Executable compression is any means of compressing an executable file and combining the compressed data with decompression code into a single executable. When this compressed executable is executed, the decompression code recreates the original code from the compressed code before executing it. In most cases this happens transparently so the compressed executable can be used in exactly the same way as the original. Executable compressors are often referred to as "runtime packers", "software packers", "software protectors" (or even "polymorphic packers" and "obfuscating tools").
A compressed executable can be considered a self-extracting archive, where a compressed executable is packaged along with the relevant decompression code in an executable file. Some compressed executables can be decompressed to reconstruct the original program file without being directly executed. Two programs that can be used to do this are CUP386 and UNP.
Most compressed executables decompress the original code in memory and most require slightly more memory to run (because they need to store the decompressor code, the compressed data and the decompressed code). Moreover, some compressed executables have additional requirements, such as those that write the decompressed executable to the file system before executing it.
Advantages and disadvantages
Software distributors use executable compression for a variety of reasons, primarily to reduce the secondary storage requirements of their software; as executable compressors are specifically designed to compress executable code, they often achieve better compression ratio than standard data compression facilities such as gzip, zip or bzip2 . This allows software distributors to stay within the constraints of their chosen distribution media (such as CD-ROM, DVD-ROM, or Floppy disk), or to reduce the time and bandwidth customers require to access software distributed via the Internet.
Executable compression is also frequently used to deter reverse engineering or to obfuscate the contents of the executable (for example, to hide the presence of malware from antivirus scanners) by proprietary methods of compression and/or added encryption. Executable compression can be used to prevent direct disassembly, mask string literals and modify signatures. Although this does not eliminate the chance of reverse engineering, it can make the process more costly.
A compressed executable requires less storage space in the file system, thus less time to transfer data from the file system into memory. On the other hand, it requires some time to decompress the data before execution begins. However, the speed of various storage media has not kept up with average processor speeds, so the storage is very often the bottleneck. Thus the compressed executable will load faster on most common systems. On modern desktop computers, this is rarely noticeable unless the executable is unusually big, so loading speed is not a primary reason for or against compressing an executable.
On operating systems which read executable images on demand from the disk (see virtual memory), compressed executables make this process less efficient. The decompressor stub allocates a block of memory to hold the decompressed data, which stays allocated as long as the executable stays loaded, whether it is used or not, competing for memory resources with other applications all along. If the operating system uses a swap file, the decompressed data has to be written to it to free up the memory instead of simply discarding unused data blocks and reloading them from the executable image if needed again. This is usually not noticeable, but it becomes a problem when an executable is loaded more than once at the same time—the operating system cannot reuse data blocks it has already loaded, the data has to be decompressed into a new memory block, and will be swapped out independently if not used. The additional storage and time requirements mean that it has to be weighed carefully whether to compress executables which are typically run more than once at the same time.
Also, some older virus scanners simply report all compressed executables as viruses because the decompressor stubs share some characteristics with those. Most modern virus scanners can unpack several different executable compression layers to check the actual executable inside, but some popular anti-virus and anti-malware scanners have had troubles with false positive alarms on compressed executables. In an attempt to solve the problem of malware obfuscated with the help of runtime packers the IEEE Industry Connections Security Group has introduced a software taggant system.
Executable compression used to be more popular when computers were limited to the storage capacity of floppy disks, which were both slow and low capacity media, and small hard drives; it allowed the computer to store more software in the same amount of space, without the inconvenience of having to manually unpack an archive file every time the user wanted to use the software. However, executable compression has become less popular because of increased storage capacity on computers. It has its use in the demoscene where demos have to stay within a size limit like 64 kilobytes to enter some competitions. Only very sophisticated compression formats, which add to load time, keep an executable small enough to enter these competitions.
List of executable packers
CP/M and MSX-DOS executable
- Realia Spacemaker (since 1982, written by Robert B. K. Dewar, SM.COM, signature "MEMORY$")
- Microsoft EXEPACK (since 1985, written by Reuben Borman, EXEPACK.EXE, LINK.EXE /E[XEPACK], signature "RB")
- LZEXE (since 1989, written by Fabrice Bellard, LZEXE.EXE)
- PKWare PKLite (since 1990, written by Phil Katz, PKLITE.EXE)
- DIET (since 1991, written by Teddy Matsumoto, DIET.EXE)
- TINYPROG (TINYPROG.EXE)
- RJS Software RJCRUSH (since 1994, written by Roland Skinner, RJCRUSH.EXE)
- XPA (since 1995, written by JauMing Tseng, XPA.EXE)
- Ibsen Software aPACK (since 1997, written by Jørgen Ibsen, APACK.EXE)
- UPX (since 1998, written by Markus F. X. J. Oberhumer and László Molnár)
- 32LiTE (since 1998, written by Oleg Prokhorov, 32LITE.EXE)
- Knowledge Dynamics LZW Compressor
- WWpack (written by Piotr Warezak and Rafal Wierzbicki, WWPACK.EXE)
- 624 (only for .COM files smaller than 25 KB, uses LZW)
- HASP Envelope
Known executable compressors under OS/2:
Known executable compressors for New Executables:
- PKWare PKLite (from version 2.01)
Known executable compressors for Portable Executables:
Note: Clients in purple are no longer in development.
|Name||Latest stable||Software license||x86-64 support|
|Armadillo||9.62 (7 June 2013)||Proprietary||Yes|
|ASPack||2.40 (7 December 2018)||Proprietary||Yes|
|ASPR (ASProtect)||2.78 (7 December 2018)||Proprietary||Yes|
|BoxedApp Packer||3.3 (26 July 2015)||Proprietary||Yes|
|CExe||1.0b (20 July 2001)||GPL||No|
|dotBundle||1.3 (4 April 2013)||Proprietary||Yes|
|Enigma Protector||6.60 (21 August 2019)||Proprietary||Yes|
|Enigma Virtual Box||9.40 (10 October 2019)||Proprietary||Yes|
|EXE Bundle||3.11 (7 January 2011)||Proprietary||?|
|EXE Stealth||4.14 (29 June 2011)||Proprietary||?|
|eXPressor||18.104.22.168 (14 January 2010)||Proprietary||?|
|FSG||2.0 (24 May 2004)||Freeware||No|
|kkrunchy src||0.23a4 (Unknown)||BSD||No|
|MPRESS||2.19 (2 January 2012)||Freeware||Yes|
|.netshrink||2.7 (2 July 2016)||Proprietary||Yes|
|Obsidium||1.6 (11 April 2017)||Proprietary||Yes|
|PELock||2.06 (15 August 2016)||Proprietary||No|
|PESpin||1.33 (3 May 2011)||Freeware||Yes|
|Petite||2.4 (22 September 2016)||Freeware||No|
|RLPack Basic||1.21 (31 October 2008)||GPL||No|
|Smart Packer Pro X||22.214.171.124 (3 June 2019)||Proprietary||Yes|
|Themida/WinLicense||3.0 (24 October 2019)||Proprietary||Yes|
|UPX||3.95 (26 August 2018)||GPL||experimental|
|VMProtect||3.4 (3 August 2019)||Proprietary||Yes|
|XComp/XPack||0.98 (18 February 2007)||Freeware||No|
Known executable compressors for ELF files:
CLI assembly files
Known executable compressors for CLI assembly files:
Mac OS Classic application files
Known executable compressors for Mac OS Classic application files:
Mach-O (Apple Mac OS X) files
Known executable compressors for Mach-O (Apple Mac OS X) files:
- HASP Envelope
Commodore 64 and VIC 20
Known executable compressors for executables on the Commodore Amiga series:
Known executable compressors for Java:
- HASP Envelope
- HASP Envelope
- Reduce the redundancy in the script (by removing comments, white space and shorten variable and functions names). This does not alter the behavior of the script.
- Compress the original script and create a new script that contains decompression code and compressed data. This is similar to binary executable compression.
These compress the original script and output a new script that has a decompressor and compressed data.
Redundancy reducing compressors
These remove white space, remove comments, and shorten variable and function names but do not alter the behavior of the script.
- YUI compressor
- Gielen, Pierre; Taylor, Johnathan (1997) . Logan, Wolverine (ed.). "PMarc help manual". Archived from the original on 2019-04-22. Retrieved 2019-02-22.
[…] PMEXE.CPM […] is a module […] in combination with PMARC […] used to make executable compressed COM files (just like LZEXE or PKLITE […] type: PMARC <archive>.COM=PMEXE2.CPM <filename> [options] The archive-name must be .COM […] not .PMA. The output file will have the extension .CPM. It's an MSX-DOS COM file […] rename file […] to run it […]
- "Expert Report of Robert B. K. Dewar In Response To The Report Of Kenneth D. Crews". Cambridge University Press et al v. Patton et al, Filing 124, Supplemental Initial Disclosures by Cambridge University Press, Oxford University Press, Inc., Sage Publications, Inc. - Cambridge University Press, Oxfort University Press, Inc., and Sage Publications, Inc. v. Mark P. Becker, Georgia State University President, et al, Civil Action No. 1:08-CV-1425-ODE (Court document). United States District Court For The Northern District Of Georgia, Atlanta Division. p. 18. Exhibit A. Archived from the original on 2018-05-01. Retrieved 2019-04-23.
[…] SPACEMAKER and TERMULATOR, commodity software for IBM PC (PC DOS file compression utility and VT-100 emulator), being marketed by Realia, Inc. R.B.K. Dewar (1982-1983), 8088 assembly language, 8,000 lines […]
- Realia, Inc. (January 1983). "If you use DOS, you need this program". PC Magazine (advertisement). Ziff-Davis Publishing. 2 (9): 417. Archived from the original on 2019-04-22. Retrieved 2019-04-22.
- Dewar, Robert Berriedale Keith (1984-03-13). "DOS 3.1 ASMB (Another Silly Microsoft Bug)". info-ibmpc@USC-ISIB.ARPA. Archived from the original on 2018-05-01. Retrieved 2019-04-23.
[…] The /E option of the linker should generate an EXE file which is logically equivalent to the uncompressed EXE file. The current version […] results in AX being clobbered. AX on entry to an EXE file has a definite meaning (it indicates drive validity for the parameters), thus it should be passed through to the uncompressed image. Given this one very obvious violation of the interface rules, there may be others, I have not bothered to investigate further […] I did write the Realia SpaceMaker program which does a similar sort of thing to the EXEPACK option (but needless to say does not have this particular […]
- Necasek, Michal (2018-04-30). "Realia SpaceMaker". OS/2 Museum. Archived from the original on 2019-01-27. Retrieved 2019-02-22.
- Parsons, Jeff (2019-01-10). "An Update on Early Norton Utilities". PCjs. Archived from the original on 2019-01-29. Retrieved 2019-02-22.
- Necasek, Michal (2019-01-12). "Yep, Norton Did It". OS/2 Museum. Archived from the original on 2019-04-22. Retrieved 2019-04-22.
- Necasek, Michal (2018-03-23). "EXEPACK and the A20-Gate". OS/2 Museum. Archived from the original on 2018-11-13. Retrieved 2019-04-20.
- Paul, Matthias R. (2002-10-07) . "Re: masm .com (PSP) related trouble". Newsgroup: alt.lang.asm. Archived from the original on 2017-09-03. Retrieved 2017-09-03.}
- Miles, Ya'akov; Nather, Ed (1986-05-17) [1986-02-05, 1986-02-09]. "Undocumented Microsoft LINK option: /E". INFO-IBMPC mailing list. Archived from the original on 2018-05-01. Retrieved 2019-04-26.
[Miles:] There exists an undocumented […] switch to Microsoft LINK.EXE […], which will cause an automatic compaction during binding. This process will eliminate storage for uninitialized arrays from the .EXE file produced by the linker […] To use this feature, specify the /E option to the command line […] [Nather:] The option does not exist in MS Link versions 3.00 and 3.01 [Miles:] By comparing the sizes of the (packed) files generated from LINK ver 3.02 and the /E option with the size of the .EXE file manually packed with […] EXEPACK, I have come to the conclusion that LINK ver 3.02 option /E generates EXACTLY the same size file as manually running EXEPACK on a regular .EXE file output by LINK […]
- Bellard, Fabrice (2003-02-09). "LZEXE home page". bellard.org. Archived from the original on 2019-03-24. Retrieved 2019-03-18.
- Salomon, David (2000) . "Chapter 3.22 EXE Compressors". Data Compression: The Complete Reference (2 ed.). Springer-Verlag. p. 212. doi:10.1007/978-3-642-86092-8. ISBN 978-3-540-78086-1. Retrieved 2019-04-26.
- "Archived copy". Archived from the original on 2013-08-21. Retrieved 2013-05-06.CS1 maint: archived copy as title (link)
- DotProtect http://site.yvansoftware.be/dotpacker1_0 Archived 22 January 2011 at the Wayback Machine
- Kiene, Steve; Mark, Dave (1999). "A Chat With Steve Kiene". MacTech. Vol. 15 no. 4. Retrieved 2017-12-10.