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| 4 | <title>HOWTO samhain+GnuPG</title>
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| 119 |
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| 120 | <body>
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| 121 | <div class="body">
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| 122 | <p style="text-align: center; background: #ccc; border: 1px solid #2d5588;"><a
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| 123 | style="text-decoration: none;"
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| 124 | href="http://www.la-samhna.de/samhain/">samhain file integrity
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| 125 | scanner</a> | <a style="text-decoration: none;"
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| 126 | href="http://www.la-samhna.de/samhain/s_documentation.html">online
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| 127 | documentation</a></p>
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| 128 | <br><center>
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| 129 | <h1>Using samhain with GnuPG</h1>
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| 130 | </center>
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| 131 | <br>
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| 132 | <hr>
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| 133 | <p>
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| 134 | This document aims to explain how to use samhain with <b>signed configuration
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| 135 | and database files</b> which are checked by invoking GnuPG.
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| 136 | </p>
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| 137 | <h2>Introduction</h2>
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| 138 | <p>
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| 139 | Samhain can be compiled to recognize PGP signatures on configuration and
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| 140 | database files and to invoke GnuPG in order to check such signatures.
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| 141 | (<b>Note:</b> while the application usually is referred to as <i>GnuPG</i>,
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| 142 | the executable itself is called <i>gpg</i>).
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| 143 | </p>
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| 144 | <p>
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| 145 | If samhain is compiled with this option, then
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| 146 | </p>
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| 147 |
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| 148 | <ol>
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| 149 | <li>
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| 150 | both the <i>configuration file</i>
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| 151 | and the <i>file signature database</i> must be signed, and
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| 152 | </li>
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| 153 | <li>
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| 154 | for both files the signatures must verify correctly,
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| 155 | </li>
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| 156 | <li>
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| 157 | otherwise samhain will abort.
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| 158 | </li>
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| 159 | </ol>
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| 160 |
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| 161 |
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| 162 | <h2>Prerequisites</h2>
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| 163 | <ul>
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| 164 | <li>
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| 165 | <p>
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| 166 | Obviously you need <i>gpg</i> (GnuPG), and you must
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| 167 | have created a key pair with:
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| 168 | </p><p>
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| 169 | <tt> gpg --gen-key</tt>
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| 170 | </p><p>
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| 171 | (it does not really matter which type of key, the defaults are ok).
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| 172 | </p><p>
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| 173 | GnuPG uses a public-key algorithm: the key pair consists of
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| 174 | </p>
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| 175 | <ul>
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| 176 | <li>
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| 177 | a <i>secret key</i> that is
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| 178 | used for signing and stored in <b>~user/.gnupg/secring.gpg</b>, and
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| 179 | </li><li>
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| 180 | a <i>public key</i> used for verifying the signature, and stored in
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| 181 | <b>~user/.gnupg/pubring.gpg</b>.
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| 182 | </li>
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| 183 | </ul>
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| 184 | <p>
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| 185 | The secret key obviously should be
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| 186 | kept secret, while the public key can be published.
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| 187 | </p>
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| 188 | </li>
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| 189 | <li>
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| 190 | <p>
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| 191 | You need to compile samhain with support for GnuPG:
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| 192 | </p><p>
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| 193 | <tt> ./configure --with-gpg=/path/to/gpg [more options]</tt>
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| 194 | </p><p>
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| 195 | </li>
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| 196 | </ul>
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| 197 |
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| 198 | <p>
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| 199 | <b>Note 1:</b> If compiled with support for GnuPG,
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| 200 | the TIGER192 checksum of the gpg
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| 201 | executable will be compiled into samhain, and the gpg executable will
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| 202 | be checksummed (to verify its integrity) before invoking it. If you
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| 203 | don't like this, you should add the <i>configure</i> option:
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| 204 | </p><p>
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| 205 | <tt> --with-checksum=no</tt>
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| 206 | </p>
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| 207 | <div class="warnblock">
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| 208 | <p>
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| 209 | Compiling in the GnuPG checksum will tie the samhain executable to
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| 210 | the gpg executable. If you upgrade GnuPG, you will need to re-compile
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| 211 | samhain. If you don't like this, use <tt>'--with-checksum=no'</tt>.
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| 212 | </p>
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| 213 | </div>
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| 214 | <p>
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| 215 | <b>Note 2:</b> The mere fact that the signature
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| 216 | is correct does not prove that it has been signed by <i>you</i> with
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| 217 | <i>your</i> key - it just proves that it has been signed by <i>somebody</i>.
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| 218 | Samhain can optionally check the <i>fingerprint</i> of the key that has been
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| 219 | used to sign the files, to verify that <i>your</i> key has been used
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| 220 | to sign the file(s). To enable this, use the <i>configure</i> option
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| 221 | </p><p>
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| 222 | <tt> --with-fingerprint=FINGERPRINT</tt>
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| 223 | </p><p>
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| 224 | where FINGERPRINT is the hexadecimal fingerprint of the key as listed
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| 225 | with
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| 226 | </p><p>
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| 227 | <tt> gpg --fingerprint</tt>
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| 228 | </p>
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| 229 |
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| 230 | <h3>Example</h3>
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| 231 |
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| 232 | <pre style="background-color:#DDDDDD; color:#000000">
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| 233 |
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| 234 | rainer$ gpg --fingerprint rainer
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| 235 | pub 1024D/0F571F6C 1999-10-31 Rainer Wichmann
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| 236 | Key fingerprint = EF6C EF54 701A 0AFD B86A F4C3 1AAD 26C8 0F57 1F6C
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| 237 | uid Rainer Wichmann
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| 238 | sub 1024g/9DACAC30 1999-10-31
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| 239 |
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| 240 | rainer$ which gpg
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| 241 | /usr/bin/gpg
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| 242 |
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| 243 | rainer$ ./configure --with-gpg=/usr/bin/gpg --with-fingerprint=EF6CEF54701A0AFDB86AF4C31AAD26C80F571F6C
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| 244 |
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| 245 | </pre>
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| 246 |
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| 247 | <h2>Signing the files</h2>
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| 248 | <p>
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| 249 | The <i>configuration file</i> and the
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| 250 | <i>file signature database</i>
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| 251 | (created by running <tt>samhain -t init</tt>) must be signed manually
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| 252 | using the command:
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| 253 | </p><p>
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| 254 | <tt> gpg -a --clearsign --not-dash-escaped /etc/samhainrc</tt><br/>
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| 255 | <tt> mv /etc/samhainrc.asc /etc/samhainrc</tt>
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| 256 | </p><p>
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| 257 | <i>Gpg</i> will create a <i>signed copy</i> of the file,
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| 258 | named <i>file.asc</i>.
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| 259 | You need to <b>rename</b> (<tt>cp/mv</tt>) this signed copy
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| 260 | to the original filename.
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| 261 | After signing the configuration file, you can initialize the database
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| 262 | and sign it likewise.
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| 263 | </p>
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| 264 | <p>
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| 265 | <b>Note 1:</b> The installation script will ask you to
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| 266 | sign the <i>configuration file</i> upon installation.
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| 267 | </p><p>
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| 268 | <b>Note 2:</b> The <i>gpg</i> option <tt>--not-dash-escaped</tt>
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| 269 | does not harm if used with the
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| 270 | <i>configuration file</i>, but is only required for the
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| 271 | <i>file signature database</i>.
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| 272 | </p>
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| 273 |
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| 274 | <h3>TIP</h3>
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| 275 | <p>
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| 276 | In the subdirectory <tt>scripts/</tt> of the source directory you will find
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| 277 | a Perl script <b>samhainadmin.pl</b> to facilitate some
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| 278 | tasks related to the administration of signed configuration and
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| 279 | database files (e.g. examine/create/remove signatures).
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| 280 | Use with <i>--help</i> to get usage
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| 281 | information.
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| 282 | </p>
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| 283 |
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| 284 | <h3>CAVEAT</h3>
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| 285 | <p>
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| 286 | When signing, the option <i>--not-dash-escaped</i> is
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| 287 | recommended, because otherwise the database might get corrupted.
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| 288 | However, this implies that after a database update,
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| 289 | you <i>must</i> remove the old signature first, before
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| 290 | re-signing the database. Without 'dash escaping',
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| 291 | gpg will not properly handle the old signature.
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| 292 | See the tip just above.
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| 293 | </p>
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| 294 |
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| 295 | <h3>Example</h3>
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| 296 |
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| 297 | <pre style="background-color:#DDDDDD; color:#000000">
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| 298 |
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| 299 | root# gpg -a --clearsign --not-dash-escaped /etc/samhainrc
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| 300 |
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| 301 | You need a passphrase to unlock the secret key for
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| 302 | user: "Rainer Wichmann"
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| 303 | 1024-bit DSA key, ID 0F571F6C, created 1999-10-31
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| 304 |
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| 305 | root# mv /etc/samhainrc.asc /etc/samhainrc
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| 306 | root# samhain -t init
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| 307 | root# gpg -a --clearsign --not-dash-escaped /var/lib/samhain/samhain_file
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| 308 |
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| 309 | You need a passphrase to unlock the secret key for
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| 310 | user: "Rainer Wichmann"
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| 311 | 1024-bit DSA key, ID 0F571F6C, created 1999-10-31
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| 312 |
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| 313 | root# mv /var/lib/samhain/samhain_file.asc /var/lib/samhain/samhain_file
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| 314 | root# samhain -D -t check
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| 315 |
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| 316 | </pre>
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| 317 |
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| 318 | <h2>Make samhain verify the signature</h2>
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| 319 | <p>
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| 320 | This is the part where some people run into problems. The point is,
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| 321 | when <i>gpg</i> is invoked by samhain, it must <i>find the public key</i>
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| 322 | needed for verification. <i>Gpg</i> expects public keys in a file
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| 323 | located at <b>~user/.gnupg/pubring.gpg</b> where <b>~user</b>
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| 324 | is the home directory of the user as that <i>gpg</i> is running.
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| 325 | </p><p>
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| 326 | It is therefore <i>crucial</i> to include the public key corresponding
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| 327 | to te secret key used for signing into the correct <b>pubring.gpg</b>
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| 328 | file (this file can hold many public keys, e.g. of people sending you
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| 329 | emails signed by them).
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| 330 | </p><p>
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| [553] | 331 | So which is the correct file? Here we have to consider two separate
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| [1] | 332 | cases:
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| 333 | </p>
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| 334 | <ol>
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| 335 | <li>The client (or standalone) samhain daemon runs with UID 0 (i.e. root),
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| 336 | thus the public key must be in <b>~root/.gnupg/pubring.gpg</b>
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| 337 | </li>
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| 338 | <li>
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| 339 | The server (yule) <i>always</i> drops root privileges (if started with), and
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| 340 | runs as a <i>non-root user</i>. The username to use is compiled in,
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| 341 | either with the <i>configure</i> option <tt>--enable-identity=USER</tt>,
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| 342 | or by default as determined by <i>configure</i> (the first existing user
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| 343 | out of the list <i>yule, daemon, nobody</i>). Thus, the public key
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| 344 | must be in <b>~root/.gnupg/pubring.gpg</b> (for startup) <i>and</i>
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| 345 | in <b>~non_root_user/.gnupg/pubring.gpg</b> (for reload with SIGHUP).
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| 346 | </li>
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| 347 | </ol>
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| 348 | <p>
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| 349 | To import a public key into the public
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| 350 | keyring (pubring.gpg) of another user, you can do:
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| 351 | </p><p>
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| 352 | <tt> gpg --export KEY-ID > filename</tt><br>
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| 353 | <tt> su another_user</tt><br>
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| 354 | <tt> gpg --import filename</tt>
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| 355 | </p>
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| 356 | <p>
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| 357 | <b>Note:</b> samhain will invoke <i>gpg</i> with the options:
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| 358 | </p><p>
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| 359 | <tt> --status-fd 1 --verify --homedir /homedir/.gnupg --no-tty -</tt>
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| 360 | </p><p>
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| 361 | and pipe the configuration/database file into <i>gpg</i>, similar to:
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| 362 | </p><p>
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| 363 | <tt>cat filename | /usr/bin/gpg --status-fd 1 --verify --homedir /root/.gnupg --no-tty -</tt>
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| 364 | </p><p>
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| 365 | (of course samhain does not invoke cat, or the shell; the example above
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| 366 | just shows how to do the same from the shell command prompt).
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| 367 | </p>
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| 368 |
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| 369 | <h3>Example for signature check</h3>
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| 370 | <p>
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| 371 | If you want to check the signature the same way samhain does, it should look
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| 372 | like (note the GOODSIG and VALIDSIG keywords in the output):
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| 373 | </p>
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| 374 | <pre style="background-color:#DDDDDD; color:#000000">
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| 375 |
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| 376 | root# cat /etc/samhainrc | gpg --status-fd 1 --verify --homedir /root/.gnupg --no-tty -
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| 377 | gpg: Signature made Sat Mar 15 16:08:21 2003 CET using DSA key ID 0F571F6C
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| 378 | [GNUPG:] SIG_ID 9hQvRhgjWLqyFzVOHi2b0uDmBFo 2003-03-15 1047740901
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| 379 | [GNUPG:] GOODSIG 1AAD26C80F571F6C Rainer Wichmann
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| 380 | gpg: Good signature from "Rainer Wichmann"
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| 381 | gpg: aka "Rainer Wichmann"
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| 382 | [GNUPG:] VALIDSIG EF6CEF54701A0AFDB86AF4C31AAD26C80F571F6C 2003-03-15 1047740901
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| 383 | [GNUPG:] TRUST_ULTIMATE
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| 384 |
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| 385 | </pre>
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| 386 |
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| 387 | <h2>Troubleshooting</h2>
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| 388 | <p>
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| 389 | First and foremost, run samhain (or yule) from the command line, in non-daemon
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| 390 | mode, and with the command-line option <tt>-p debug</tt> for debug-level
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| 391 | output. This will print
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| 392 | descriptive information on setup errors and/or relevant output from
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| 393 | the GnuPG subprocess.
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| 394 | </p>
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| 395 | <p>
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| 396 | Output from the GnuPG subprocess is marked by <b>[GNUPG:]</b>, and
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| 397 | may show the following errors:
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| 398 | </p>
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| 399 |
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| 400 | <ul>
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| 401 | <li><b>ERRSIG</b> and/or <b>NO_PUBKEY</b> indicates that gpg did not find
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| 402 | the public key to verify the signature. You should import that key
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| 403 | into the keyrings of root and (for yule additionaly) the yule user.
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| 404 | </li>
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| 405 | <li><b>BADSIG</b> indicates that the public key was found by gpg, but
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| 406 | the signature is invalid. Either the file has been modified after
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| 407 | signing, or a previous signature has not been removed.
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| 408 | </li>
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| 409 | <li><b>NODATA</b> indicates that there is no signed data, i.e. the
|
|---|
| 410 | configuration or database file is not signed at all.
|
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| 411 | </li>
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| 412 | </ul>
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| 413 | </div>
|
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| 414 | </body>
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| 415 | </html>
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