1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3 <head> 4 <meta http-equiv="content-type" content="text/html;charset=iso-8859-1"> 5 <meta name="generator" content="HTML Tidy, see www.w3.org"> 6 <title>Authentication Support</title> 7 <!-- Changed by: Harlan Stenn, 24-Jul-2018 --> 8 <link href="scripts/style.css" type="text/css" rel="stylesheet"> 9 <style type="text/css"> 10 <!-- 11 <style1 { 12 color: #FF0000; 13 font-weight: bold; 14 } 15 .style1 {color: #FF0000} 16 --> 17 </style> 18 </head> 19 <body> 20 <h3>Authentication Support</h3> 21 <img src="pic/alice44.gif" alt="gif" align="left"><a href="http://www.eecis.udel.edu/%7emills/pictures.html">from <i>Alice's Adventures in Wonderland</i>, Lewis Carroll</a> 22 <p>Our resident cryptographer; now you see him, now you don't.</p> 23 <p>Last update: 24 <!-- #BeginDate format:En2m -->24-Jul-2018 09:12<!-- #EndDate --> 25 UTC</p> 26 <br clear="left"> 27 <h4>Related Links</h4> 28 <script type="text/javascript" language="javascript" src="scripts/hand.txt"></script> 29 <script type="text/javascript" language="javascript" src="scripts/authopt.txt"></script> 30 <h4>Table of Contents</h4> 31 <ul> 32 <li class="inline"><a href="#auth">Introduction</a></li> 33 <li class="inline"><a href="#symm">Symmetric Key Cryptography</a></li> 34 <li class="inline"><a href="#windows">Microsoft Windows Authentication</a></li> 35 <li class="inline"><a href="#pub">Public Key Cryptography</a></li> 36 </ul> 37 <hr> 38 <h4 id="auth">Introduction</h4> 39 <p>This page describes the various cryptographic authentication 40 provisions in NTPv4. Authentication support allows the NTP client to 41 verify that servers are in fact known and trusted and not intruders 42 intending accidentally or intentionally to masquerade as a legitimate 43 server. A detailed discussion of the NTP multi-layer security model 44 and vulnerability analysis is in the white 45 paper <a href="http://www.eecis.udel.edu/~mills/security.html">NTP 46 Security Analysis</a>.</p> 47 <p>The NTPv3 specification (RFC-1305) defined an authentication scheme 48 properly described as <em>symmetric key cryptography</em>. It used 49 the Data Encryption Standard (DES) algorithm operating in cipher-block 50 chaining (CBC) mode. Subsequently, this algorithm was replaced by the 51 RSA Message Digest 5 (MD5) algorithm commonly called keyed-MD5. 52 Either algorithm computes a message digest or one-way hash which can 53 be used to verify the client has the same message digest as the 54 server. The MD5 message digest algorithm is included in the 55 distribution, so without further cryptographic support, the 56 distribution can be freely exported.</p> 57 <p>If the OpenSSL cryptographic library is installed prior to building 58 the distribution, all message digest algorithms included in the 59 library may be used, including SHA and SHA1. However, if conformance 60 to FIPS 140-2 is required, only a limited subset of these algorithms 61 can be used. This library is available 62 from <a href="http://www.openssl.org">http://www.openssl.org</a> and 63 can be installed using the procedures outlined in 64 the <a href="build.html">Building and Installing the Distribution</a> 65 page. Once installed, the configure and build process automatically 66 detects the library and links the library routines required.</p> 67 <p>In addition to the symmetric key algorithms, this distribution 68 includes support for the Autokey public key algorithms and protocol 69 specified in RFC-5906 "Network Time Protocol Version 4: Autokey 70 Specification". This support is available only if the OpenSSL 71 library has been installed and the <tt>--enable-autokey</tt> option is 72 used when the distribution is built.</p> 73 <p> Public key cryptography is generally considered more secure than 74 symmetric key cryptography, since the security is based on private and 75 public values which are generated by each participant and where the 76 private value is never revealed. Autokey uses X.509 public 77 certificates, which can be produced by commercial services, the 78 OpenSSL application program, or 79 the <a href="keygen.html"><tt>ntp-keygen</tt></a> utility program in 80 the NTP software distribution.</p> 81 <p>Note that according to US law, NTP binaries including OpenSSL library 82 components, including the OpenSSL library itself, cannot be exported 83 outside the US without license from the US Department of Commerce. 84 Builders outside the US are advised to obtain the OpenSSL library 85 directly from OpenSSL, which is outside the US, and build outside the 86 US.</p> 87 <p>Authentication is configured separately for each association using 88 the <tt>key</tt> or <tt>autokey</tt> option of the <tt>server</tt> 89 configuration command, as described in 90 the <a href="confopt.html">Server Options</a> page. 91 The <a href="keygen.html">ntp-keygen</a> page describes the files 92 required for the various authentication schemes. Further details are 93 in the briefings, papers and reports at the NTP project page linked 94 from <a href="http://www.ntp.org">www.ntp.org</a>.</p> 95 <p>By default, the client sends non-authenticated packets and the server 96 responds with non-authenticated packets. If the client sends 97 authenticated packets, the server responds with authenticated packets 98 if correct, or a crypto-NAK packet if not. In the case of unsolicited 99 packets which might consume significant resources, such as broadcast 100 or symmetric mode packets, authentication is required, unless 101 overridden by a <tt>disable auth</tt> command. In the current climate 102 of targeted broadcast or "letterbomb" attacks, defeating 103 this requirement would be decidedly dangerous. In any case, 104 the <tt>notrust </tt>flag, described on 105 the <a href="authopt.html">Access Control Options</a> page, can be 106 used to disable access to all but correctly authenticated clients.</p> 107 <h4 id="symm">Symmetric Key Cryptography</h4> 108 <p>The original NTPv3 specification (RFC-1305), as well as the current 109 NTPv4 specification (RFC-5905), allows any one of possibly 65,535 110 message digest keys (excluding zero), each distinguished by a 32-bit 111 key ID, to authenticate an association. The servers and clients 112 involved must agree on the key ID, key type and key to authenticate 113 NTP packets.</p> 114 <p>The message digest is a cryptographic hash computed by an algorithm 115 such as MD5, SHA, or AES-128 CMAC. When authentication is specified, 116 a message authentication code (MAC) is appended to the NTP packet 117 header. The MAC consists of a 32-bit key identifier (key ID) followed 118 by a 128- or 160-bit message digest. The algorithm computes the 119 digest as the hash of a 128- or 160- bit message digest key 120 concatenated with the NTP packet header fields with the exception of 121 the MAC. On transmit, the message digest is computed and inserted in 122 the MAC. On receive, the message digest is computed and compared with 123 the MAC. The packet is accepted only if the two MACs are identical. 124 If a discrepancy is found by the client, the client ignores the 125 packet, but raises an alarm. If this happens at the server, the 126 server returns a special message called a <em>crypto-NAK</em>. Since 127 the crypto-NAK is protected by the loopback test, an intruder cannot 128 disrupt the protocol by sending a bogus crypto-NAK.</p> 129 <p>Keys and related information are specified in a keys file, which must 130 be distributed and stored using secure means beyond the scope of the 131 NTP protocol itself. Besides the keys used for ordinary NTP 132 associations, additional keys can be used as passwords for 133 the <tt><a href="ntpq.html">ntpq</a></tt> 134 and <tt><a href="ntpdc.html">ntpdc</a></tt> utility programs. 135 Ordinarily, the <tt>ntp.keys</tt> file is generated by 136 the <tt><a href="keygen.html">ntp-keygen</a></tt> program, but it can 137 be constructed and edited using an ordinary text editor.</p> 138 <p> Each line of the keys file consists of three or four fields: a 139 key ID in the range 1 to 65,535, inclusive, a key type, a 140 message digest key consisting of a printable ASCII string up to 141 20 characters or a hex digit string with more than 20 142 characters, and an optional comma-separated list of IPs that are 143 allowed to serve time. If the OpenSSL library is installed, the 144 key type can be any message digest algorithm supported by the 145 library. If the OpenSSL library is not installed, the only 146 permitted key type is MD5.</p> 147 <table> 148 <caption style="caption-side: bottom;"> 149 Figure 1. Typical Symmetric Key File 150 </caption> 151 <tr><td style="border: 1px solid black; border-spacing: 0;"> 152 <pre style="color:grey;"> 153 # ntpkey_MD5key_bk.ntp.org.3595864945 154 # Thu Dec 12 19:22:25 2013 155 156 1 MD5 L";Nw<`.I<f4U0)247"i # MD5 key 157 2 MD5 &>l0%XXK9O'51VwV<xq~ # MD5 key 158 3 MD5 lb4zLW~d^!K:]RsD'qb6 # MD5 key 159 4 MD5 Yue:tL[+vR)M`n~bY,'? # MD5 key 160 5 MD5 B;fxlKgr/&4ZTbL6=RxA # MD5 key 161 6 MD5 4eYwa`o}3i@@V@..R9!l # MD5 key 162 7 MD5 `A.([h+;wTQ|xfi%Sn_! # MD5 key 163 8 MD5 45:V,r4]l6y^JH6"Sh?F # MD5 key 164 9 MD5 3-5vcn*6l29DS?Xdsg)* # MD5 key 165 10 MD5 2late4Me # MD5 key 166 11 SHA1 a27872d3030a9025b8446c751b4551a7629af65c # SHA1 key 167 12 SHA1 21bc3b4865dbb9e920902abdccb3e04ff97a5e74 # SHA1 key 168 13 SHA1 2b7736fe24fef5ba85ae11594132ab5d6f6daba9 # SHA1 key 169 14 SHA a5332809c8878dd3a5b918819108a111509aeceb # SHA key 170 15 MD2 2fe16c88c760ff2f16d4267e36c1aa6c926e6964 # MD2 key 171 16 MD4 b2691811dc19cfc0e2f9bcacd74213f29812183d # MD4 key 172 17 MD5 e4d6735b8bdad58ec5ffcb087300a17f7fef1f7c # MD5 key 173 18 MDC2 a8d5e2315c025bf3a79174c87fbd10477de2eabc # MDC2 key 174 19 RIPEMD160 77ca332cafb30e3cafb174dcd5b80ded7ba9b3d2 # RIPEMD160 key 175 20 AES128CMAC f92ff73eee86c1e7dc638d6489a04e4e555af878 # AES128CMAC key 176 21 MD5 sampo 10.1.2.3/24 177 </pre></td></tr></table> 178 <p>Figure 1 shows a typical symmetric keys file used by the reference 179 implementation when the OpenSSL library is installed. Each line of 180 the file contains three or four fields. The first field is an integer 181 between 1 and 65535, inclusive, representing the key identifier. The 182 second field is the digest algorithm, which in the absence of the 183 OpenSSL library must be <tt>MD5</tt>, which designates the MD5 message 184 digest algorithm. The third field is the key. The optional fourth 185 field is one or more comma-separated IPs. An IP may end with an 186 optional <tt>/subnetbits</tt> suffix, which limits the acceptance of 187 the key identifier to packets claiming to be from the described IP 188 space. In this example, for the key IDs in the range 1-10 the key is 189 interpreted as a printable ASCII string. For the key IDs in the range 190 11-20, the key is a 40-character hex digit string. In either case, 191 the key is truncated or zero-filled internally to either 128 or 160 192 bits, depending on the key type. The line can be edited later or new 193 lines can be added to change any field. The key can be changed to a 194 password, such as <tt>2late4Me</tt> for key ID 10. Note that two or 195 more keys files can be combined in any order as long as the key IDs 196 are distinct.</p> 197 <p>When <tt>ntpd</tt> is started, it reads the keys file specified by 198 the <tt>keys</tt> command and installs the keys in the key cache. 199 However, individual keys must be activated with 200 the <tt>trustedkey</tt> configuration command before use. This 201 allows, for instance, the installation of possibly several batches of 202 keys and then activating a key remotely using <tt>ntpq</tt> 203 or <tt>ntpdc</tt>. The <tt>requestkey</tt> command selects the key ID 204 used as the password for the <tt>ntpdc</tt> utility, while 205 the <tt>controlkey</tt> command selects the key ID used as the 206 password for the <tt>ntpq</tt> utility.</p> 207 <h4 id="windows">Microsoft Windows Authentication</h4> 208 <p>In addition to the above means, <tt>ntpd</tt> now supports Microsoft 209 Windows MS-SNTP authentication using Active Directory services. This 210 support was contributed by the Samba Team and is still in development. 211 It is enabled using the <tt>mssntp</tt> flag of the <tt>restrict</tt> 212 command described on the <a href="accopt.html#restrict">Access Control 213 Options</a> page. <span class="style1">Note: Potential users should 214 be aware that these services involve a TCP connection to another 215 process that could potentially block, denying services to other users. 216 Therefore, this flag should be used only for a dedicated server with 217 no clients other than MS-SNTP.</span></p> 218 <h4 id="pub">Public Key Cryptography</h4> 219 <p>See the <a href="autokey.html">Autokey Public-Key Authentication</a> 220 page.</p> 221 <hr> 222 <script type="text/javascript" language="javascript" src="scripts/footer.txt"></script> 223 </body> 224</html> 225
