S/Key 是基于单向 hash 功能的一次性密码管理方式。 为了考虑兼容性而使用了 MD4 散列， 而其他系统则使用了 MD5 和 DES-MAC。 S/Key 从 1.1.5 版之后就一直是 FreeBSD 基本系统的一部分， 包含这一特性的操作系统也日益增多。 S/Key 是 Bell Communications Research, Inc. 的注册商标。
从 FreeBSD 的 5.0 版开始， S/Key 被功能相同的 OPIE (One-time Passwords In Everything) 取代了。 OPIE 默认使用 MD5 散列。
下面将介绍三种不同的口令。 第一种是您常用的 UNIX® 风格或 Kerberos 口令; 我们在后面的章节中将称其为 ``UNIX 口令''。 第二种是使用 S/Key key 程序或 OPIE opiekey(1) 程序生成， 并为 keyinit 或 opiepasswd(1) 以及登录提示所接受的一次性口令，我们称其为 ``一次性口令''。 最后一类口令是您输入给 key/opiekey 程序 (有些时候是 keyinit/opiepasswd 程序) 用以产生一次性口令的秘密口令，我们称其为 ``秘密口令'' 或简称为 ``秘密口令''。
秘密口令和您的 UNIX 口令毫无关系， 尽管可以设置为相同的， 但不推荐这么做。 S/Key 和 OPIE 秘密口令并不像旧式的 UNIX 口令那样只能限于8位以内[1]。 您想要用多长的口令都可以。 有六、七个词的短句是很常见的选择。 在绝大多数时候， S/Key 或 OPIE 系统和 UNIX 口令系统完全相互独立地工作。
除了口令之外， 对于 S/Key 和 OPIE 还有两组至关重要的数据。 其一被称作 ``种子'' 或 ``key''， 它包括两个字符和五个数字。 另一个被称作 ``迭代轮数''， 这是一个 1 到 100 之间的数字。 S/Key 通过将种子加到秘密口令后面， 并执行迭代轮数那么多次的 MD4/MD5 散列运算来得到结果， 并将结果表示为 6 个短的英文单词。 这 6 个英文单词就是您的一次性口令。 验证系统 (主要是 PAM) 会记录上次使用的一次性口令， 如果用户提供的口令的散列值与上次一致， 则可以通过身份验证。 由于使用了单向的散列函数， 因此即使截获了上次使用的口令， 也没有办法恢复出下次将要使用的口令； 每次成功登录都将导致迭代轮数递减， 这样用户和登录程序将保持同步。 当迭代轮数减少到 1 时， 都必须重新初始化 S/Key 和 OPIE。
There are three programs involved in each system which we will discuss below. The key and opiekey programs accept an iteration count, a seed, and a secret password, and generate a one-time password or a consecutive list of one-time passwords. The keyinit and opiepasswd programs are used to initialize S/Key and OPIE respectively, and to change passwords, iteration counts, or seeds; they take either a secret passphrase, or an iteration count, seed, and one-time password. The keyinfo and opieinfo programs examine the relevant credentials files (/etc/skeykeys or /etc/opiekeys) and print out the invoking user's current iteration count and seed.
There are four different sorts of operations we will cover. The first is using keyinit or opiepasswd over a secure connection to set up one-time-passwords for the first time, or to change your password or seed. The second operation is using keyinit or opiepasswd over an insecure connection, in conjunction with key or opiekey over a secure connection, to do the same. The third is using key/opiekey to log in over an insecure connection. The fourth is using key or opiekey to generate a number of keys which can be written down or printed out to carry with you when going to some location without secure connections to anywhere.
To initialize S/Key for the first time, change your password, or change your seed while logged in over a secure connection (e.g., on the console of a machine or via ssh), use the keyinit command without any parameters while logged in as yourself:
% keyinit Adding unfurl: Reminder - Only use this method if you are directly connected. If you are using telnet or rlogin exit with no password and use keyinit -s. Enter secret password: Again secret password: ID unfurl s/key is 99 to17757 DEFY CLUB PRO NASH LACE SOFT
For OPIE, opiepasswd is used instead:
% opiepasswd -c [grimreaper] ~ $ opiepasswd -f -c Adding unfurl: Only use this method from the console; NEVER from remote. If you are using telnet, xterm, or a dial-in, type ^C now or exit with no password. Then run opiepasswd without the -c parameter. Using MD5 to compute responses. Enter new secret pass phrase: Again new secret pass phrase: ID unfurl OTP key is 499 to4268 MOS MALL GOAT ARM AVID COED
At the Enter new secret pass phrase: or Enter secret password: prompts, you should enter a password or phrase. Remember, this is not the password that you will use to login with, this is used to generate your one-time login keys. The ``ID'' line gives the parameters of your particular instance: your login name, the iteration count, and seed. When logging in the system will remember these parameters and present them back to you so you do not have to remember them. The last line gives the particular one-time password which corresponds to those parameters and your secret password; if you were to re-login immediately, this one-time password is the one you would use.
To initialize or change your secret password over an insecure connection, you will need to already have a secure connection to some place where you can run key or opiekey; this might be in the form of a desk accessory on a Macintosh®, or a shell prompt on a machine you trust. You will also need to make up an iteration count (100 is probably a good value), and you may make up your own seed or use a randomly-generated one. Over on the insecure connection (to the machine you are initializing), use the keyinit -s command:
% keyinit -s Updating unfurl: Old key: to17758 Reminder you need the 6 English words from the key command. Enter sequence count from 1 to 9999: 100 Enter new key [default to17759]: s/key 100 to 17759 s/key access password: s/key access password:CURE MIKE BANE HIM RACY GORE
For OPIE, you need to use opiepasswd:
% opiepasswd
Updating unfurl:
You need the response from an OTP generator.
Old secret pass phrase:
otp-md5 498 to4268 ext
Response: GAME GAG WELT OUT DOWN CHAT
New secret pass phrase:
otp-md5 499 to4269
Response: LINE PAP MILK NELL BUOY TROY
ID mark OTP key is 499 gr4269
LINE PAP MILK NELL BUOY TROY
To accept the default seed (which the keyinit program confusingly calls a key), press Return. Then before entering an access password, move over to your secure connection or S/Key desk accessory, and give it the same parameters:
% key 100 to17759 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> CURE MIKE BANE HIM RACY GORE
Or for OPIE:
% opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT
Now switch back over to the insecure connection, and copy the one-time password generated over to the relevant program.
Once you have initialized S/Key or OPIE, when you login you will be presented with a prompt like this:
% telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> s/key 97 fw13894 Password:
Or for OPIE:
% telnet example.com Trying 10.0.0.1... Connected to example.com Escape character is '^]'. FreeBSD/i386 (example.com) (ttypa) login: <username> otp-md5 498 gr4269 ext Password:
As a side note, the S/Key and OPIE prompts have a useful feature (not shown here): if you press Return at the password prompt, the prompter will turn echo on, so you can see what you are typing. This can be extremely useful if you are attempting to type in a password by hand, such as from a printout.
At this point you need to generate your one-time password to answer this login prompt. This must be done on a trusted system that you can run key or opiekey on. (There are versions of these for DOS, Windows® and Mac OS® as well.) They need both the iteration count and the seed as command line options. You can cut-and-paste these right from the login prompt on the machine that you are logging in to.
On the trusted system:
% key 97 fw13894 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: WELD LIP ACTS ENDS ME HAAG
For OPIE:
% opiekey 498 to4268 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: GAME GAG WELT OUT DOWN CHAT
Now that you have your one-time password you can continue logging in:
login: <username> s/key 97 fw13894 Password: <return to enable echo> s/key 97 fw13894 Password [echo on]: WELD LIP ACTS ENDS ME HAAG Last login: Tue Mar 21 11:56:41 from 10.0.0.2 ...
Sometimes you have to go places where you do not have access to a trusted machine or secure connection. In this case, it is possible to use the key and opiekey commands to generate a number of one-time passwords beforehand to be printed out and taken with you. For example:
% key -n 5 30 zz99999 Reminder - Do not use this program while logged in via telnet or rlogin. Enter secret password: <secret password> 26: SODA RUDE LEA LIND BUDD SILT 27: JILT SPY DUTY GLOW COWL ROT 28: THEM OW COLA RUNT BONG SCOT 29: COT MASH BARR BRIM NAN FLAG 30: CAN KNEE CAST NAME FOLK BILK
Or for OPIE:
% opiekey -n 5 30 zz99999 Using the MD5 algorithm to compute response. Reminder: Don't use opiekey from telnet or dial-in sessions. Enter secret pass phrase: <secret password> 26: JOAN BORE FOSS DES NAY QUIT 27: LATE BIAS SLAY FOLK MUCH TRIG 28: SALT TIN ANTI LOON NEAL USE 29: RIO ODIN GO BYE FURY TIC 30: GREW JIVE SAN GIRD BOIL PHI
The -n 5 requests five keys in sequence, the 30 specifies what the last iteration number should be. Note that these are printed out in reverse order of eventual use. If you are really paranoid, you might want to write the results down by hand; otherwise you can cut-and-paste into lpr. Note that each line shows both the iteration count and the one-time password; you may still find it handy to scratch off passwords as you use them.
S/Key can place restrictions on the use of UNIX passwords based on the host name, user name, terminal port, or IP address of a login session. These restrictions can be found in the configuration file /etc/skey.access. The skey.access(5) manual page has more information on the complete format of the file and also details some security cautions to be aware of before depending on this file for security.
If there is no /etc/skey.access file (this is the default on FreeBSD 4.X systems), then all users will be allowed to use UNIX passwords. If the file exists, however, then all users will be required to use S/Key unless explicitly permitted to do otherwise by configuration statements in the skey.access file. In all cases, UNIX passwords are permitted on the console.
Here is a sample skey.access configuration file which illustrates the three most common sorts of configuration statements:
permit internet 192.168.0.0 255.255.0.0 permit user fnord permit port ttyd0
The first line (permit internet) allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX passwords. This should not be considered a security mechanism, but rather, a means to remind authorized users that they are using an insecure network and need to use S/Key for authentication.
The second line (permit user) allows the specified username, in this case fnord, to use UNIX passwords at any time. Generally speaking, this should only be used for people who are either unable to use the key program, like those with dumb terminals, or those who are uneducable.
The third line (permit port) allows all users logging in on the specified terminal line to use UNIX passwords; this would be used for dial-ups.
OPIE can restrict the use of UNIX passwords based on the IP address of a login session just like S/Key does. The relevant file is /etc/opieaccess, which is present by default on FreeBSD 5.0 and newer systems. Please check opieaccess(5) for more information on this file and which security considerations you should be aware of when using it.
Here is a sample opieaccess file:
permit 192.168.0.0 255.255.0.0
This line allows users whose IP source address (which is vulnerable to spoofing) matches the specified value and mask, to use UNIX passwords at any time.
If no rules in opieaccess are matched, the default is to deny non-OPIE logins.
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在 FreeBSD 中标准的登录口令最长可达 128 个字符那么长。 |