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Edit File: init_creds.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Initial credentials — MIT Kerberos Documentation</title> <link rel="stylesheet" href="../_static/agogo.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="stylesheet" href="../_static/kerb.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '1.15.1', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="MIT Kerberos Documentation" href="../index.html" /> <link rel="up" title="For application developers" href="index.html" /> <link rel="next" title="Principal manipulation and parsing" href="princ_handle.html" /> <link rel="prev" title="Differences between Heimdal and MIT Kerberos API" href="h5l_mit_apidiff.html" /> </head> <body> <div class="header-wrapper"> <div class="header"> <h1><a href="../index.html">MIT Kerberos Documentation</a></h1> <div class="rel"> <a href="../index.html" title="Full Table of Contents" accesskey="C">Contents</a> | <a href="h5l_mit_apidiff.html" title="Differences between Heimdal and MIT Kerberos API" accesskey="P">previous</a> | <a href="princ_handle.html" title="Principal manipulation and parsing" accesskey="N">next</a> | <a href="../genindex.html" title="General Index" accesskey="I">index</a> | <a href="../search.html" title="Enter search criteria" accesskey="S">Search</a> | <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__Initial credentials">feedback</a> </div> </div> </div> <div class="content-wrapper"> <div class="content"> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="initial-credentials"> <h1>Initial credentials<a class="headerlink" href="#initial-credentials" title="Permalink to this headline">¶</a></h1> <p>Software that performs tasks such as logging users into a computer when they type their Kerberos password needs to get initial credentials (usually ticket granting tickets) from Kerberos. Such software shares some behavior with the <a class="reference internal" href="../user/user_commands/kinit.html#kinit-1"><em>kinit</em></a> program.</p> <p>Whenever a program grants access to a resource (such as a local login session on a desktop computer) based on a user successfully getting initial Kerberos credentials, it must verify those credentials against a secure shared secret (e.g., a host keytab) to ensure that the user credentials actually originate from a legitimate KDC. Failure to perform this verification is a critical vulnerability, because a malicious user can execute the “Zanarotti attack”: the user constructs a fake response that appears to come from the legitimate KDC, but whose contents come from an attacker-controlled KDC.</p> <p>Some applications read a Kerberos password over the network (ideally over a secure channel), which they then verify against the KDC. While this technique may be the only practical way to integrate Kerberos into some existing legacy systems, its use is contrary to the original design goals of Kerberos.</p> <p>The function <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_password()</span></tt> will get initial credentials for a client using a password. An application that needs to verify the credentials can call <tt class="xref c c-func docutils literal"><span class="pre">krb5_verify_init_creds()</span></tt>. Here is an example of code to obtain and verify TGT credentials, given strings <em>princname</em> and <em>password</em> for the client principal name and password:</p> <div class="highlight-python"><pre>krb5_error_code ret; krb5_creds creds; krb5_principal client_princ = NULL; memset(&creds, 0, sizeof(creds)); ret = krb5_parse_name(context, princname, &client_princ); if (ret) goto cleanup; ret = krb5_get_init_creds_password(context, &creds, client_princ, password, NULL, NULL, 0, NULL, NULL); if (ret) goto cleanup; ret = krb5_verify_init_creds(context, &creds, NULL, NULL, NULL, NULL); cleanup: krb5_free_principal(context, client_princ); krb5_free_cred_contents(context, &creds); return ret;</pre> </div> <div class="section" id="options-for-get-init-creds"> <h2>Options for get_init_creds<a class="headerlink" href="#options-for-get-init-creds" title="Permalink to this headline">¶</a></h2> <p>The function <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_password()</span></tt> takes an options parameter (which can be a null pointer). Use the function <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_opt_alloc()</span></tt> to allocate an options structure, and <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_opt_free()</span></tt> to free it. For example:</p> <div class="highlight-python"><pre>krb5_error_code ret; krb5_get_init_creds_opt *opt = NULL; krb5_creds creds; memset(&creds, 0, sizeof(creds)); ret = krb5_get_init_creds_opt_alloc(context, &opt); if (ret) goto cleanup; krb5_get_init_creds_opt_set_tkt_life(opt, 24 * 60 * 60); ret = krb5_get_init_creds_password(context, &creds, client_princ, password, NULL, NULL, 0, NULL, opt); if (ret) goto cleanup; cleanup: krb5_get_init_creds_opt_free(context, opt); krb5_free_cred_contents(context, &creds); return ret;</pre> </div> </div> <div class="section" id="getting-anonymous-credentials"> <h2>Getting anonymous credentials<a class="headerlink" href="#getting-anonymous-credentials" title="Permalink to this headline">¶</a></h2> <p>As of release 1.8, it is possible to obtain fully anonymous or partially anonymous (realm-exposed) credentials, if the KDC supports it. The MIT KDC supports issuing fully anonymous credentials as of release 1.8 if configured appropriately (see <a class="reference internal" href="../admin/pkinit.html#anonymous-pkinit"><em>Anonymous PKINIT</em></a>), but does not support issuing realm-exposed anonymous credentials at this time.</p> <p>To obtain fully anonymous credentials, call <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_opt_set_anonymous()</span></tt> on the options structure to set the anonymous flag, and specify a client principal with the KDC’s realm and a single empty data component (the principal obtained by parsing <tt class="docutils literal"><span class="pre">@</span></tt><em>realmname</em>). Authentication will take place using anonymous PKINIT; if successful, the client principal of the resulting tickets will be <tt class="docutils literal"><span class="pre">WELLKNOWN/ANONYMOUS@WELLKNOWN:ANONYMOUS</span></tt>. Here is an example:</p> <div class="highlight-python"><pre>krb5_get_init_creds_opt_set_anonymous(opt, 1); ret = krb5_build_principal(context, &client_princ, strlen(myrealm), myrealm, "", (char *)NULL); if (ret) goto cleanup; ret = krb5_get_init_creds_password(context, &creds, client_princ, password, NULL, NULL, 0, NULL, opt); if (ret) goto cleanup;</pre> </div> <p>To obtain realm-exposed anonymous credentials, set the anonymous flag on the options structure as above, but specify a normal client principal in order to prove membership in the realm. Authentication will take place as it normally does; if successful, the client principal of the resulting tickets will be <tt class="docutils literal"><span class="pre">WELLKNOWN/ANONYMOUS@</span></tt><em>realmname</em>.</p> </div> <div class="section" id="user-interaction"> <h2>User interaction<a class="headerlink" href="#user-interaction" title="Permalink to this headline">¶</a></h2> <p>Authenticating a user usually requires the entry of secret information, such as a password. A password can be supplied directly to <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_password()</span></tt> via the <em>password</em> parameter, or the application can supply prompter and/or responder callbacks instead. If callbacks are used, the user can also be queried for other secret information such as a PIN, informed of impending password expiration, or prompted to change a password which has expired.</p> <div class="section" id="prompter-callback"> <h3>Prompter callback<a class="headerlink" href="#prompter-callback" title="Permalink to this headline">¶</a></h3> <p>A prompter callback can be specified via the <em>prompter</em> and <em>data</em> parameters to <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_password()</span></tt>. The prompter will be invoked each time the krb5 library has a question to ask or information to present. When the prompter callback is invoked, the <em>banner</em> argument (if not null) is intended to be displayed to the user, and the questions to be answered are specified in the <em>prompts</em> array. Each prompt contains a text question in the <em>prompt</em> field, a <em>hidden</em> bit to indicate whether the answer should be hidden from display, and a storage area for the answer in the <em>reply</em> field. The callback should fill in each question’s <tt class="docutils literal"><span class="pre">reply->data</span></tt> with the answer, up to a maximum number of <tt class="docutils literal"><span class="pre">reply->length</span></tt> bytes, and then reset <tt class="docutils literal"><span class="pre">reply->length</span></tt> to the length of the answer.</p> <p>A prompter callback can call <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_prompt_types()</span></tt> to get an array of type constants corresponding to the prompts, to get programmatic information about the semantic meaning of the questions. <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_prompt_types()</span></tt> may return a null pointer if no prompt type information is available.</p> <p>Text-based applications can use a built-in text prompter implementation by supplying <tt class="xref c c-func docutils literal"><span class="pre">krb5_prompter_posix()</span></tt> as the <em>prompter</em> parameter and a null pointer as the <em>data</em> parameter. For example:</p> <div class="highlight-python"><pre>ret = krb5_get_init_creds_password(context, &creds, client_princ, NULL, krb5_prompter_posix, NULL, 0, NULL, NULL);</pre> </div> </div> <div class="section" id="responder-callback"> <h3>Responder callback<a class="headerlink" href="#responder-callback" title="Permalink to this headline">¶</a></h3> <p>A responder callback can be specified through the init_creds options using the <tt class="xref c c-func docutils literal"><span class="pre">krb5_get_init_creds_opt_set_responder()</span></tt> function. Responder callbacks can present a more sophisticated user interface for authentication secrets. The responder callback is usually invoked only once per authentication, with a list of questions produced by all of the allowed preauthentication mechanisms.</p> <p>When the responder callback is invoked, the <em>rctx</em> argument can be accessed to obtain the list of questions and to answer them. The <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_list_questions()</span></tt> function retrieves an array of question types. For each question type, the <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_get_challenge()</span></tt> function retrieves additional information about the question, if applicable, and the <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_set_answer()</span></tt> function sets the answer.</p> <p>Responder question types, challenges, and answers are UTF-8 strings. The question type is a well-known string; the meaning of the challenge and answer depend on the question type. If an application does not understand a question type, it cannot interpret the challenge or provide an answer. Failing to answer a question typically results in the prompter callback being used as a fallback.</p> <div class="section" id="password-question"> <h4>Password question<a class="headerlink" href="#password-question" title="Permalink to this headline">¶</a></h4> <p>The <tt class="xref c c-macro docutils literal"><span class="pre">KRB5_RESPONDER_QUESTION_PASSWORD</span></tt> (or <tt class="docutils literal"><span class="pre">"password"</span></tt>) question type requests the user’s password. This question does not have a challenge, and the response is simply the password string.</p> </div> <div class="section" id="one-time-password-question"> <h4>One-time password question<a class="headerlink" href="#one-time-password-question" title="Permalink to this headline">¶</a></h4> <p>The <tt class="xref c c-macro docutils literal"><span class="pre">KRB5_RESPONDER_QUESTION_OTP</span></tt> (or <tt class="docutils literal"><span class="pre">"otp"</span></tt>) question type requests a choice among one-time password tokens and the PIN and value for the chosen token. The challenge and answer are JSON-encoded strings, but an application can use convenience functions to avoid doing any JSON processing itself.</p> <p>The <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_otp_get_challenge()</span></tt> function decodes the challenge into a krb5_responder_otp_challenge structure. The <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_otp_set_answer()</span></tt> function selects one of the token information elements from the challenge and supplies the value and pin for that token.</p> </div> <div class="section" id="pkinit-password-or-pin-question"> <h4>PKINIT password or PIN question<a class="headerlink" href="#pkinit-password-or-pin-question" title="Permalink to this headline">¶</a></h4> <p>The <tt class="xref c c-macro docutils literal"><span class="pre">KRB5_RESPONDER_QUESTION_PKINIT</span></tt> (or <tt class="docutils literal"><span class="pre">"pkinit"</span></tt>) question type requests PINs for hardware devices and/or passwords for encrypted credentials which are stored on disk, potentially also supplying information about the state of the hardware devices. The challenge and answer are JSON-encoded strings, but an application can use convenience functions to avoid doing any JSON processing itself.</p> <p>The <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_pkinit_get_challenge()</span></tt> function decodes the challenges into a krb5_responder_pkinit_challenge structure. The <tt class="xref c c-func docutils literal"><span class="pre">krb5_responder_pkinit_set_answer()</span></tt> function can be used to supply the PIN or password for a particular client credential, and can be called multiple times.</p> </div> <div class="section" id="example"> <h4>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h4> <p>Here is an example of using a responder callback:</p> <div class="highlight-python"><pre>static krb5_error_code my_responder(krb5_context context, void *data, krb5_responder_context rctx) { krb5_error_code ret; krb5_responder_otp_challenge *chl; if (krb5_responder_get_challenge(context, rctx, KRB5_RESPONDER_QUESTION_PASSWORD)) { ret = krb5_responder_set_answer(context, rctx, KRB5_RESPONDER_QUESTION_PASSWORD, "open sesame"); if (ret) return ret; } ret = krb5_responder_otp_get_challenge(context, rctx, &chl); if (ret == 0 && chl != NULL) { ret = krb5_responder_otp_set_answer(context, rctx, 0, "1234", NULL); krb5_responder_otp_challenge_free(context, rctx, chl); if (ret) return ret; } return 0; } static krb5_error_code get_creds(krb5_context context, krb5_principal client_princ) { krb5_error_code ret; krb5_get_init_creds_opt *opt = NULL; krb5_creds creds; memset(&creds, 0, sizeof(creds)); ret = krb5_get_init_creds_opt_alloc(context, &opt); if (ret) goto cleanup; ret = krb5_get_init_creds_opt_set_responder(context, opt, my_responder, NULL); if (ret) goto cleanup; ret = krb5_get_init_creds_password(context, &creds, client_princ, NULL, NULL, NULL, 0, NULL, opt); cleanup: krb5_get_init_creds_opt_free(context, opt); krb5_free_cred_contents(context, &creds); return ret; }</pre> </div> </div> </div> </div> <div class="section" id="verifying-initial-credentials"> <h2>Verifying initial credentials<a class="headerlink" href="#verifying-initial-credentials" title="Permalink to this headline">¶</a></h2> <p>Use the function <tt class="xref c c-func docutils literal"><span class="pre">krb5_verify_init_creds()</span></tt> to verify initial credentials. It takes an options structure (which can be a null pointer). Use <tt class="xref c c-func docutils literal"><span class="pre">krb5_verify_init_creds_opt_init()</span></tt> to initialize the caller-allocated options structure, and <tt class="xref c c-func docutils literal"><span class="pre">krb5_verify_init_creds_opt_set_ap_req_nofail()</span></tt> to set the “nofail” option. For example:</p> <div class="highlight-python"><pre>krb5_verify_init_creds_opt vopt; krb5_verify_init_creds_opt_init(&vopt); krb5_verify_init_creds_opt_set_ap_req_nofail(&vopt, 1); ret = krb5_verify_init_creds(context, &creds, NULL, NULL, NULL, &vopt);</pre> </div> <p>The confusingly named “nofail” option, when set, means that the verification must actually succeed in order for <tt class="xref c c-func docutils literal"><span class="pre">krb5_verify_init_creds()</span></tt> to indicate success. The default state of this option (cleared) means that if there is no key material available to verify the user credentials, the verification will succeed anyway. (The default can be changed by a configuration file setting.)</p> <p>This accommodates a use case where a large number of unkeyed shared desktop workstations need to allow users to log in using Kerberos. The security risks from this practice are mitigated by the absence of valuable state on the shared workstations—any valuable resources that the users would access reside on networked servers.</p> </div> </div> </div> </div> </div> </div> <div class="sidebar"> <h2>On this page</h2> <ul> <li><a class="reference internal" href="#">Initial credentials</a><ul> <li><a class="reference internal" href="#options-for-get-init-creds">Options for get_init_creds</a></li> <li><a class="reference internal" href="#getting-anonymous-credentials">Getting anonymous credentials</a></li> <li><a class="reference internal" href="#user-interaction">User interaction</a><ul> <li><a class="reference internal" href="#prompter-callback">Prompter callback</a></li> <li><a class="reference internal" href="#responder-callback">Responder callback</a><ul> <li><a class="reference internal" href="#password-question">Password question</a></li> <li><a class="reference internal" href="#one-time-password-question">One-time password question</a></li> <li><a class="reference internal" href="#pkinit-password-or-pin-question">PKINIT password or PIN question</a></li> <li><a class="reference internal" href="#example">Example</a></li> </ul> </li> </ul> </li> <li><a class="reference internal" href="#verifying-initial-credentials">Verifying initial credentials</a></li> </ul> </li> </ul> <br/> <h2>Table of contents</h2> <ul class="current"> <li class="toctree-l1"><a class="reference internal" href="../user/index.html">For users</a></li> <li class="toctree-l1"><a class="reference internal" href="../admin/index.html">For administrators</a></li> <li class="toctree-l1 current"><a class="reference internal" href="index.html">For application developers</a><ul class="current"> <li class="toctree-l2"><a class="reference internal" href="gssapi.html">Developing with GSSAPI</a></li> <li class="toctree-l2"><a class="reference internal" href="y2038.html">Year 2038 considerations for uses of krb5_timestamp</a></li> <li class="toctree-l2"><a class="reference internal" href="h5l_mit_apidiff.html">Differences between Heimdal and MIT Kerberos API</a></li> <li class="toctree-l2 current"><a class="current reference internal" href="">Initial credentials</a><ul class="simple"> </ul> </li> <li class="toctree-l2"><a class="reference internal" href="princ_handle.html">Principal manipulation and parsing</a></li> <li class="toctree-l2"><a class="reference internal" href="refs/index.html">Complete reference - API and datatypes</a></li> </ul> </li> <li class="toctree-l1"><a class="reference internal" href="../plugindev/index.html">For plugin module developers</a></li> <li class="toctree-l1"><a class="reference internal" href="../build/index.html">Building Kerberos V5</a></li> <li class="toctree-l1"><a class="reference internal" href="../basic/index.html">Kerberos V5 concepts</a></li> <li class="toctree-l1"><a class="reference internal" href="../formats/index.html">Protocols and file formats</a></li> <li class="toctree-l1"><a class="reference internal" href="../mitK5features.html">MIT Kerberos features</a></li> <li class="toctree-l1"><a class="reference internal" href="../build_this.html">How to build this documentation from the source</a></li> <li class="toctree-l1"><a class="reference internal" href="../about.html">Contributing to the MIT Kerberos Documentation</a></li> <li class="toctree-l1"><a class="reference internal" href="../resources.html">Resources</a></li> </ul> <br/> <h4><a href="../index.html">Full Table of Contents</a></h4> <h4>Search</h4> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <div class="clearer"></div> </div> </div> <div class="footer-wrapper"> <div class="footer" > <div class="right" ><i>Release: 1.15.1</i><br /> © <a href="../copyright.html">Copyright</a> 1985-2017, MIT. </div> <div class="left"> <a href="../index.html" title="Full Table of Contents" >Contents</a> | <a href="h5l_mit_apidiff.html" title="Differences between Heimdal and MIT Kerberos API" >previous</a> | <a href="princ_handle.html" title="Principal manipulation and parsing" >next</a> | <a href="../genindex.html" title="General Index" >index</a> | <a href="../search.html" title="Enter search criteria" >Search</a> | <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__Initial credentials">feedback</a> </div> </div> </div> </body> </html>