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<title>Challenge-Handshake Authentication Protocol</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Challenge-Handshake Authentication Protocol</span></span>

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<p>In <a href="Computing" title="Computing">computing</a>, the <b>Challenge-Handshake Authentication Protocol</b> (<b>CHAP</b>) is an <a href="Authentication_protocol" title="Authentication protocol">authentication protocol</a> originally used by <a href="Point-to-Point_Protocol" title="Point-to-Point Protocol">Point-to-Point Protocol</a> (PPP) to validate users. CHAP is also carried in other authentication protocols such as <a href="RADIUS" title="RADIUS">RADIUS</a> and <a href="Diameter_(protocol)" title="Diameter (protocol)">Diameter</a>.

</p><p>Almost all <a href="Network_operating_system" title="Network operating system">network operating systems</a> support PPP with CHAP, as do most <a href="Network_access_server" title="Network access server">network access servers</a>. CHAP is also used in <a href="PPPoE" class="mw-redirect" title="PPPoE">PPPoE</a>, for authenticating DSL users.

</p><p>As the PPP sends data unencrypted and "in the clear", CHAP is vulnerable to any attacker who can observe the PPP session. An attacker can see the user's name, CHAP challenge, CHAP response, and any other information associated with the PPP session. The attacker can then mount an offline <a href="Dictionary_attack" title="Dictionary attack">dictionary attack</a> in order to obtain the original password. When used in PPP, CHAP also provides protection against <a href="Replay_attack" title="Replay attack">replay attacks</a> by the peer through the use of a challenge which is generated by the authenticator, which is typically a <a href="Network_access_server" title="Network access server">network access server</a>.

</p><p>Where CHAP is used in other protocols, it may be sent in the clear, or it may be protected by a security layer such as <a href="Transport_Layer_Security" title="Transport Layer Security">Transport Layer Security</a> (TLS). For example, when CHAP is sent over <a href="RADIUS" title="RADIUS">RADIUS</a> using <a href="User_Datagram_Protocol" title="User Datagram Protocol">User Datagram Protocol</a> (UDP), any attacker who can see the RADIUS packets can mount an offline <a href="Dictionary_attack" title="Dictionary attack">dictionary attack</a>, as with PPP.

</p><p>CHAP requires that both the client and server know the clear-text version of the password, although the password itself is never sent over the network. Thus when used in PPP, CHAP provides better security as compared to <a href="Password_Authentication_Protocol" title="Password Authentication Protocol">Password Authentication Protocol</a> (PAP) which is vulnerable for both these reasons.

</p>

<div class="mw-heading mw-heading2"><h2 id="Benefits_of_CHAP">Benefits of CHAP</h2></div>

<p>When the peer sends CHAP, the authentication server will receive it, and obtain the "known good" password from a database, and perform the CHAP calculations. If the resulting hashes match, then the user is deemed to be authenticated. If the hashes do not match, then the user's authentication attempt is rejected.

</p><p>Since the authentication server has to store the password in clear-text, it is impossible to use different <a href="Password#Form_of_stored_passwords" title="Password">formats for the stored password</a>. If an attacker were to steal the entire database of passwords, all of those passwords would be visible "in the clear" in the database.

</p><p>As a result, while CHAP can be more secure than PAP when used over a PPP link, it prevents more secure storage "at rest" than with other methods such as <a href="Password_authentication_protocol" class="mw-redirect" title="Password authentication protocol">PAP</a>.

</p>

<div class="mw-heading mw-heading2"><h2 id="Variants">Variants</h2></div>

<p><a href="MS-CHAP" title="MS-CHAP">MS-CHAP</a> is similar to CHAP but uses a different hash algorithm, and allows for each party to authenticate the other.

</p>

<div class="mw-heading mw-heading2"><h2 id="Working_cycle">Working cycle</h2></div>

<p>CHAP is an authentication scheme originally used by <a href="Point-to-Point_Protocol" title="Point-to-Point Protocol">Point-to-Point Protocol</a> (PPP) servers to validate the identity of remote clients. CHAP periodically verifies the identity of the <a href="Client_(computing)" title="Client (computing)">client</a> by using a <a href="Handshake_(computing)" title="Handshake (computing)">three-way handshake</a>. This happens at the time of establishing the initial <a href="Link_Control_Protocol" title="Link Control Protocol">link (LCP)</a>, and may happen again at any time afterwards. The verification is based on a <a href="Shared_secret" title="Shared secret">shared secret</a> (such as the client's password).<sup id="cite_ref-Forouzan2007_1-0" class="reference"><a href="#cite_note-Forouzan2007-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

</p>

<ol><li>After the completion of the link establishment phase, the authenticator sends a "challenge" message to the peer.</li>

<li>The peer responds with a value calculated using a <a href="One-way_function" title="One-way function">one-way hash function</a> on the challenge and the secret combined.</li>

<li>The authenticator checks the response against its own calculation of the expected hash value. If the values match, the authenticator acknowledges the authentication; otherwise it should terminate the connection.</li>

<li>In PPP, the authenticator may send a new challenge at random intervals to the peer and repeats steps 1 through 3. However, when CHAP is used in most situations (e.g. <a href="RADIUS" title="RADIUS">RADIUS</a>), this step is not performed.</li></ol>

<div class="mw-heading mw-heading2"><h2 id="CHAP_packets">CHAP packets</h2></div>

<th>Description

</th>

<th>1 byte

</th>

<th>1 byte

</th>

<th>2 bytes

</th>

<th>1 byte

</th>

<th>Variable

</th>

<th>Variable

</th></tr>

<tr>

<td>Challenge

</td>

<td>Code = 1

</td>

<td>ID

</td>

<td>Length

</td>

<td>Challenge length

</td>

<td>Challenge value

</td>

<td>Name

</td></tr>

<tr>

<td>Response

</td>

<td>Code = 2

</td>

<td>ID

</td>

<td>Length

</td>

<td>Response length

</td>

<td>Response value

</td>

<td>Name

</td></tr>

<tr>

<td>Success

</td>

<td>Code = 3

</td>

<td>ID

</td>

<td>Length

</td>

<td>

</td>

<td>Message

</td>

<td>

</td></tr>

<tr>

<td>Failure

</td>

<td>Code = 4

</td>

<td>ID

</td>

<td>Length

</td>

<td>

</td>

<td>Message

</td>

<td>

</td></tr></tbody></table>

<p>The ID chosen for the random challenge is also used in the corresponding response, success, and failure packets. A new challenge with a new ID must be different from the last challenge with another ID. If the success or failure is lost, the same response can be sent again, and it triggers the same success or failure indication. For <a href="MD5" title="MD5">MD5</a> as hash the response value is <code>MD5(ID||secret||challenge)</code>, the MD5 for the concatenation of ID, secret, and challenge.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

</p>

<ul><li><a href="List_of_authentication_protocols" class="mw-redirect" title="List of authentication protocols">List of authentication protocols</a></li>

<li><a href="Password_Authentication_Protocol" title="Password Authentication Protocol">Password Authentication Protocol</a></li>

<li><a href="Challenge%E2%80%93response_authentication" title="Challenge–response authentication">Challenge–response authentication</a></li>

<li><a href="Cryptographic_hash_function" title="Cryptographic hash function">Cryptographic hash function</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFForouzan2007" class="citation book cs1">Forouzan (2007). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=6HaNKmfBK1oC&pg=PA352"><i>Data Communications & Networking 4E Sie</i></a>. McGraw-Hill Education (India) Pvt Limited. pp. 352–. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-07-063414-5</bdi><span class="reference-accessdate">. Retrieved <span class="nowrap">24 November</span> 2012</span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a00800b4131.shtml">"Understanding and Configuring PPP CHAP Authentication"</a>. <a href="Cisco_Systems" class="mw-redirect" title="Cisco Systems">Cisco tech note</a>. 2005<span class="reference-accessdate">. Retrieved <span class="nowrap">2011-08-14</span></span>.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

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<div class="side-box-text plainlist">Wikibooks has a book on the topic of: <i><b><a href="https://en.wikibooks.org/wiki/Network_Plus_Certification/Security/User_Authentication" class="extiw external" title="wikibooks:Network Plus Certification/Security/User Authentication">Network Plus Certification/Security/User Authentication</a></b></i></div></div>

</div>

<ul><li><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc1994">1994</a> PPP Challenge Handshake Authentication Protocol (CHAP)</li>

<li><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc2865">2865</a> Remote Authentication Dial In User Service (<a href="RADIUS" title="RADIUS">RADIUS</a>): <i>uses <a href="Password_authentication_protocol" class="mw-redirect" title="Password authentication protocol">PAP</a> or CHAP</i></li>

<li><a href="RFC_(identifier)" class="mw-redirect" title="RFC (identifier)">RFC</a> <a rel="nofollow" class="external text" href="https://www.rfc-editor.org/rfc/rfc3748">3748</a> Extensible Authentication Protocol (<a href="Extensible_Authentication_Protocol" title="Extensible Authentication Protocol">EAP</a>): <i>discusses CHAP</i></li></ul>

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</style><div id="Authentication330" style="font-size:114%;margin:0 4em"><a href="Authentication" title="Authentication">Authentication</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Authentication<br>APIs</th><td class="navbox-list-with-group navbox-list navbox-odd" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="BSD_Authentication" title="BSD Authentication">BSD Authentication</a> (BSD Auth)</li>

<li><a href="EAuthentication" class="mw-redirect" title="EAuthentication">eAuthentication</a> (eAuth)</li>

<li><a href="Generic_Security_Services_Application_Program_Interface" class="mw-redirect" title="Generic Security Services Application Program Interface">Generic Security Services API</a> (GSSAPI)</li>

<li><a href="Java_Authentication_and_Authorization_Service" title="Java Authentication and Authorization Service">Java Authentication and Authorization Service</a> (JAAS)</li>

<li><a href="Pluggable_authentication_module" class="mw-redirect" title="Pluggable authentication module">Pluggable Authentication Modules</a> (PAM)</li>

<li><a href="Simple_Authentication_and_Security_Layer" title="Simple Authentication and Security Layer">Simple Authentication and Security Layer</a> (SASL)</li>

<li><a href="Security_Support_Provider_Interface" title="Security Support Provider Interface">Security Support Provider Interface</a> (SSPI)</li>

<li><a href="XUDA" title="XUDA">XCert Universal Database API</a> (XUDA)</li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Authentication_protocol" title="Authentication protocol">Authentication<br>protocols</a></th><td class="navbox-list-with-group navbox-list navbox-even" style="width:100%;padding:0"><div style="padding:0 0.25em">

<li><a href="Authentication_and_Key_Agreement_(protocol)" class="mw-redirect" title="Authentication and Key Agreement (protocol)">Authentication and Key Agreement</a> (AKA)</li>

<li><a href="CAVE-based_authentication" title="CAVE-based authentication">CAVE-based authentication</a></li>

<li> (CHAP)

<li><a href="Central_Authentication_Service" title="Central Authentication Service">Central Authentication Service</a> (CAS)</li>

<li><a href="Diameter_(protocol)" title="Diameter (protocol)">Diameter</a></li>

<li><a href="Extensible_Authentication_Protocol" title="Extensible Authentication Protocol">Extensible Authentication Protocol</a> (EAP)</li>

<li><a href="Host_Identity_Protocol" title="Host Identity Protocol">Host Identity Protocol</a> (HIP)</li>

<li><a href="IndieAuth" title="IndieAuth">IndieAuth</a></li>

<li><a href="Kerberos_(protocol)" title="Kerberos (protocol)">Kerberos</a></li>

<li><a href="LAN_Manager" title="LAN Manager">LAN Manager</a></li>

<li><a href="NT_LAN_Manager" class="mw-redirect" title="NT LAN Manager">NT LAN Manager</a> (NTLM)</li>

<li><a href="OAuth" title="OAuth">OAuth</a></li>

<li><a href="OpenID" title="OpenID">OpenID</a></li>

<li><a href="OpenID_Connect" class="mw-redirect" title="OpenID Connect">OpenID Connect</a> (OIDC)</li>

<li><a href="Password-authenticated_key_agreement" title="Password-authenticated key agreement">Password-authenticated key agreement</a> protocols</li>

<li><a href="Password_Authentication_Protocol" title="Password Authentication Protocol">Password Authentication Protocol</a> (PAP)</li>

<li><a href="Protected_Extensible_Authentication_Protocol" title="Protected Extensible Authentication Protocol">Protected Extensible Authentication Protocol</a> (PEAP)</li>

<li><a href="RADIUS" title="RADIUS">Remote Access Dial In User Service</a> (RADIUS)</li>

<li><a href="Resource_Access_Control_Facility" title="Resource Access Control Facility">Resource Access Control Facility</a> (RACF)</li>

<li><a href="Secure_Remote_Password_protocol" title="Secure Remote Password protocol">Secure Remote Password protocol</a> (SRP)</li>

<li><a href="TACACS" title="TACACS">TACACS</a></li>

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<ul><li><span class="noviewer" typeof="mw:File"><span title="Category"></span></span> Category</li>

<li><span class="noviewer" typeof="mw:File"><span title="Commons page"></span></span> <a href="https://commons.wikimedia.org/wiki/Category:Authentication" class="extiw external" title="commons:Category:Authentication">Commons</a></li></ul>

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