Exe Encrypter/Packer

This is my first post to the forum..

Anyway I was wondering if any people here could help me understand how exe packers and crypters work. I get the basics of it that a packed exe
includes a loader stub & then exe data; the unpacking process is what
I don't understand, is the program unpacked to disk and run like that or is it
launched from memory (how?) also when a file is being packed is anything removed (Like the header).

Thanks for the help.

[dj-siba]

is the program unpacked to disk and run like...

No, it's unpacked to memory...

for more info read this:

hxxp://dasomnetwork.com/~leedw/pub/writing_your_own_packer.html

Regards

Packers basically serves two pupose
1. Reduce the file size
2. Make the code uninteligible to disassemblers and as such dissude prospective reverser from reversing programmes. This is further extended by use f anti-debuggng tricks
packers when run in right environment like no debuggers serves its pupose by first unpacking or decrypting the code it has crypted/packed and run the exe