alt="Private Messages with Private Message Key" align="center" border"0">
+<a href="silc_priv2.png">
+<img src="s_silc_priv2.png" alt="Private Messages with Private Message
+Key" align="center" border="0"></a>
</object>
<p><br>
<p>
Using this method of private messages delivery is recommended if the
-clients cannot trust the servers and routers in the SILC Network. The
+clients cannot trust the servers and routers in the SILC Network. The
drawback is the extra phase of setting the private message key before
starting the conversation. However, using the SKE protocol is the
recommended way to negotiate the private message key since it can be
automatized and does not cause any extra tasks for end user.
+<!--
<p><br>
<h2>Private Message Delivery With Public Key Encryption</h2>
<p>
and routers cannot decrypt the messages since they do not have the
receiver's private key. The receiver on the other hand has the private
key which it uses to decrypt the message.
-
-<p><br>
-<object data="silc_priv3.jpg" type="application/postscript">
-<img src="silc_priv3.png" alt="Private Messges with Public Key Cryptosystem" align="center" border"0">
-</object>
-<p><br>
-
-As the diagram above shows the Client A has the Client B's public key.
-It will encrypt the message with that key and sends the message to the
-SILC Network. All servers and routers pass the message through since
-they cannot decrypt it. The Client B then uses its private key to
-decrypt the message. The Client B has also the Client A's public key
-that it can use to encrypt messages that it will send to Client A.
<p>
Even this method of private message delivery is not perfect. The drawbacks
-of this method is that the public key encryption process, as being
-asymmetric cryptosystem, is much slower than encryption process with
-symmetric cryptosystems. This is not probably problem with short messages
-but may be inconvenient with long messages. The other drawback is that the
-sender must first assure that the public key it is using in the encryption
-is actually the receiver's public key. This is a absolute requirement
-in this method. If the sender cannot authenticate the receiver's public
-key this method of private message delivery should not be used. In SILC
-protocol clients can fetch other clients public keys from servers.
-However, the servers may not have authenticated the fetched public key so
-that should not be fully trusted. Use of certificates can solve the
-problem. The receiver's certificate could be authenticated by a third
-party Certification Authority (CA).
+of this method is that the sender must first assure that the public key
+it is using in the encryption is actually the receiver's public key.
+This is a absolute requirement in this method. If the sender cannot
+authenticate the receiver's public key this method of private message
+delivery should not be used. In SILC protocol clients can fetch other
+clients public keys from servers, but the client still must verify the
+key. Use of certificates can solve the problem. The receiver's certificate
+could be authenticated by a third party Certification Authority (CA).
<p>
Usually verifying the public key is not a problem since the receiver
can provide the public key on some website, or verify the fingerprint of
the key over email, or phone call. The clients can also fetch the
-public keys from SILC servers if they trust that the keys are authentic.
-If both of the clients trust that the public keys are authentic using this
-method of private message delivery is very simple and recommended.
+public keys from SILC servers. If both of the clients trust that the
+public keys are authentic using this method of private message delivery
+is very simple and recommended.
+-->
<p><br>
-<h1>Conclusion</h1>
+<h1>MIME Messages</h1>
+
+SILC Protocol supports MIME messages as normal channel and private
+messages. By using MIME messages it is possible to send for example
+images, music and video and audio stream in SILC. Any MIME type that is
+supported by the application can be sent via SILC network.
+<p>
+
+The MIME messages are utilized by using so called Message Flags in the
+message payload that is used in SILC protocol. The Message Flags
+indicates the recipient that the message is a MIME message and it then
+knows how to interpret the message. Using Message Flags it possible also
+to send other kind of messages and to augment features of normal channel
+and private messages.
-The Secure Internet Live Conferencing (SILC) protocol is a new generation
-chat protocol that provides all the common conferencing services with
-strong support for security. It has wide range of security properties
-that should meet the highest levels of security requirements, while not
-forgetting easy of use. The network topology offers new architectural
-solution with better scalability over traditional chat protocols.
<p><br>
-<h1>Further Information</h1>
+<h1>Secure File Transfers</h1>
+
+The file transfer support in chat protocols are a absolute requirement
+nowadays, and chat protocol without one is no chat protocol at all. SILC
+also supports file transfer with the addition that the file transfer
+stream is secured. When a user wants to transfer a file to another
+user, the SILC Key Exchange (SKE) protocol is first executed to negotiate
+a session key for the file transfer stream. This key is then used to
+protect the peer to peer stream between users.
<p>
-More detailed information about the SILC protocol is available in the
-SILC protocol specification documents. There exists currently four
-Internet Drafts that defines the protocol in great detail. The Internet
-Drafts are available from the following sources but also from the
-<a href="http://www.ietf.org">IETF website</a>.
+
+The file transfer protocol used in SILC protocol is the SSH File Transfer
+protocol (SFTP). Even though the name of the protocol relates to SSH,
+the actual file transfer protocol has nothing to do with Secure Shell.
+The SFTP is totally independent file transfer protocol and its stream
+is secured using SILC. The SFTP is very good protocol because in addition
+of providing simple file transfer support, it can also support complex
+file and directory manipulation.
<p>
-- <a href="http://silcnet.org/docs/draft-riikonen-silc-spec-03.txt">
-Secure Internet Live Conferencing (SILC), Protocol Specification</a>
-<br>
+The support for file transfer in SILC has been designed so that using
+practically any file transfer protocol is possible. The mandatory protocol
+is SFTP but in the future adding support for other protocols is also
+possible.
+
+
+<p><br>
+<h1>Future of the Protocol</h1>
+
+The protocol has matured into the version 1.2 over the past few years.
+It has reached a level where it is the most rich featured conferencing
+protocol as of today. It is the SILC Project's intention to standardize
+the SILC protocol in the IETF and this is where the focus is now moving.
+
+
+<p><br>
+<h1>Conclusion</h1>
-- <a href="http://silcnet.org/docs/draft-riikonen-silc-pp-03.txt">
-SILC Packet Protocol</a>
-<br>
+Secure Internet Live Conferencing is a modern conferencing protocol which
+provides rich conferencing features with high security. It has a wide
+range of security properties and features that should meet the highest
+levels of security requirements, while not forgetting ease of use. The
+network topology offers new architectural solution with better scalability
+over traditional chat protocols.
-- <a href="http://silcnet.org/docs/draft-riikonen-silc-ke-auth-03.txt">
-SILC Key Exchange and Authentication Protocols</a>
-<br>
-- <a href="http://silcnet.org/docs/draft-riikonen-silc-commands-01.txt">
-SILC Commands</a>
+<p><br>
+<h1>Further Information</h1>
+<p>
+More detailed information about the SILC protocol is available in the
+SILC protocol specification documents. There exists currently six
+Internet Drafts that defines the protocol in great detail. The Internet
+Drafts are available from the <a href="http://silcnet.org">SILC Project
+website</a> but also from the <a href="http://www.ietf.org">IETF website</a>.
<p>
For comprehensive introduction to cryptography refer to the
- Certificate
<p>
-Certificate is a digital document which can be used to verify the
+Certificate is a digital document which can be used to verify the
identity of a person or host. In SILC, certificates can be used to prove
identity of clients, servers and routers. Basically certificate is a public
key with subject name. SILC supports X.509, OpenPGP and SPKI certificates.
projects / silc.git / blobdiff