"Unable to execute file for security reasons" - background?

[theusualsuspect]

I have been tinkering around with .sisx files, modifying the E32Image files and the likes and stumbled upon this error. I used Google and I even found a few topics in this forum but none actually helped me any.

My actual scenario: I unpacked a .sisx file using sisinfo. I decompressed the contained .exe file using petran -nocompress. I took the uncompressed file, copied it back on my mass storage replacing the original .exe file and tried to run it, giving the error specified in the title.
I have read on the E32Image specifications but I couldn't really find anything related to a checksum that would really matter.
I tried something else, modified a single byte, in the decompressed file, recompressed it leaving the code size the same, leaving the header intact and its CRC checksum etc. but it would still trigger the "security reasons" error.

So, my question: What hidden detail makes Symbian OS know I had my fingers on the executable?

[wizard_hu_]

This is normal. The whole platform security and the signing process is about that you can not execute arbitrary binaries on the device. Prior to execution, the OS checks the integrity of the binaries using checksums generated on installation time (note that despite you are mentioning .sisx files, actually you have copied but not installed the modified one).

[theusualsuspect]

Oh alright, I expected something like that. So basically I have to repack the file into a .sis, resign it and then install it again to test it.

[ltomuta]

Actually, you can simply install the original sis file, without any changes ...