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╓────────────╖ ║ Sentry 2.5 ║ ╙────────────╜ Michael A. Bobbitt Mike.Bobbitt@AcadiaU.Ca http://dragon.acadiau.ca/~910318b/Sentry.html TABLE OF CONTENTS 1.0 Introduction 1.1 Purpose of Sentry 1.1.1 In the Home 1.1.2 In the Business Environment 1.2 Features of Sentry 1.2.1 General 1.2.2 Expiry Dates 1.3 Distribution of Sentry 2.0 Setup 2.1 General Setup Issues 2.2 The Initial Login 2.3 First Priorities 2.4 Using the Sentry.ini file 2.5 Modifying Your AUTOEXEC.BAT 3.0 Securing Your Computer 3.1 BIOS Password 3.2 Boot Sequence 3.3 Switches in CONFIG.SYS 3.4 Passwords 3.5 The Password File 3.6 Placement in AUTOEXEC.BAT 3.7 Keep a Backup 4.0 Logging In 4.1 The Login Procedure 4.2 Changing Passwords 5.0 The SuperUser Menu 5.1 Create User 5.2 Delete User 5.3 View Users 5.4 Toggle SuperUser Status 5.5 Change Account Exipry Date 5.6 Change Password Expiry Date 5.7 View Log File 5.8 Change Account Password 5.9 Assign Max Invalid Logins 5.10 Edit Initialization Settings 5.11 Exit 6.0 Using Sentry With Windows 6.1 General 6.2 Installation Procedure 6.3 Additional Windows Security 6.4 Windows 95 7.0 Securing Other Programs 7.1 General 7.2 Setup 8.0 Creating a new Instance of Sentry 9.0 Registration 10.0 Revision History 10.1 Pre-Release Notes 10.2 Sentry V1.0 10.3 Sentry V1.1 10.4 Sentry V1.2 10.5 Sentry V1.3 10.6 Sentry V1.4 10.7 Sentry V2.0 10.8 Sentry V2.1 10.9 Sentry V2.2 10.10 Sentry V2.3 10.11 Sentry V2.4 11.0 Technical Notes 11.1 Encoding Algorithm 11.2 Time Stamps 11.3 Files 11.4 File_id.diz 11.5 General 12.0 Potential Threats To Security 12.1 The Password File 12.2 Hardware Loopholes 13.0 Troubleshooting 13.1 Error messages 13.1.1 Error opening password file! 13.1.2 Error opening temp file! 13.1.3 Error in creating log file! 13.1.4 Error opening log file! 13.1.5 Error opening Sentry.ini file! 13.1.6 Error in Sentry.ini file! [XXXXXXXX] 13.1.7 Registration Error - Program Aborted! 13.1.8 Error in time stamps. 13.1.9 Out of memory! 13.1.10 This account has expired. 13.2 Other problems 15.0 Standard Disclaimer 16.0 Credits ======================== 1.0 Introduction ================ Currently, almost all fields of computer security are growing and advancing, with a few notable exceptions. Networks, servers and public access systems are all tightening their access to avoid potential problems. But what of the single-user PC? Is the information contained on these any less valuable? Often times not, yet adequate security programs do not exist for PC's. In my search for security, I discovered that PC's were virtually ignored, and those programs that did exist were weak and faulty. Faced with this situation, I decided to write my own security program from scratch, incorporating the tightest security measures possible, while allowing flexible, easy use. 1.1 Purpose of Sentry Sentry is meant for one basic purpose only: to keep unwanted people off your PC. It is flexible enough that this single purpose can be used in many different ways, for many different reasons. Here are just a few. 1.1.1 In the Home Most people have something on their home PC that they don't want others to see. Maybe it's a sensitive document, maybe it's copies of e-mail, or maybe you just want to keep track of who is using your system. Sentry has the solution for all of these problems, by restricting access and logging all attempts to use the system. Alternatively, sometimes you just want to keep people out, period. It could be your nosy room mate, your little brother, or your boss at work. In any case, Sentry will keep them out, while letting a select group of people in. 1.1.2 In the Business Environment Businesses will find Sentry valuable to guard against unauthorized entry into PC's. With Sentry, you can leave your terminals unattended while knowing that nobody can access the information held on them, unless you want them to. Many businesses have cleaners or other independant contractors come in during silent hours. A lot of businesses have PC's in open areas where a "passer-by" could use them. There's no guarantee that these people won't attempt to use your PC's when your're not around. For businesses that work in groups, Sentry is useful for keeping track of who used the PC's and when. Supervisors could have SuperUser access on all PC's in their group, allowing them to manage and control access as required. Also, the log that Sentry keeps could be a valuable tool in determining who was doing what, and when. You may not need to use Sentry on your system. You might never have a security problem to worry about. But the truth is that Security isn't something to take a chance on. It is a serious problem in today's computing society, and many legitimate users find themselves helpless or confused about the real issues. It is a shame that the immoral few have ruined it for the rest, but we cannot let them get in the way of our daily business. With the proper tools and knowledge, we can fight back by closing up the common loopholes that attackers use. 1.2 Features of Sentry 1.2.1 General Sentry will allow you to set up accounts on your computer, one account for each person you want to have access. If someone doesn't have an account, they don't get in. It's that simple. Each person has their own account, with their own password, making it easy to track who logs in and when. Since Sentry doesn't use a "master access" password, you can wipe a single users account without affecting any other users. Sentry records each login attempt in a log file which can be viewed by a SuperUser at any time. 1.2.2 Expiry Dates You can set accounts to expire on a certain date, effectively barring access to the specified user after that. For example, if you know that Joe will be leaving on April 14th, you can set his account to expire on April 15th. That means you don't have to remember to delete his account on the 15th... It will expire on it's own, and you can delete it whenever you remember. You can also set expiry dates for passwords, meaning that a user will have to enter a new password once his old one has expired. In addition, you can set the period of time that new passwords are good for. Sentry can be used to restrict access when you first turn on your PC, to block DOS access from within Windows, to secure a single program from general use, or any combination of the above. 1.3 Distribution of Sentry Sentry is a copywritten piece of work, however distribution of the ShareWare version is allowed and encouraged. The only stipulation is that it must be unmodified, and must contain all of the original files (and no others). Essentially, the .zip file you initially recieved is the only format that Sentry is distributable in. DO NOT distribute any registered versions whatsoever. (By distributing a registered version, you are giving out copies of a specific encoding scheme, which can be used against the registered user). 2.0 Setup ========= Since you are reading this, I can safely assume you have unzipped Sentry. Along with that, I will also assume that you have created a directory for Sentry, and that all Sentry files are currently in it. (If this is not true, do it now). You may want to read the section entitled "Creating a new Instance of Sentry" below. 2.1 General Setup Issues The very first thing you should do is make sure you keep a copy of all the Sentry files somewhere safe. Copying them to a floppy disk and storing it is a good idea. The second thing you have to do is to set up the Sentry.ini file for your system. You should load the file into a text editor (like MS-DOS's edit) and make the changes from there. There are instructions and tips provided for each item. Make sure you read and understand these instructions before making any changes, as an improperly set up Sentry.ini file can cause errors and security loopholes. 2.2 The Initial Login Now the next thing you must do is to change the password and login provided with the initial copy of Sentry. To do this, you must run the Sentry.exe program. If it displays an error message, check the troubleshooting section at the end of this file. When it asks for a login, use "Sentry", with the password "Sentry". You can turn case sensitivity off in the Sentry.ini file, but for logins only. The password must be entered exactly as shown (sentry or SENTRY will not work). The Sentry account has SuperUser access, meaning you can create and delete accounts while logged in as Sentry. To log in as a SuperUser, enter Sentry for a login, and instead of pressing enter, hold down the CONTROL key and press enter. This tells the Sentry program that you want to log in as a SuperUser. Then enter your password normally, and instead of dropping to DOS, you will go to the SuperUser menu. (See the section below on "The SuperUser Menu" for more information). From here, you should create a new user (you) and grant yourself SuperUser access. You can do this during creation by holding the CTRL key while pressing enter after typing your login name. You can also do this by using the "Create User" option normally and then using the "Toggle SuperUser Access" menu option to give yourself SuperUser privileges. 2.3 First Priorities Once you have your account set up, you should delete the Sentry account, so nobody else can use it. This is very important as a potential intruder will probably try to enter through the default Sentry account first. (If you delete it, that closes the loophole). Sentry will not let you delete the last SuperUser, so you must create a new user with SuperUser access before deleting the Sentry account. From here, you can create the other users of your system, and define their starting passwords, expiry dates and access rights. See the section below on "The SuperUser Menu" for more information. 2.4 Using the Sentry.ini file Sentry can be configured to your needs by way of the Sentry.ini file. This file contains information that you may wish to change, allowing Sentry to adapt to many different situations. The Sentry.ini file follows a very specific format, and if you deviate from it, Sentry will not work! Comments are allowed, as long as the comment line begins with a * character. Blank lines are also permitted. The order of the items in the Sentry.ini file is critical. Do not move items around. The comments provided with the Sentry.ini file will guide you through this stage. Feel free to add your own comment lines as you see fit. If this is too much for you, then you can edit the Sentry.ini file from the SuperUser menu. You can do this by selecting the "Edit Initialization Settings" option (see section 5.10). Many people find this easier than editing the actual file. Any changes you make will not take effect until the next time you use Sentry. 2.5 Modifying Your AUTOEXEC.BAT Simply add the following line to the top of your AUTOEXEC.BAT file: ----------------- CUT ---------------- \Sentry\Sentry ----------------- CUT ---------------- The above example assumes you have installed Sentry on c:\Sentry, which may not be true. Adjust the path in the first line to reflect the location of Sentry on your system. NOTE: These lines MUST be the first lines in your autoexec.bat file. If not, the user may be given a chance to bypass Sentry by pressing CTRL-C or CTRL-BREAK. If Sentry is installed on a drive other than the one you boot from (drive D in the example below), then add these lines instead: ----------------- CUT ---------------- d: \Sentry\Sentry c: ----------------- CUT ---------------- 3.0 Securing Your Computer ========================== Sentry is essentially useless unless you take the appropriate additional security measures for your machine. Below I have outlined some of the things you can do to increase the security of your system. 3.1 BIOS Password Protect your BIOS! It has a built in password, so set it! Without the password, any user can get into your BIOS and change your critical system settings. I have seen several different types of BIOS setups, but generally, you use the CHANGE PASSWORD command (Sometimes also listed as SUPERVISOR PASSWORD) from the main menu and set the SECURITY OPTION to setup (as opposed to system) in the BIOS FEATURES SETUP screen. Depending on the layout of your BIOS, you may have a CHANGE SETUP PASSWORD option right on the main menu. 3.2 Boot Sequence Change your boot sequence. Again, in your BIOS, under the BIOS FEATURES SETUP screen, set your BOOT SEQUENCE to boot from your hard drive first. (This usually means set it to C,A as opposed to A,C). This will ensure that no-one can bypass Sentry by means of a boot disk. 3.3 Switches in CONFIG.SYS Add "switches /n/f" as the first line of your CONFIG.SYS file. The /n will dis-allow pressing F8 to step through the CONFIG.SYS and AUTOEXEC.BAT files. This means that users cannot bypass Sentry by this method. The /f switch is optional, it simply speeds up your boot-up time (by about 2 seconds). 3.4 Passwords Pick a good password. Short passwords are a bad idea, as well as birthdays, girlfriends names, etc. Anything that is easy to guess should be avoided. Good passwords are not words at all, but made up from a "mnemonic" sentence. For example, the sentence "I'll wait for you" turns into "Illw84u" (trust me). Mixing lower case letters, upper case letters, and numbers increases the security of a password, and passwords of this type are near impossible to guess or crack. Do not write your password down, especially near your computer. Try to memorize it if possible. Change your password, but not too often. People who change their passwords too often (less than 6 months or so) tend to write them down, which is a bigger risk. Don't enter your password with someone looking over your shoulder. If someone is in the room, block their view, or wait until they leave. 3.5 The Password File Put your password file in a safe place. Although passwords are encoded, usernames and time stamps are not. Also remember, the encoding scheme is one way only, so if I encode login names, you will never be able to view who is in your password file (IE: you would have to "guess" which person to delete). Users can change time stamps in the password file, and this can cause serious problems, so KEEP IT SAFE! NOTE: Sentry automatically hides your password file for you. After you have run Sentry once, your password file (and log file) will not appear in directory listings. This will help your security, but is not a fool-proof method. 3.6 Placement in AUTOEXEC.BAT When you are installing Sentry, make sure it is the FIRST file in your AUTOEXEC.BAT! Otherwise, the user may be able to exit when another program is running and avoid Sentry all together! 3.7 Keep a Backup Keep a backup copy of all your Sentry files! Especially your password file! If something happens to these files, you may be locked out your computer if you don't have a backup. 4.0 Logging In ============== 4.1 The Login Procedure When you first run Sentry, you should see the standard startup screen. Your registration information is contained here. (The shareware release simply says it is registered to Shareware). You should also see a prompt asking you to enter your login (or user name). If instead you see an error message, check at the end of this file for help. While entering your user name, if you have SuperUser access, you can log in as such. You can do this by holding down the CTRL key while pressing enter. If you do not have SuperUser access, or do not wish to log in as such, simply press enter. In either case, this will send you to the password prompt. Once you have reached the password prompt, you simply enter your password, and hit enter when you are finished. Your password is not echoed to the screen. At this time, the date and time of your last login will be displayed. The minimum and maximum length of both the login and password are set by the "MinPasswordLen" and "MaxPasswordLen" attributes in the Sentry.ini file respectively. If there have been any invalid login attempts made against your account since the last valid login, you will be told so, and how many. If the number of invalid login attempts against your account has exceeded the maximum allowed, your account will be locked out until the SuperUser resets it. If your account has expired, it will say so, and subsequently lock you out. If your password has expired, it will also say so, but will then prompt you to enter a new password. You may not re-use your old password, and you must pick a password that corresponds to the length limits set out in the Sentry.ini file. Once you have entered and verified your new password, it is given a new expiry date according to the "PasswordExpiresIn" value in the Sentry.ini file. If you have logged in as a SuperUser, and have SuperUser access, you will now be in the SuperUser menu (see the section below). If not, you will simply be dropped to DOS, and the login procedure is complete. 4.2 Changing Passwords If you wish to change your password at any time, you can do so by entering the pass key. The pass key is defined in the Sentry.ini file, and is displayed when Sentry starts up. To change your password, simply enter your user name as usual, and when prompted for your password, enter the pass key. You will then be prompted for your old password (to make sure it is really you), and then you will be asked for your new password, which will be verified and saved to disk. Your new password will expire in the number of days designated by "PasswordExpiresIn" in the Sentry.ini file. Changing your password has no effect on SuperUser access. 5.0 The SuperUser Menu ====================== 5.1 Create User This option allows you to create users on your system. First of all, you must enter the username (or login) you wish to assign to that user. If you wish to create a user with SuperUser status, type in the username and hold down the CONTROL key when pressing enter. Otherwise, just press enter when you're finished at the login prompt. Once that is complete, Sentry asks for the password. The password is not echoed to the screen. The user should choose and enter their own password. SuperUsers need not know what the passwords are since they can still manage the accounts without knowing them. The password is entered twice to ensure no typos were made, and then saved to the password file in encoded form. The ESCAPE key will abort this operation at any time. Note: There is currently a maximum of 100 users allowed. This limit can be bypassed if required (contact me for a larger capacity version). 5.2 Delete User The delete option is very simple. It brings up a list of all users, and you simply use the cursor keys to highlight the user you wish to delete. SuperUsers are denoted by a * to the right of their username. Press enter to select the appropriate user. If the selected user is a SuperUser, Sentry will give a warning. SuperUser or not, Sentry will then ask if you are sure you want to delete them. Any input other than a 'y' will not perform the delete. The ESCAPE key will abort this option at any time. 5.3 View Users This option allows you to view all users who currently have accounts on your system. SuperUsers are again denoted by a * to the right of their login name. The last login date/time, account expiry date and password expiry date are also shown to the right of the user's login name. In addition, the number of invalid logins since the users last valid login are shown with the number of invalid logins allowed before an account is locked up. An "X" in the "Max Inv Log" (Maximum Number of Invalid Logins Allowed) column means there is no limit. You can use the PAGE UP and PAGE DOWN keys to scroll forward and back if there are multiple pages of users. Arrows ( and ) will be present if there are additional pages above and/or below. The ESCAPE key will exit back to the main menu. 5.4 Toggle SuperUser Status When this option is selected, it first brings up a list of all users on the system. Again, SuperUsers are denoted by a *. Once you have selected a user, Sentry will ask you if you want to grant/revoke SuperUser access to/from the appropriate user. Any input other than a 'y' will not change that users status. Now the user must enter a password. (Since the SuperUser status is encoded in the password, and the password can never be decoded, I can't change SuperUser access without resetting the password). The user can re-enter their old password, or enter a new one (the old one will be over-written). In a worst case scenario, the SuperUser can re-assign a new password to the user if he is unavailable to enter a new password himself. (A hostile user can have SuperUser access revoked without having to enter a new password; you can do it for him). 5.5 Change Account Expiry Date This option lets you define when an account will expire on your system. Expired accounts no longer have access. This option is useful if a user will be leaving. Then you don't have to remember to delete their account on the day they leave. You can set it to expire, and delete it when you remember. This option also works in conjunction with the "Assign Max Invalid Logins" option. Once an account reaches it's maximum number of sequential invalid logins, it expires. The only way to re-activate the account is to change the expiry date with this option. Once you select this option, some information about the account will be displayed. If it is a SuperUser account, Sentry will tell you so. It will then display the account's current expiry date. You will be asked if you are sure you want to change that user's expiry date. Any input other than a 'y' will abort the process, otherwise you will be prompted for the year the account will expire. The year must be entered as 4 digits (IE: 1997). If you enter 'N' at the year prompt, no expiry date is assigned to that account (it is valid forever). If you enter a valid year, you will then be prompted for the expiry month, which is entered as 2 digits (IE: 06 for June) followed by the expiry day, which is also entered as 2 digits. Accounts expire at one second past midnight on the date of expiry. 5.6 Change Password Expiry Date This option lets you define when a users password will expire. Once the password has expired, the user must enter a new one. This forces the user to change their password. Once a password has expired, the next time the user logs in he will be forced to enter a new password. The new password is valid for the number of days set in the "PasswordExpiresIn" option in the Sentry.ini file. Once a password has expired, the user cannot re-enter it. He must select a new password. (NOTE: Sentry does not keep historical records on passwords, so a user may alternate back and forth between 2 passwords. This is not a secure practice and should be avoided). Once you select this option, some information about the account will be displayed. If it is a SuperUser account, Sentry will tell you so. It will then display the current expiry date for the password. You will be asked if you are sure you want to change that user's expiry date. Any input other than a 'y' will abort the process, otherwise you will be prompted for the year the password will expire. The year must be entered as 4 digits (IE: 1997). If you enter 'N' at the year prompt, no expiry date is assigned to that password (it is valid forever). If you enter a valid year, you will then be prompted for the expiry month, which is entered as 2 digits (IE: 06 for June) followed by the expiry day, which is also entered as 2 digits. Passwords expire at one second past midnight on the date of expiry. 5.7 View Log File Every time a user logs in, a record is kept on disk. If you want to view that online record, select this option. The log file will be displayed, one screen at a time. Once the entire log file has been displayed, Sentry will ask you if you want to clear the log file. Any input other than 'y' will exit, leaving the log file in tact. If you answer with a 'y', Sentry will clear out the old entries. This should be done fairly frequently, depending on how busy your system is. Large log files take up more disk space. You should always keep a close eye on the log file as this will often tell you when something is wrong on your system. All error messages are saved to the log file, so you can see if Sentry has run into any problems. Also, it records the current time, and the username of the user attempting to log in. This will help you to identify any potential attacks on your system. The log file is hidden by Sentry, but you should also place it somewhere safe so that users cannot tamper with it. You can set the location of the log file with the Sentry.ini file. 5.8 Change Account Password This option allows the SuperUser to change an account's password, in case the user forgot it, or some other strange disaster has occurred. When assigning a new password to an account, you should set the expiry date to be immediatly, so the user is forced to choose a new one. If the previous password had no expiry date, none is assigned to the new password. Otherwise the password expires in the number of days assigned to "PasswordExpiresIn" in the Sentry.ini file. You can abort at any time by pressing the ESCAPE key. 5.9 Assign Max Invalid Logins This allows you to set the maximum number of invalid logins allowed before an account is disabled. The default number is defined in the Sentry.ini file ("InvalidLogins") and is assigned to all accounts when they are first used. To assign a new maximum, simply select the "Assign Max Invalid Logins" option from the SuperUser menu. From here you will be shown the complete user list, and asked to select the user you wish to change. Pressing the ESCAPE key will abort the operation here. Once you have selected the user, you will be informed if that user is a SuperUser. NOTE: You should not assign a maximum number of invalid logins to your last SuperUser account. If you do, and someone attempts to break in to that account, you could be locked out of the SuperUser menu! Next you will be told what the user's current max invalid login setting is, and asked if you want to change it. Any input other than a 'Y' will abort the operation. Now you will be asked to enter the number of invalid login attempts before an account is disabled. Entering 'N' or a 0 will mean that there can be unlimited invalid login attempts made. Please note that a value of less than 10 may cause you more trouble than good. You may be spending a lot of time re-setting accounts if you pick too low a value, so consider this carefully. Also note that when an account is locked up, it is actually set to expire immediately. As a result, if you wish to re-activate an account, you must change the account's expiry date (see above). This is also handy for determining when the account was actually de-activated. The expiry date for that account it set the the day it was locked out. Every time a successful login is made to an account, the invalid login counter is reset. This means that an account will not be locked out if it has a valid login before the maximum is reached. (For example, say an account has a maximum of 10 invalid logins. If there are 7 invalid logins before a successful login, and then 5 more invalid logins, the account will not be locked up. There must be 10 sequential invalid logins for the account to be disabled.) Once you have finished making the change, you can view the user list to make sure it is acceptable. 5.10 Edit Initialization Settings This option allows you to edit the Sentry.ini file from within Sentry. Once this option is selected, all of the attributes from the Sentry.ini file are displayed. Simply select the attribute you wish to modify, and you will be given the following information: - A one line description of the attribute. - The name of the attribute as it appears in the Sentry.ini file. This will appear in brackets under the one-line description. - A brief description of the attribute and it's uses. - Valid settings for the attribute, if applicable. - Any security notes, if applicable. - The default value for the attribute. - The current value for the attribute. At this time, you will be prompted to enter a new value for the attribute. Pressing ENTER on a blank line, or pressing the ESC key abort any changes the current value. Sentry performs strict checking on the values you enter, and will not save an invalid value. For this reason, you should change your Sentry.ini settings from the SuperUser menu whenever possible. 5.11 Exit This simply returns you to the DOS prompt. It is the same as pressing the ESCAPE key. 6.0 Using Sentry With Windows ============================= 6.1 General Sentry is also adaptable to Windows, allowing you to keep users away from DOS. This may be desirable if you enter Windows immediatly upon startup (I.E.: your autoexec.bat contains "win" as a command). If you want to limit access to your system as a whole, you could install Sentry in the usual way. This would keep out unwanted users all together. If you want to also limit access to the DOS prompt from Windows, you can do that too. All you must do is create a new instance of Sentry (see below), and then simply set up your windows to run Sentry when you drop to DOS. The example setup below assumes you have installed an instance of Sentry in c:\Sentry\Inst1, but you can substitute your actual directory names in where applicable. 6.2 Installation Procedure Installing Sentry in the Windows environment is simple. Included in the Sentry zip file are 2 files: Sentry.grp Sentry.pif Copy both of these files into your windows directory. Now, enter Windows and from the Program Manager, select [F]ile, then [N]ew. Next select Program Group. When prompted, enter "Sentry" for both description and file name. Now you should see a new program group called Sentry. The Sentry program group should contain a single icon, labelled MS-DOS. This icon actually points to sentry.pif, which contains specific information about running Sentry. You can edit the new icon (highlight it and press ALT-ENTER) and change the working directory to point to your Sentry files. IE: c:\Sentry, or c:\Sentry\Inst1, etc. From here, run the Pif Editor program (the icon is a little tag), and open Sentry.pif. Now change the "Program Filename" to point to your Sentry.exe file. IE: c:\Sentry\Sentry.exe, or c:\Sentry\Inst1\Sentry.exe, etc. In Addition, change the "Working Directory" to be the same as the working directory for the icon (see above). Once you save the .pif file, you are all set. At this point you should test out the new Sentry icon to make sure it works ok. When you double click on the new icon, it shoould take you directly to Sentry. Once you have entered a correct username and password, it will then drop you to a regular DOS shell. You can type "EXIT" to return to Windows at any time. Once you are sure it works, remove your old MS-DOS icon, so that users cannot use it to drop straight to DOS. You can drag your new icon into the same location as your old MS-DOS icon, so everything will look the same. Now there is one last step. If a user were to exit Windows, they would be at the DOS level, which is not what we want. Since your autoexec.bat file is running windows on startup, you can block people from exiting by forcing them to go through Sentry. You can do this by adding a call to Sentry after the win command in your autoexec.bat. For example, the last few lines of your autoexec.bat might look like this: ----------------- CUT ---------------- win \Sentry\Inst1\Sentry ----------------- CUT ---------------- Alternatively, you can add the security measures described in section 6.3 below. And that's it. If it seems like a complicated process, just try the steps one at a time, and make sure everything works ok. The end result is that when you click on the MS-DOS icon, it will run Sentry before dropping you to the DOS shell. That means that you can limit access to the operating system, without pulling any fancy tricks in Windows. Since you have made a new instance of Sentry, you could have one instance run when you boot up (giving access to windows), and have another instance run when you click on the DOS icon (giving access to the operating system). That way, a user that has access to windows may not be able to drop to DOS. Be warned however that Sentry will still lock up your system when a user fails to log in. This may cause you to lose any information you have not saved in your Windows session, if Windows becomes unstable. If Sentry does lock up your system, you may be able to recover back to Windows by pressing CTRL-ALT-DELETE and closing the DOS prompt. This will still not allow access to DOS, but you will not lose anything you were working on. 6.3 Additional Windows Security There are additional security measures built into Windows that should be considered when using Sentry. After all, securing the MS-DOS icon won't do any good if someone can edit it back to the way it was. To add more security to your Windows system, just add this Restriction section to your progman.ini file: ----------------- CUT ---------------- [Restrictions] EditLevel=4 rem Stops the creation, movement, copying, deletion, or rem modification of ANY groups or icons. Noclose=1 rem Prevents a user from exiting windows. NoRun=1 rem Disables the run command selection from the file menu. NoSave=1 rem Stops the selection of save settings on exit from program rem manager. NoSaveSettings=1 rem Disables the save settings on exit command, so any changes rem made to the your program manager group icons and windows rem cannot be saved upon exiting windows. ----------------- CUT ---------------- To nulify any of these entries, remove it or change the value from 1 to 0. You can delete the file manager Icon as an additonal security precation. 6.4 Windows 95 Sentry is easily adaptable to use in a Windows 95 environment. In most respects, the setup is the same as in a DOS/Windows environment. As before, ensure that the call to Sentry is at the beginning of your AUTOEXEC.BAT file. The Sentry.pif file and Sentry.grp file will still work with Windows 95, although at this time I have not drawn up specific instructions for installation. I have not yet conducted thorough tests in a Windows 95 environment, however documentation for this should be forthcoming in a future revision. 7.0 Securing Other Programs =========================== 7.1 General In some cases, it might be beneficial to secure a single program. It might be a word-processor, a mail program, or even Windows. In any case, you can secure it with Sentry, even if you don't use Sentry during boot-up. You will probably want to create a new instance of Sentry (see below) for each program you want to secure. That will allow you to have different accounts and passwords for each program. 7.2 Setup To set this up, all you have to do is write a batch file of the following format, and stick it in a directory in your path. (C:\DOS is almost always in your path, so you could stick these batch files there). In this example, we will secure the program called RUNME (loacted at C:\prog\runme.exe): ----------BEGIN RUNME.BAT----------------- @echo off c: \Sentry\Inst2\Sentry cd \prog runme cd \ -----------END RUNME.BAT------------------ The program as shown above will run Sentry before it runs RUNME.EXE. As long at the batch file is in your path ahead of RUNME.EXE, RUNME.BAT will execute first, barring the user from running RUNME.EXE without running Sentry first. C:\DOS is almost always first in your path, so this will work for everything but DOS programs. You can add a new directory to you path by editing your AUTOEXEC.BAT. If you add it to the beginning, and place all your batch files there, they will run first. The only exception to this rule is that if the user is in the directory containing RUNME.EXE they will not execute RUNME.BAT first. This is not by any means a perfect method of securing a program. It will work in most cases, but with the proper knowledge and patience, this method can be defeated. If your users are skilled, don't rely on this method to be "bullet-proof." (Sentry was not designed for this purpose, it is merely an additional use). Future versions of Sentry may have an "automated" method for doing this if the need exists. 8.0 Creating a new Instance of Sentry ===================================== In some of the scenarios listed above, you may be required to create a new "instance" of Sentry. That is to say, a completely new copy of Sentry, that works independantly of all other copies. This is a fairly simple process, that you can perform as many times as required. The first thing you must do when creating a new instance is to create the directory you wish to place it under. If you plan to have several instances, you may want to create a Sentry main directory, with your instances branching off of that. For example: c:\Sentry | +----+------- Inst1 | +------- Inst2 This will allow you to keep all your Sentry files well organized, and seperate from your other programs. Create a directory for each instance you anticipate you will need. From there, simply copy ALL of the Sentry files into each directory. The final step is to set up the accounts of each instance according to your needs. To do this, you must go into each directory individually and run Sentry. Now log in as a SuperUser and set up all the required accounts. Remember that each instance is independant of the others. Your password file should not be the same for any 2 instances (otherwise there is no advantage to having 2 instances). That means that the PassFile setting in each Sentry.ini must be different, and your password files must be in different locations. For simplicity's sake, you may want to keep your password file for each instance in the same directory as the other related Sentry files. (The password file for Instance 1 goes into c:\Sentry\Inst1, and so on). You should always test out each instance and make sure it works before using it. 9.0 Registration ================ If you use Sentry on your machine(s), I urge you to register. I have put a lot of time and effort into making Sentry a viable security program, and I would appreciate the effort very much. On the other hand, I realize that not everyone can afford to register. In that case, please feel free to continue to use the ShareWare version. There are no limitations on how long or how many times it can be used. My intention with Sentry was to make a contribution to PC security, and to make it available to everyone. All I ask is that if you use Sentry, and are able, please register. The shareware version of Sentry has all the functionality of the registered version, with one exception: it does not encode passwords. This will allow you to test out all the functions of Sentry before you commit to purchasing it. The ShareWare version in itself is a very secure program as is, however if you are serious about Security, plaintext passwords are not a viable option. To order, simply fill out the order form provided (Order.frm) and e-mail or snail-mail a copy to me. As a registered user, you will receive: - The full Sentry program (with password encryption) and related files on 3.5" disk. - A laser printed user's manual (essentially this file, with a few changes in format and content). - Online internet support via e-mail. - Free upgrades as soon as they are available. - Notices about any potential security risks, and instructions on how to protect yourself. - The ability to request specific features in future versions of Sentry. NOTE: Registered versions are not inter-compatible. That is, user X's password file is encrypted differently than user Y's password file. The version of Sentry that you recieve is good only for you and your files. (Likewise, nobody else can use their copy of Sentry with your password file). See the order form for information on obtaining compatible versions of Sentry. 10.0 Revision History ===================== 10.1 Pre-Release Notes March 95 - My search for DOS security programs is unsuccessful. The general idea for Sentry is formed. April 95 - Coding for Sentry begins. May 95 - Still coding... June 95 - Sentry is now a complete program. July 95 - Beta testing. Many updates made. 10.2 Sentry V1.0 Release Date: 20 July 95 - Sentry V1.0 (Shareware) is released. Contains basic functionality. At this point, Sentry is approximately 1700 lines of code. 10.3 Sentry V1.1 Release Date: 27 July 95 - Sentry V1.1 Released. V1.1 fixes some serious bugs in V1.0, and has these additions: * Date format is selectable. * Users with no password expiration are not assigned an expiry date on entering a new password. * Incorrect password verification displayed immediately when creating a new user, or toggling SuperUser status. 10.4 Sentry V1.2 Release Date: 29 July 95 - Sentry V1.2 Released. Some more minor bugs are repaired and several areas are updated. Additions are: * Memory management improved. * All user stats now shown instead of just login name. * The keyboard buffer is flushed after an invalid login attempt. 10.5 Sentry V1.3 Release Date: 01 Aug 95 - Sentry V1.3 Released. Again, more bugs were fixed in this version. Additions are: * The log file is now much more detailed than before, and includes a time stamp on every action. * A bug with deleting a user listed as 15th or later in the password file has been fixed. (Regardless of what page the user you selected to delete was on, a user from the first page was always deleted). * A bug in the multi-user display was fixed. (If the last page contained one user, you could not page down to see him). 10.6 Sentry V1.4 Release Date: 08 Aug 95 - Sentry V1.4 Released. Some minor changes have been made, mostly for the sake of appearance. Additions are: * The Sentry.ini file now allows the user to select the colours to be used for normal and highlighted text. * Input routines have been improved and simplified. Inapropriate characters have been stripped from the input stream. 10.7 Sentry V2.0 Releaase Date: 11 Aug 95 - Sentry V2.0 Released. Many revisions, fixes and additions have been made to the program for this release. Most of the update ideas came from Bret Jacobsen. Additions include: * Invalid login/password length messages (both to the screen and log file) are user-definable in the Sentry.ini file. * If a normal user attempts to log in as a SuperUser, it is recorded in the log file. * The SuperUser Login flag is cleared after an invalid login. (This is a bug fix: previously if CTRL-ENTER was pressed during an invalid login attempt, the next successful login attempt would be as a SuperUser, as long as the user had access). * A bug with the Toggle SuperUser Access option corrupting the time stamps has been fixed. * The log file is again re-organized to be more "readable." * You can no longer delete the last SuperUser on your system (thereby locking yourself out). Sentry performs a check previous to deletion and will abort if you are deleting the last SuperUser. * A bug with the input skipping characters was fixed. * After a successful login, the number of invalid login attempts since your last successful login is now shown. * The user can now abort a password change, unless the password has expired. * The option to reset an account's password has been added to the SuperUser menu. * You can now define the number of invalid logins permissible before an account expires (locks up). The default setting is defined in the Sentry.ini file, and the settings for individual accounts can be modified from the SuperUser menu. 10.8 Sentry V2.1 Releaase Date: 20 Aug 95 - Sentry V2.1 Released. Changes include: * Input no longer automatically ends after "MaxPasswordLen" characters have been entered at the login or password prompts. Input continues as required, however only "MaxPasswordLen" characters are used. Any extra characters are discarded. * Case sensitivity for logins can now be turned on and off via the Sentry.ini file. This is applicable to logins only, passwords are still case sensitive. 10.9 Sentry V2.2 Releaase Date: 10 Sep 95 - Sentry V2.2 Released. Changes include: * Sentry now uses windowed screens for all output, as opposed to simply directing output to the entire screen. * When entering dates, the year is now checked. If it is not 4 characters, the user is forced to re-enter it. This was causing problems with Sentry accepting '96' as a valid year, and subsequently not translating correctly. * A bug with the password encoding scheme has been fixed. The encryption used to truncate passwords at length 8, causing only the first 8 characters to be recognized. For SuperUsers, this meant that passwords over 8 would not contain the SuperUser stamp (it was truncated). This problem has now been rectified, and passwords are significant for MaxPasswordLen characters. All in all, this seriously increases the amount of security available from Sentry. NOTE: Because of the above modification, older versions of Sentry cannot use password files from V2.2 and up. The reverse is not true however, as Sentry V2.2 can use password files all the way back to V1.0. Also note that to use passwords over 8 characters, you must re-create the old passwords. This can be easily done by selecting the "Change Account Password" option from the SuperUser menu. 10.10 Sentry V2.3 Releaase Date: 24 Sep 95 - Sentry V2.3 Released. Changes include: * Small bugs with windowed mode have been fixed. * Windowed mode is now optional. For those that prefer "normal" operation, you can set that in the Sentry.ini file. * Log file viewing can now handle long lines, and you can skip to the end of the file by pressing the ESC key once. * Quickstart instructions are now included in the Qstart.txt file. 10.11 Sentry V2.4 Releaase Date: 06 Oct 95 - Sentry V2.4 Released. Changes include: * The Sentry.ini file can now be edited from the SuperUser menu. The user can get information on each attribute, including a brief description, security notes, default setting, and the current setting just by selecting which attribute they wish to change. 10.12 Sentry V2.5 Releaase Date: 10 Oct 95 - Sentry V2.5 Released. Changes include: * Sentry can now be run from any location. The user does not have to be in the Sentry home directory for it to work properly. 11.0 Technical Notes ==================== 11.1 Encoding Algorithm The encoding algorithm used is the standard UNIX crypt() algorithm. It is a one-way encoding algorithm that incorporates the Data Encryption Standard (DES) and RSA technology. It is used on UNIX systems to secure passwords. (As a note, the encryption code is not included with the ShareWare version, so no amount of examining the code will reveal the algorithm). The users' passwords are never decoded. They are stored on disk and in memory in an encoded format. The entered passwords are encoded using the same algorithm and matched in an encoded form. This prevents disk or memory scans from revealing the password to prying eyes. I have begun running some tests on cracking the passwords, and I will include my results. So far, this is what I have found: Password Length Maximum Time to Break ------------------------------------------------ 4 22 days 5 1368 days or 3.75 years 6 232.4 years 7 14409 years 8 893357 years All times listed are approximated as using a Pentium 90MHz CPU and an alpha-numeric password. These figures may be adjusted as my testing becomes more accurate. The above figures also assume you know the length of the password, which cannot be determined by looking at the encrypted version. As a result, the search time may be much greater. SuperUser access is also encoded in the password. I tried many different ways before finally settling on this. It is the most secure method. Actually, SuperUser access is stored on the password (which is then encoded), and then stored again on the encoded password. That looks like: password <-- Password as entered. super(password) <-- Password with SuperUser stamp. * This is the stamp that is used to determine access. crypt(super(password)) <-- Encoded password password with SuperUser stamp. super(crypt(super(password))) <-- SuperUser stamped encoded password with SuperUser stamp. * This is the stamp that is used to "see" who has SuperUser access. That way when you use the "View Users" command, you can see which ones are SuperUsers. However, since the non-encoded SuperUser stamp can be edited, it is only used for viewing. The encoded SuperUser stamp is used for access. If anyone attempts to alter the non-encoded stamp, a warning will be displayed in the log file each time that user logs in. 11.2 Time Stamps The time stamps used in Sentry are in the standard UNIX format. That is, the number of seconds since 01 Jan 1970 00:00:00. NOTE: I have found one date which always seems to cause an error. 01/01/1997 translates to 01/15/1995 (1st day of the 15th month of 1997). This is obviously incorrect. I have determined it is a bug in Borland's date conversion routines. It is the ONLY date that causes problems, to my knowledge, all others translate fine. I advise against using 01/01/1997 for any expiry dates. 11.3 Files The following files are included with this release of Sentry: Sentry.exe This is the executable program. Sentry.ini The initialization file. Sentry.pwd The password file. Sentry.pif The Sentry .pif file for Windows. Sentry.grp The Sentry Group file for Windows. File_id.diz Short description file. Order.frm The order form. Readme.com Displays this file. Readme.txt This file. Qstart.txt Quickstart instructions. Whatsnew.txt A short description of modifications to the latest version. If you do not have all of these files, Sentry will probably not work for you. You can pick up a complete copy of Sentry (and updates as they become available) at: http://dragon.acadiau.ca/~910318b/Sentry.html 11.4 File_id.diz The actual contents of the file_id.diz file are shown below. If any modification has been made to the original file, please re-create it from the following section. NOTE: this is primarily for SysOps of BBS's. Single users can delete the file_id.diz if they wish. (However, please make sure that all files are present if you distribute the program.) -------------------CUT---------------------- (V2.5) Sentry - Security for DOS/Windows Sentry is a DOS based security program that allows you to control and monitor access to your PC. It can support up to 100 users and has normal and "SuperUser" access. It can also be used in Windows to secure the MS-DOS icon. Installed correctly, Sentry can be an extremely powerful security tool for your PC. http://dragon.acadiau.ca/~910318b/Sentry.html Mike Bobbitt [Mike.Bobbitt@AcadiaU.Ca] -------------------CUT---------------------- 11.5 General Sentry is written entirely in Borland C for DOS. As of version 2.5, Sentry contains over 2600 lines of code. Portability between machines is not an issue, since Sentry has been designed and tested on standard MS-DOS machines. Sentry is verified to be compatible with Windows 3.x and Windows 95. 12.0 Potential Threats To Security ================================== Never underestimate your users. And never be satisfied that your system is completely "air-tight". Users are incredibly apt at finding loopholes in security, and once found, these holes can be expoited. Because of this, I am listing below all of the security loopholes that I am aware of at this time. 12.1 The Password File Let me once again stress again that the time stamps and user names are not encoded in the password file. This means that if a malicious user found the password file, he could edit it, and effectively wreak havoc on your system. Bear in mind that the password file is automatically hidden by Sentry, so finding it is not always easy. Also, most of these methods require a working knowledge of how Sentry operates, and that is not commonly available. The best source for that information would be from this file, which is deliberately missing some key pieces of "technical" information. If a user did manage to find the password file however, he could do any of the following: - Add/Remove an expiry date (account or password) This is not a serious issue for passwords, as the user would still have to enter their old one before being notified that their password has expired. An attacker cannot expire a password and then log in to that account. - Edit a username This would effectively lock out that user, unless they could guess their new user name. - Delete a user Users can be deleted, but not created. - Destroy a password Since passwords are encoded, there is no way to change a password to something usable. An edited password will likely lock that user out of their account, until the password is reset. - Change "last login" information Not critical, but could be used to cover an attackers tracks. - Change "number of invalid logins since last login" information Same as above. - Add/Remove restrictions on the number of invalid login attempts Could allow a "brute force" technique to work on an account password if the restriction was lifted. Also, if an attacker set this restriction to 1 invalid login, that account would be disabled if a single failed login attempt was made. It is important to know however, that a renegade user can never grant himself SuperUser privileges, nor can he ever view any users password, or attempt to effectively modify one. To perform any of these functions, he must be logged in as a SuperUser. In fact, not even SuperUsers can view passwords. Nobody can. (They can never be decrypted, remember?) In essence: KEEP YOUR PASSWORD FILE SOMEWHERE SAFE, AND GUARD YOUR SUPERUSER PASSWORDS! 12.2 Hardware Loopholes Although it is unlikely that a user will do this, there is a potential security risk to Sentry. Fortunately, this method can only be implemented by technical users. If a user really wants to get into your system, he can disassemble it, and take the battery out of your BIOS. This will reset your BIOS to the standard setup, which does not include password protection. From there, the user can enter your BIOS, and change the boot sequence from C,A to A,C. This means that your computer will search for a boot disk before booting from your hard drive. Therefore, the user can get in if he has a pre-made boot disk. Like I said, it's unlikely, but possible. If you want to fix up this back door, you can re-wire your floppy disk drive so that it is never used on boot up. I do not have instructions for that at this time, however I am looking, and will include them in future. Along the same lines, a user could replace your hard drive (the one containing Sentry) with another hard-drive. This way he could then boot up using the new hard drive, and never have to worry about Sentry. Also, if he kept your hard drive on the system (as drive D for example), he could still access your data. This kind of trickery is highly improbable, but not impossible. The above two methods take more technical skill than the average user posesses, and should not be considered a serious threat. I include them simply so you can be aware such things exist. If you are concerned about attacks of this nature, you should secure your system's case to the frame (IE: make it impossible to open the case with a screwdriver). It is critical to realize that it is EXTREMELY difficult to repel a determined and well-organized attack. Using Sentry does not guarentee your computer's safety. It does however greatly reduce the threat of a successful attack, and more importantly, it can alert you to potential threats before they become a serious problem. Your ability to defend is much greater once you know you are under attack. Sentry can assist in keeping you informed of suspicious actions on your PC, and it is a powerful tool in defeating most threats. 13.0 Troubleshooting ==================== 13.1 Error messages Below are listed all possible error messages you can get while running Sentry. With each is a brief description of what it means, probable causes, and how to fix it. Please be aware that ALL of these errors cause Sentry to lock-up. This may be an inconvenience at times, but it is done for security reasons (that way an attacker can't "induce" an error and get into the system). 13.1.1 Error opening password file! This means your password file cannot be found or opened. Make sure your password file is at the location specified by "PassFile" in your Sentry.ini file. As long as it's there, you should have no problems. 13.1.2 Error opening temp file! A temporary storage file cannot be opened. Make sure you have at least a little disk space left when you run Sentry. 13.1.3 Error in creating log file! The file that logs all transactions to your computer cannot be created. Make sure you have specified a valid pathname for "LogFile" in the Sentry.ini file, and make sure you have some free space on your drive. If a log file already exists, it will be appended to. Otherwise it will be created. 13.1.4 Error opening log file! There is a problem with the log file. Make sure that the log file pointed to by "LogFile" in the Sentry.ini file is not write-protected. 13.1.5 Error opening Sentry.ini file! This means that your Sentry.ini file cannot be found. Make sure the Sentry.ini file is in the same directory as your Sentry.exe file, and that it is named correctly. 13.1.6 Error in Sentry.ini file! [XXXXXXXX] Your Sentry.ini file does not follow the correct format. The error message should have a word in square brackets after it (IE: [PassFile], [PasswordExpiresIn], etc). This is the attribute in the Sentry.ini file that is causing the problems. (Sentry expects to find that attribute but doesn't). Use an editor to set the attribute correctly. If you are completely lost, restore the Sentry.ini file from you backup, or use the default attribute setting listed in the comments of the Sentry.ini file. 13.1.7 Registration Error - Program Aborted! Someone (probably you) has tried unsuccessfully to change the registration information. Very naughty, but if you want to fix it, simply restore Sentry.exe from your original copy. 13.1.8 Error in time stamps. This means there is a problem with the time stamps on a user's account. You can attempt to manually edit a user's record in the Sentry.pwd file, but I don't recommend this. If you don't know what you're doing you are more likely to cause additional problems than to fix the user. Otherwise, you can try restoring your password file from backup. If you continue to use a password file that is corrupt, you will probably find you are locked out quite frequently. It should be a top priority to fix the password file. 13.1.9 Out of memory! When this error appears, it means that for some reason, Sentry didn't have enough memory to run. This is extremely rare, since Sentry requires very little memory. If this error occurs, try freeing up some memory by unloading some un-needed programs, or rebooting. 13.1.10 This account has expired. Sorry, you're out of luck. The SuperUser(s) have set your account to expire, and so it has. If you ARE the SuperUser, silly you (you shouldn't let your own account expire!). In that case, you will have to restore your password file from a backup, and make the necessary changes to get your system running. 13.2 Other problems When I run Sentry, is displays a warning saying I am using a newer/older version of the Sentry.ini file. A: You should probably get the latest version of Sentry (see section 11.3 for info on obtaining the latest version). Replace all your current files with the files you retrieve. This warning may not be a problem in itself, however it is not a good idea to use a Sentry.ini file from a different version. I get a warning about users being tampered with in the log file. A: Someone has been tampering with your users (obviously). They thought they could change the SuperUser access on your system, but really couldn't. The only side effect of this is that some users may appear to have SuperUser access when they don't (and vice-versa) when you view them from the SuperUser menu. The users' actual access has not changed. To remedy this situation, you can get the user to enter another password (this can easily be done by expiring their current password). As soon as the new password is entered, the problem will disappear. I can't log in at all. A: Make sure you are a user on the system. Make sure your Sentry.ini file has been set up correctly. Make sure your password file is in the right location. I can't log in as a SuperUser. A: Did you give yourself SuperUser access? Are you remembering to hold down CTRL when you hit enter? My password file is corrupt or deleted. A: Pray you kept a backup somewhere. At the very least, you should have the password file sent with Sentry. In that case, re-install, and log in as Sentry. Since Sentry is initially a SuperUser, you can re-create your users (don't forget to delete the Sentry user when you are finished!). I can't create a c:\Sentry directory on my drive. A: If you are using MS-DOS's Undelete program, it may create a directory called "Sentry" off of your root directory. This directory is hidden, and can only be seen with the "dir /a" command. (This only occurs if you are using the "delete sentry" mode). Possible fixes are to stop using "delete sentry" mode, or to create your Sentry directory as something else (eg: c:\Sentry2, c:\Secur\Sentry, etc etc). My password file disappeared, but Sentry still works fine! A: Your password file is still there, but Sentry has hidden it from normal view. When you type "dir" your password file will no longer show up. This is done to confuse and mislead any potential attackers. If you want to check to make sure it is really there, type "dir /a" and it should show up. The log file is protected with the same measures. I copied my Sentry files to another drive/directory, and now it says my password file is not found! A: This is (once again) because the password file is hidden. When the copy operation was performed, it didn't "find" the hidden file and therefore was not copied with the other files. Older versions of the DOS copy command can copy hidden files, but that function has been removed in later versions. The best bet to avoiding this is to re-install Sentry on the destination drive/directory and create your accounts from scratch. (There are ways to move the password file, but I won't discuss them here). 15.0 Standard Disclaimer ======================== Inexperienced users should take care this program, as you may lock yourself out of your computer! If you feel you want to use it, but don't feel completely confident, leave a "back door" for you to use. (IE: don't implement all of the security measures listed in "Securing Your Computer"), so that if you DO get locked out, you can still get in somehow. I have not included a nice simple install program on purpose. If you aren't familiar with the basics of DOS, then you probably shouldn't be installing Sentry anyway. If there is a need for an install program, I may include it in a future release. I take no responsibility for how you use this program, or any effects it may have on your system. Having said that, I would greatly appreciate any comments you might have about my program (either positive or negative). If you find any problems, or have a suggestion for making Sentry better, please let me know, and I'll try to put it in a future release. I look forward to hearing from you. You can contact me via e-mail at Mike.Bobbitt@AcadiaU.Ca and I will reply as quickly as I can. If you do not have internet access, you can contact me via surface mail at: Michael A. Bobbitt P.O. Box 1336 Wolfville, NS B0P 1X0 16.0 Credits ============ My thanks go out to these people: Mark Saarinen for the encryption algorithm. D.J. Houghton, Rob Coombs and Jack Hill for testing and advising during the design phase. Bret Jacobsen for finding some serious errors in the first release, as well as continually making suggestions for improvement. Bret has given invaluable assistance in the development of Sentry. ...And my wife for listening to me talk incessantly about it.