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Lethe is a tiny and well-simple library/tool for data wiping. In Greek mythology Lethe is one of the five rivers from
Hades underworld. According to the myth, who touch, drink or even take a single drop from this river will experience
forgetfulness. You can use this data wiping tool on Linux, FreeBSD, NetBSD, MINIX, OpenBSD , Solaris
and Windows.
This is my implementation of a suggestion given by the worldwide known cryptographer and information security specialist Bruce Schneier in his book "Applied Cryptography", about destroying information:
"Most commercial programs that claim to implement the DoD standard overwrite three times: first with all ones, then with all zeros and finally with a repeating one-zero pattern. Given my general level of paranoia, I recommend overwriting a deleted file seven times: the first time with all ones, the second time with all zeros, and five times with a cryptographically secure pseudo-random sequence. Recent developments at the National Institute of Standards and Technology with electron-tunneling microscopes suggest even that might not be enough. Honestly, if your data is sufficiently valuable, assume that it is impossible to erase data completely off magnetic media. Burn or shred the media; it's cheaper to buy media new than lose your secrets."
-- Bruce Schneier ["Applied Cryptography" 228-229 pp.]
This book was written at 90's. DoD additionally states that:
"The number of times an overwrite must be accomplished depends on the storage media, sometimes on its sensitivity, and sometimes on different DoD component requirements."
-- National Computer Security Center ["A Guide to Understanding Data Rememberance in Automanted Information Systems"]
Here, overwrite times are configured by the user. Anyway, I think that burn-and-shred advices given by Mr. Schneier should not be discarded.
WARNING: This general DoD information destruction method does not work with flash based filesystems. It probably will
not work with your smartphone. Do not use this tool for doing this! Maybe in future I may extend this code to give some kind of
support for flash stuff too, but by now, I won't do it.
For ensuring that the implemented data destruction here is working fine on your system the well-known forensic tool
Foremost is used (if you are on Linux, in this case and have it installed). There are ports of Foremost for
other Unix-like than Linux, if you have a port of it installed, it will be used. If you do not have Foremost installed
and still are on some Unix-like, the test binary will try to perform a MacGyver with cat + strings + grep for all necessary
recoverying stuff.
I strongly suggest you to build the software on your machine by running all tests before installing the tool or using it as a library into your own stuff. Be sure that it is actually working instead of just believing on it.
Finally, I am not reponsible for any misuse of information or code here. I am not responsible for any possible damage, data destruction or loss (partial or total) done by this software, too. Who knows your babysitter! Use it at your own risk!
Well,
you@Hades:~/src# git clone https://github.com/rafael-santiago/lethe --recursive
you@Hades:~/src# _
All done.
Lethe's build is based on another tool of mine called Hefesto. After following all steps for making Hefesto works on your system, if you want to build the library:
you@Hades:~/src# cd lethe/src
you@Hades:~/src/lethe/src# hefesto --forge-type=lib
you@Hades:~/src/lethe/src# _
If the build succeeds the ar file will be at ../lib/liblethe.a.
Now, supposing you want the tool:
you@Hades:~/src# cd lethe/src
you@Hades:~/src/lethe/src# hefesto --forge-type=tool
you@Hades:~/src/lethe/src# _
If the build succeeds the binary will be at ../bin/lethe.
On Unix-likes when running the build at the first time it will try to detect the device path where your / is stored.
This device path will be saved to src/DEV_PATH. This device path is very important to ensure that data wiping done
by Lethe is really working on your system. If you have cloned this repository to a place where it will not be stored into the
detected device path, you need to change the content of src/DEV_PATH to the correct device path where your repository copy is
stored.
Your device will not be written. It will only scanned trying to find test data during lethe_drop tests. Since your device
is being read on-line (it will not be umounted for sure) try to minimize disk activity on your system when building Lethe. Otherwise
your disk activities can screw up all tests.
Depeding on the size of your storage device the tests can take some time, go walk your dog (good luck if you have a cat instead), stroll out
there, etc. Come back later! If you are in a rush add to the build command the option --quick-tests. Anyway, I strongly suggest you to
test if the data wiping is actually working on your environment before installing and using Lethe.
Yes, there is.
After running the tool's build, all you should do is:
you@Hades:~/src/lethe/src# hefesto --install
you@Hades:~/src/lethe/src# _
If you want to uninstall it:
you@Hades:~/src/lethe/src# hefesto --uninstall
you@Hades:~/src/lethe/src# _
On Windows the install script will try to export the lethe's binary path. With it you will be able to call lethe
from anywhere within your system. However, you should run the install command with administrator privileges.
Lethe is a well-simple tool. It works based on commands and command options. The general idea when using lethe from your
command line is: lethe <command> [options].
Until now lethe has the commands listed in Table 1.
Table 1: Current commands supported.
| Command | Utility |
|---|---|
drop |
Removes files and directories |
help |
Offers quick help guide for commands |
man |
Manual reader |
version |
Displays the tool version |
It removes files. Its synopsis is: lethe drop <file name and/or glob patterns> [options].
Supposing you want to remove the file thanks-for-nothing.txt and all files containing crimson-ballroom in their names:
you@Hades:~/tmp# lethe drop thanks-for-nothing.txt *crimson-ballroom*
you@Hades:~/tmp# _
By default Lethe will ask you if do you really want to delete a found file. If you prefer skipping all possible confirmations
you must use --ask-me-nothing bool option.
you@Hades:~/tmp# lethe drop *make-it-alright* televison-addict* \
> --ask-me-nothing
you@Hades:~/tmp# _
The removing process basically consists on repeated overwrites and renames passes. By default you have five overwrite passes
and ten renaming passes. In order to change those default values use the options --overwrite-passes=<n> and/or --rename-passes=<n>.
The total of passes must be one at least or a greater value. Let's use 200 renaming and 1000 overwrites passes.
you@Hades:~/tmp# lethe drop * --ask-me-nothing \
> --overwrite-passes=1000 --rename-passes=200
you@Hades:~/tmp# _
Warning: The given sample command can be dangerous depending on where you are testing it. Lethe always recursively
removes any found directory. I meant that it will be emptied by using an implicit "*" glob pattern. Be careful when using this
tool.
The renaming and overwriting stuff uses random data. By default those data is provided by the internal Lethe's randomizer. If you prefer
providing your own randomizer you need to use the option --dyn-randomizer=<lib-path>:<function-name>.
Supposing you have a dynamic local library called my-awesome-csprng.so. This library has the function mac well-exported
and it is "your awesome csprng":
you@Hades:~/tmp# lethe drop [AB]-REPORTS-*-1995.DAT \
> --dyn-randomizer=my-awesome-csprng.so:mac --ask-me-nothing
you@Hades:~/tmp# _
The prototype for a Lethe randomizer implementation is:
unsigned char randomizer(void);If during a drop process you regret about your remove choices, try to hit CTRL + c as soon as possible. It will prevent of
removing more files by aborting the application. You can also interrupt lethe by sending a SIGINT or SIGTERM to its
process.
It shows the content of this manual at your terminal screen by using your environment pager:
you@Hades:~/tmp# lethe man
Nothing special. It only gives you a quick command synopsis. You only need to provide as option a command as the help topic you are looking for:
you@Hades:~/tmp# lethe help drop
It reports the version of your Lethe binary:
you@Hades:~/tmp# lethe version
But you can also use --version if you prefer:
you@Hades:~/tmp# lethe --version
By the way, Lethe is GPLv2. If this license fits to you, for using Lethe as a library, you only need to include
src/lethe.h and "call" lethe_drop(). For building it you need to link your stuff with liblethe.a.
Now, lethe_drop basically takes two arguments:
- The filename or a glob pattern.
- The drop type.
If it returns zero all was done, otherwise some error has occurred during the process.
Take a look:
// test-lethe.c
#include <lethe.h>
int main(int argc, char **argv) {
...
if (lethe_drop("sensible-data.txt", kLetheDataOblivion) == 0) {
printf("done!\n");
}
return 0;
}Supposing the Lethe's code base is located at ~src/lethe:
you@Hades:~/src/test# cc test-lethe.c -I~/src/lethe/src -L~/src/lethe/lib -llethe
you@Hades:~/src/test# _
Pretty simple. Now explaining:
lethe_drop("sensible-data.txt", kLetheDataOblivion)When you call lethe_drop by passing kLetheDataOblivion the content of the file will "fall into oblivion" but the
file will remain.
If you want to remove the file too, use:
// test-lethe.c
#include <lethe.h>
int main(int argc, char **argv) {
...
if (lethe_drop("sensible-data.txt", kLetheDataOblivion | kLetheFileRemove) == 0) {
printf("done!\n");
}
return 0;
}Lethe needs some randomizer to perform all file oblivion stuff. If you prefer using your custom randomizer you must pass
it in lethe_drop call. The randomizer must have the following prototype:
unsigned char your_custom_randomizer(void);Now you must add to drop type argument the mask kLetheCustomRandomizer, as follows:
if (lethe_drop("sensible?data-[01]*.txt",
kLetheDataOblivion | kLetheFileRemove | kLetheCustomRandomizer,
ur_awesome_csprng) == 0) {
printf("done!\n");
}Be careful when using lethe_drop, by default it does not prompt you about removing or not what was found.
After all you are taking a drop from Lethe, right? ;)
Anyway, if you want to have the possibility of changing your mind after calling lethe_drop, you must add to the
drop-type argument kLetheUserPrompt mask.
if (lethe_drop("picture.jpeg",
kLetheDataOblivion | kLetheFileRemove | kLetheUserPrompt) == 0) {
printf("done!\n");
}It is also possible to set some internal components of implemented data wiping system:
- Total of overwrites.
- Total o file renamings.
- The used 'stat' function.
When those functions succeeds it returns zero.
Setting total of overwrites:
if (lethe_set_overwrite_nr(2) != 0) {
fprintf(stderr, "ERROR: Overwrite numbers not set.\n");
}The passed number must be a number from 1 up to n.
Setting total of file renamings:
if (lethe_set_rename_nr(312) != 0) {
fprintf(stderr, "ERROR: Rename numbers not set.\n");
}Setting 'stat' function:
if (lethe_set_stat(ur_custom_stat) != 0) {
fprintf(stderr, "ERROR: Stat function not set.\n");
}Certain applications are more careful for avoiding situations where some explotation could be harmful for theirs users. For example, privacy
could be compromised if some well-basic libc functions were hooked. In order to mitigate it Lethe offers the option of explicitly
indicating the address of some libc functions. The functions currently available are: memcpy, memset and memcmp.
For setting them you need to pass the pointer to your own implementation. If the set was done the function returns zero:
if (lethe_set_memcpy(builtin_memcpy) != 0) {
fprintf(stderr, "ERROR: Unable to set memcpy.\n");
exit(1);
}
if (lethe_set_memset(builtin_memset) != 0) {
fprintf(stderr, "ERROR: Unable to set memset.\n");
exit(1);
}
if (lethe_set_memcmp(builtion_memcmp) != 0) {
fprintf(stderr, "ERROR: Unable to set memcmp.\n");
exit(1);
}