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# Bob checks the received password against the one stored in his database. Then he sends a packet saying "Authentication successful" or "Authentication failed" based on the result.<ref>{{Cite book|title = Fundamentals of Cryptology|last = van Tilborg|first = Henk C.A.|publisher = Kluwer Academic Publishers|year = 2000|isbn = 0-7923-8675-2|location = Massachusetts|pages = 66–67}}</ref>
This is an example of a very basic authentication protocol vulnerable to many threats such as [[eavesdropping]], [[replay attack]], [[man-in-the-middle]] attacks, dictionary attacks or brute-force attacks. Most authentication protocols are more complicated in order to be resilient against these attacks.<ref>{{Cite book|title = Internet Cryptography|last = Smith|first = Richard E.|publisher = Addison Wesley Longman|year = 1997|isbn = 0-201-92480-3|location = Massachusetts|pages = [https://archive.org/details/internetcryptogr0000smit/page/1 1–27]|url = https://archive.org/details/internetcryptogr0000smit/page/1}}</ref>
==Types==
===Authentication protocols developed for PPP [[Point-to-Point Protocol]]===
Protocols are used mainly by [[Point-to-Point Protocol]] (PPP) servers to validate the identity of remote clients before granting them access to server data. Most of them use a password as the cornerstone of the authentication. In most cases, the password has to be shared between the communicating entities in advance.<ref>{{cite document|title = Public-key cryptography and password protocols|last = Halevi|first = Shai|citeseerx = 10.1.1.45.6423}}</ref>
[[File:PAP 2way handshake.png|thumb|PAP 2-way handshake scheme|461x461px]]
====PAP - Password Authentication Protocol====
[[Password Authentication Protocol]] is one of the oldest authentication protocols. Authentication is initialized by the client sending a packet with [[credentials]] (username and password) at the beginning of the connection, with the client repeating the authentication request until acknowledgement is received.<ref>{{cite web|url = http://data.cedupoint.cz/oppa_e-learning/2_KME/044.pdf|title = Autentizacní telekomunikacních a datových sítích|access-date = 31 October 2015|publisher = CVUT Prague|last = Vanek|first = Tomas|archive-url = https://web.archive.org/web/20160304080620/http://data.cedupoint.cz/oppa_e-learning/2_KME/044.pdf|archive-date = 4 March 2016|url-status = dead}}</ref> It is highly insecure because credentials are sent "[[Plaintext|in the clear]]" and repeatedly, making it vulnerable even to the most simple attacks like [[eavesdropping]] and [[man-in-the-middle]] based attacks. Although widely supported, it is specified that if an implementation offers a stronger authentication method, that method ''must'' be offered before PAP. Mixed authentication (e.g. the same client alternately using both PAP and CHAP) is also not expected, as the CHAP authentication would be compromised by PAP sending the password in plain-text.
====CHAP - [[Challenge-handshake authentication protocol]]====
The authentication process in this protocol is always initialized by the server/host and can be performed anytime during the session, even repeatedly. Server sends a random string (usually 128B long). The client uses password and the string received as parameters for MD5 hash function and then sends the result together with username in plain text. Server uses the username to apply the same function and compares the calculated and received hash. An authentication is successful or unsuccessful.
====EAP - Extensible Authentication Protocol====
EAP was originally developed for PPP(Point-to-Point Protocol) but today is widely used in [[IEEE 802.3]], [[IEEE 802.11]](WiFi) or [[IEEE 802.16]] as a part of [[IEEE 802.1x]] authentication framework. The latest version is standardized in RFC 5247. The advantage of EAP is that it is only a general authentication framework for client-server authentication - the specific way of authentication is defined in its many versions called EAP-methods. More than 40 EAP-methods exist, the most common are:
*[[EAP-MD5]]
*[[EAP-TLS]]
*[[EAP-TTLS]]
*[[EAP-FAST]]
*EAP-[[Protected Extensible Authentication Protocol|PEAP]]
===AAA architecture protocols (Authentication, Authorization, Accounting)===
Complex protocols used in larger networks for verifying the user (Authentication), controlling access to server data (Authorization) and monitoring network resources and information needed for billing of services (Accounting).
====[[TACACS]], [[XTACACS]] and [[TACACS+]]====
The oldest AAA protocol using IP based authentication without any encryption (usernames and passwords were transported as plain text). Later version XTACACS (Extended TACACS) added authorization and accounting. Both of these protocols were later replaced by TACACS+. TACACS+ separates the AAA components thus they can be segregated and handled on separate servers (It can even use another protocol for e.g. Authorization). It uses [[Transmission Control Protocol|TCP]] (Transmission Control Protocol) for transport and encrypts the whole packet. TACACS+ is Cisco proprietary.
====[[RADIUS]]====
[[Remote Authentication Dial-In User Service]] (RADIUS) is a full [[AAA (computer security)|AAA protocol
]] commonly used by [[ISP]]. Credentials are mostly username-password combination based, it uses [[Network access server|NAS]] and [[User Datagram Protocol|UDP]] protocol for transport.<ref>{{cite web|url = http://www.cisco.com/c/en/us/td/docs/net_mgmt/cisco_secure_access_control_system/5-1/user/guide/acsuserguide/rad_tac_phase.html|title = AAA protocols|access-date = 31 October 2015|website = www.cisco.com|publisher = CISCO}}</ref>
===Other===
[[File:Kerberos sch en.png|thumb|321x321px|Kerberos authentication scheme]]
====[[Kerberos (protocol)]]====
Kerberos is a centralized network authentication system developed at [[MIT]] and available as a free implementation from MIT but also in many commercial products. It is the default authentication method in [[Windows 2000]] and later. The authentication process itself is much more complicated than in the previous protocols - Kerberos uses [[symmetric key cryptography]], requires a [[trusted third party]] and can use [[public-key cryptography]] during certain phases of authentication if need be.<ref>{{cite web|url = http://web.mit.edu/kerberos/|title = Kerberos: The Network Authentication Protocol|date = 10 September 2015|access-date = 31 October 2015|website = web.mit.edu|publisher = MIT Kerberos}}</ref><ref>{{Cite book|title = Applied Cryptography|last = Schneier|first = Bruce|publisher = John Wiley & Sons, Inc.|year = 1997|isbn = 0-471-12845-7|location = New York|pages = 52–74}}</ref><ref>{{cite web|url = http://srp.stanford.edu/history.html|title = Protocols of the Past|access-date = 31 October 2015|website = srp.stanford.edu|publisher = Stanford University}}</ref>
==List of various other authentication protocols==
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