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AEFS - an encrypting file system for OS/2 and eComStation

By Robert Henschel © May 2004


There are a lot of ways to protect personal information under OS/2. For example, some programs offer the possibility to password-protect files. Lotus Word Pro and are among them. When saving a document, one has to enter a password and now this file can only be opened by entering the same password. (Of course, there is always the possibility to use crack programs to gain access to password-protected files. Such tools may not be available for OS/2, but that does not stop people from using them against OS/2 documents!)
If valuable information is not stored in simple documents, one can always use "ZIP" to protect any kind of data. All files are stored in a ZIP-Archive and this archive can be protected with a password. Under OS/2, one can use PKZIP. (ZIP-Archives are especially vulnerable against password cracking programs. There are ready-to-use cracking tools available in the Internet!)

Both procedures do have some disadvantages. On one hand one has to trust the companies that create office productivity applications. There is always the possibility that the encryption is not implemented correctly or that it can be circumvented in some way. On the other hand one has to extract the data from an archive into a temporary directory and after using it one has to recreate the archive. (And the temporary copies of the files need to be deleted!)
A more elegant solution to those problems is an encrypting file system. AEFS is just one of the available solutions; the Cipher Plug-In for NetDrive is another. Encrypting file systems encrypt all data that is stored on a particular drive without interaction by the user. They do not depend on the applications that are used to manipulate the data.

Description of AEFS

AEFS is an installable file system for OS/2 and eComStation. It can be used just like HPFS or JFS except that partitions do not need to be formatted. AEFS-drives can be used by any program that can access files. To make this possible, AEFS simply creates a new drive letter. All files that are stored on this new drive are automatically encrypted. There is no user-interaction required. To avoid having to partition the hard drive for this new drive, AEFS maps the new drive letter to an existing directory. It does not matter what file system is used on this drive. This will be explained in more details later.
Encryption is performed using well known algorithms like Rijndael and Twofish. The current version of AEFS limits the length of the key to 256 bit. It has to be said that 256 bit is still considered safe by current standards, but that could change rather fast in the future.


AEFS is available for OS/2 and Unix-like operating systems. Because of this, the EMX runtime is required (version 0.9d Fix 4).
The archive contains the source code of AEFS as well as a binary version. If the source code is not of interest to you, only the directory \bin and the files readme.txt or readme.inf are needed. After extracting the files to the final directory two lines have to be added to CONFIG.SYS.

IFS=[bin-directory]\stubfsd.ifs AEFS

[bin-directory] is the \bin subdirectory of the installation directory of AEFS.
To make working with the AEFS utilities easier, it is a good idea to add the [bin-directory] to the PATH of CONFIG.SYS. This is not necessary, but makes it easier. A reboot is required to activate the changes.

Using AEFS

To work with AEFS it is necessary to have an empty directory that takes the encrypted data, for example D:\Crypto. The directory can be located on any drive, no matter what the file system is. It can also be a network drive. If you want to use the mkaefs utility without a second command line parameter, the directory must not exist. The second thing that is needed is a new drive letter, for example "x".

Now one can create the AEFS file system. To do that, enter the following:

mkaefs d:\crypto

To create the AEFS file system, it is necessary to enter the password now.

To use the new AEFS file system, it has to be assigned to a drive letter. This can be done with the following command:

mntaefs.exe x: d:\crypto

When assigning the AEFS file system to a drive letter, the password is needed. One can only open the file system if the password is correct. After entering the correct password all programs can access the new drive "x".

Releasing the file system can be done with the following command:

umntaefs.exe x:

After releasing the file system, the data that was stored on drive "x" can only be accessed through the encrypted files stored in D:\Crypto.
It is a good idea to back up personal data. This is also true for encrypted personal data! Backing up the data is as easy as burning a copy of the encrypted directory D:\Crypto to a CD or packing everything in this directory in to a ZIP archive. Of course, it is also possible to back up the original not encrypted data directly from drive "x".

Mozilla - an example

Mozilla creates a profile for every user. Among other things, this profile holds the bookmark file and history. Placing this profile on an AEFS drive is quite easy. First of all, one has to start the profile manager of Mozilla and create a new profile.

Fig.1: Creating a new profile

The path for the new profile should be changed so that it points to the root of drive "x".

Fig.2: Changing the path for the profile

(Drive "x" is an AEFS file system. It has been created using the AEFS utilities.)
The Mozilla profile must only be accessible while Mozilla is running. To increase security, it will be opened just before Mozilla is started and released right after Mozilla is closed. One can do this manually every time, but using a CMD file is more convenient.

@echo off

mntaefs.exe x: d:\crypto

umntaefs.exe x:

@echo Wrong Password ?!


This CMD file should be saved into the Mozilla directory, the same directory where mozilla.exe resides. (A shadow of the file can be placed on the desktop.)
Starting the CMD file will ask the user for the password for the AEFS file system.

Fig.3: Enter password

Entering the correct password will start Mozilla. If the wrong password is entered, a short message is displayed and after hitting a key the window will be closed.

Fig.4: Wrong password

After Mozilla is closed, the AEFS file system is released and the profile directory is no longer accessible via drive "x".
The profile directory is on drive "x" and contains the personal settings for Mozilla.

Fig.5: Mozilla profile - not encrypted

The real data is stored in the directory d:\crypto and is encrypted.

Fig.6: Mozilla profile - encrypted

File system utilities

All utilities accept --help as a command line argument. So it is possible to get detailed help for all utilities on the command line. Even more information is available in the ReadME of AEFS.


Creates a new, empty AEFS file system. One needs to specify the path where the encrypted files will be stored. One can also specify the encryption algorithm, but this is optional.
Example: mkaefs d:\crypto


Checks an AEFS file system and corrects errors if possible. (This can be compared to chkdsk for HPFS or JFS) It is a good idea to create a backup of the encrypted data before using this utility!
Example: aefsck d:\crypto


Assigns a drive letter to an AEFS file system. It is also possible to mount the file system read only!
Example: mntaefs x: d:\crypto


Releases an AEFS file system.
Example: umntaefs x:


Decrypts a specific file of an AEFS file system.
This utility should only be used by experienced users!

aefsdmn and aefsparm

aefsdmn is started in CONFIG.SYS and its parameters are discussed in more detail in the ReadME. aefsparm can be used to change the parameters of aefsdmn while it is running.

Problems and Limitations

Compiling AEFS

The ReadME of AEFS lists all required programs and files for compiling AEFS. Useful MAKEFILEs are provided as well. AEFS does compile with the latest GCC of InnoTek as well as older GCC compilers. Among other things, one needs:

All MAKEFILEs are based on the Unix standard. To use them instead of creating new MAKEFILEs one needs a full Unix environment on OS/2. (SH, BINUTILS, GNUUTILS, GNUMAKE, AUTOCONF, etc.)


EMX Runtime 0.9d Fix 4:
Homepage of Eelco Dolstra (Author of AEFS):
AEFS version 2.1:
AEFS older version 2.0:
OS/2 e-Zine article about IFS's under OS/2:
Good website about IFS's under OS/2 (in german):
NetDrive and Cipher Plug-In:

Robert Henschel is the organizer of the OS/2 User Group Dresden and studies computer science at the Dresden University of Technology.

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