I don't use EXE packers, but I would not mind trying 😀

So - if I understand you correctly, I could try packing say Adobe Acrobat Reader and other downloadable EXE's, because if something goes wrong in the packing/unpacking process, I can always download a fresh copy?

Executable compression is an excellent idea. I use UPX whenever I output something for general consumption. In any case, compression doesn't damage anything or make it harder if a program is corrupt -- a corrupt program will not run (or will fuck up and die) regardless.

Or fuck up and wipe out your hard drive. I've had it happen before, but a loooong time ago. With modern OSes it's much harder for a program to do something like that by accident.

A nice thing about UPX-compressed executables is that they will verify their integrity before running (checksum), so if the checksum is wrong they won't run. (Hah! I just said the same thing twice in one sentence!)

What's really nice about executable PE compresses is you can compress anything that's PE. Very nice to get as much space as possible on BartPE.

PE?

Sorry,

PE header

and

BartPE
http://www.nu2.nu/pebuilder/

PE = Portable Executable (the executable format for Win32 exes)
NE = (the executable format for Win16 exes)
LE = Linear Executable? (MS-DOS 32-bit Exes (DOS4GW, DOS32A...)
LX = OS/2?
MZ (Sometimes credited as ZM) = MS-DOS exes.

The two byte values are the first ones in the EXE header used for recognizing EXE type.

Here is a good free PE analyzer

And here is a short list of most known exepackers...

MS-DOS (MZ): Fabrice Bellard's LZEXE, PKLite, WWPack, aPACK, UPX. There are LOTS of them. Best ones are aPACK (for small-medium exes) and UPX (good compression ratios and lots of options). Normally, newer ones yield much better compression ratios.

MS-DOS 32-bit (LE): UPX again seems to be the absolute winner.

OS/2 (LX): LXLite is a well known packer.

Win16 (LE): They are very tricky for packing. Always try them thoroughfully after packing them. I have seen just a few packers for LE being PKLite the most known.

Win32 (PE): There are several packers available. Compressing PEs is an easy and almost safe thing (at least as EXEcutables are concerned). UPX is very good too (and safe), while Dwing UPack yields an astonishing compression ratio, but it's still in ßeta development.

There's is a catch when compressing Win32 DLLs (which are PEs too, also .OCX, .CPL and such can be packed) and is that when you pack a Dynamic Link Library it can't be shared in the same space of memory anymore, so when you use a compressed DLL keep in mind that it will spawn as many copies as needed in memory, taking up more space. Thus, it's recommended to pack just .exe files, not .dll - Not to care if they are small in size, though.

Note on UPX: It can also pack certain "uncommon" executables as Atari ST, C64 and such, commonly used in emulators.

To sum it up: PROs and CONs of executable compression:

PROs: Smaller file sizes make loading times a snap. Also you need less cache memory for storing them. Decompression times are negligible.
Packed executables are more reliable when running (self crc-check) and are harder to reverse-engineering.

CONs: Packed executables take up a little more memory than their unpacked counterparts. Some files need to be unpacked for accessing certain resources (rare) and some of them that perform self-checking routines need to be unpacked (antivir executables are examples).
Virus makers sometimes use not-well-know exe packers/encryptors/scramblers to disguise their evil code from antivirus programs.

Ah a familiar topic of mine. I've used a variety of packers from the most known like UPX to like ... some unknowns. There is a very popular EXE Packer compression test page somewhere around here.

Um.. Ah, [here] is the page you guys may want to look on for the best PE Compressors. There's a LOT of them, most unknown until I read this page. It's also the most up-to-date page.

Have fun boys
Greets
~Paladinate~

I do not trust the results shown on the comparison test link posted above. Why do they run some packers at default settings while others not? I don't get it.

For UPACK 0.24ß one should use -c6 -f255 parameters.