What we mean by NOPs generator and payload encoders

[metafan at intern0t.net (MaXe)]

Deependra Singh Shekhawat wrote:
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> Hello guys,
>
> Started with metasploit few weeks back. Still learning and reading about
> it. While reading about the architecture of metasploit I came across NOP
> generators.
>
> Now I have found that these are no operation generators (correct me if I
> am wrong) used to let the exploit get un-detected from IDS.
>
> Can some one please elaborate more on NOPs and how they are generated as
> well as how one can use them in creating one's own exploit or porting
> any existing exploit to the metasploit framework ?
>
>
> Also I have one doubt about Encoders , I read that encoders are used to
> encode the payloads and probably they get decoded at the target machine,
> can you please explain how this process happens and what basically
> triggers this ?
>
> Thanks for the great stuff , guys . Really appreciate it as I read more
> about it.
>
> ~jeevanullas
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>   
A good way to learn more about NOP Generators and also get more 
information about NOP sledges is to read about Buffer Overflows! :-) 
http://en.wikipedia.org/wiki/Buffer_overflow
Of course there are many other good resources and i guess even one from 
here could explain how it works as well though shortly said it's used to 
make sure you will execute your payload :-) Stability for payloads is 
also another phrase that could fit. It's simply due to different kinds 
of systems may not execute an exploit 100% the same, so we add NOP 
sledges / slides :-D

About encoding and decoding payloads which is a bit more tough, there 
are a few resources on Wikipedia which elaborates about Polymorphic 
Code: http://en.wikipedia.org/wiki/Polymorphic_code
Of course there are other ways to encode a payload and my knowledge 
about this topic doesn't go that far, except that there is an encoded 
section and a section with a "decryption key". There is another 
"concept" which is called Mutating Code, that's something that is quite 
interesting as well ;-)

I hope i answered your questions sufficiently. I might be a little 
wrong, but not totally though i must admit that Polymorphic Code doesn't 
have much to do with Metasploit, not sure..


Best Regards,
MaXe