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SILC (Secure Internet Live Conferencing protocol) is a protocol which provides secure synchronous conferencing services over the Internet.

The SILC protocol can be divided in three main parts: SILC Key Exchange (SKE) protocol, SILC Authentication protocol and SILC Packet protocol. SILC protocol additionally defines SILC Commands that are used to manage the SILC session. As many other conferencing and chat protocols SILC too provides channels (groups), nicknames, private messages, and other common features. However, SILC nicknames, in contrast to many other protocols (e.g. IRC), are not unique; a user is able to use any nickname, even if one is already in use. The real identification in the protocol is performed by unique Client ID. The SILC protocol uses this to overcome nickname collision, a problem present in many other protocols. All messages sent in a SILC network are binary, allowing them to contain any type of data, including text, video, audio, and other multimedia data. The SKE protocol is used to establish session key and other security parameters for protecting the SILC Packet protocol. The SKE itself is based on the Diffie-Hellman key exchange algorithm (a form of asymmetric cryptography) and the exchange is protected with digital signatures. The SILC Authentication protocol is performed after successful SKE protocol execution to authenticate a client and/or a server. The authentication may be based on passphrase or on digital signatures, and if successful gives access to the relevant SILC network. The SILC Packet protocol is intended to be a secure binary packet protocol, assuring that the content of each packet (consisting of a packet header and packet payload) is secured and authenticated. The packets are secured using algorithms based on symmetric cryptography and authenticated by using Message Authentication Code algorithm, HMAC.

SILC channels (groups) are protected by using symmetric channel keys. It is optionally possible to digitally sign all channel messages. It is also possible to protect messages with a privately-generated channel key that has been previously agreed upon by channel members. Private messages between users in a SILC network are protected with session keys. It is, however, possible to execute SKE protocol between two users and use the generated key to protect private messages. Private messages may be optionally digitally signed. When messages are secured with key material generated with the SKE protocol or previously agreed upon key material (for example, passphrases) SILC provides security even when the SILC server may be compromised.

SILC was designed by Pekka Riikonen between 1996 and 1999 and first released in public in summer 2000. A client and a server were written. Protocol specifications have been iterated through the IETF. At present time, there are several clients, the most advanced being the official SILC client and an irssi plugin. SILC protocol is also integrated to the popular Pidgin instant messaging client. Other GUI clients are Silky and Colloquy.

• The SILC Project
• Silky, GUI SILC client
• Silc Improved, minimalistic SILC client using FIFOs
• Kopete SILC Plugin
• Silsa, GUI SILC Client, part of Equinox Desktop Environment (EDE)