How to class-dump AppStore app

I installed "Class Dump" from Cydia to get application header files. But there is a problem.

I can use class-dump in default app. For example, I ran this command:
class-dump -H /Applications/MobileSafari.app/MobileSafari -o /Headers/safari and could get header files in "/Headers/safari/".

But AppStore app, for example, I ran this command:class-dump -H /var/mobile/Applications/BFF...../Dropbox.app/Dropbox -o /Headers/dropbox and terminal works, but garbled characters appeared (ScreenShot) and then terminal crashed.

How can I get AppStore app header files?

All AppStore apps are encrypted. In order to use class-dump you need to decrypt (crack) them. You have two options:

1. Download cracked ipa of the app. Cracked apps are already decrypted and you can class-dump or disassemble them. This will work only for paid apps. You will not find cracked free apps for obvious reasons.
2. Crack it yourself. There are tools in Cydia for that - Rasticrac, for example. They are easy to use and will do all the work. When finished you will get cracked ipa. For free apps it's your only option.

On the other hand, built-in apps like Safari are not encrypted.