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The Shadow
Registered: Oct 2007
Posts: 304 |
EOR file coders
Someone once told me that it is impossible to open a file which was coded with an EOR coder. With todays machines, is there any conceivable way that an EOR coded file can be placed into a PC and descrambled? |
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Ymgve
Registered: May 2002
Posts: 84 |
Could you explain a bit more what an EOR coded file is, or provide examples? Unless it's some strange form of one-time-pad, you should be able to analyze and possibly decrypt the data (And I'd gladly help you out just for the challenge of it). |
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Ymgve
Registered: May 2002
Posts: 84 |
After some quick digging I've found one tool that does EOR (EOR Maker V1.0), and also it seems like Turbo Assembler has support for EORing source files. In both cases only a single byte is used as the EOR value, so even brute forcing should be easy to do. |
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Marauder/GSS
Account closed
Registered: Jul 2006
Posts: 224 |
If you just pack your files with a simple RLE-packer, it's still possible to change bytes in the file, so a lamer still could alter text in the file - like it was done in some releases *g*
That's the reason why I have put a EOR-code into some of my old crunchers, like Marauder Cruncher-Coder-Linker V2.0
Ofcoz breaking a file EORed with a single value is very easy... (c;
If you use more EOR values it's ofcoz "harder" to break, but still possible to do if you find/know the decrypt-routine, hehe |
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AlexC
Registered: Jan 2008
Posts: 299 |
If you consider EOR loops in some protection software than most of code I've seen is based on 1 byte value (key to be correct with cryptography theory) so bruteforcing it is not a problem but usually finding the key by analyzing loader is also easy.
Another case is when you have just data file without any decryption code. If the encryption key is the same length as input file than it is not possible to decrypt it. However most c64 tools I've seen either were based on 1 byte key or key based on user input (password) with limited length. This means that you can use trivial attacks to decrypt file and recover key.
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Zyron
Registered: Jan 2002
Posts: 2381 |
tlr made De-Coder that can decode files protected by some programs. |
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Marauder/GSS
Account closed
Registered: Jul 2006
Posts: 224 |
if you know/have some bytes of the original (decrypted) content, then I guess you could find out the keys... if you don't know I think it's hard to find out if the key uses more than one value...
*edit: last question was a silly one...*LOL* where's that black 'n hot brew... coffeeeeee (c;
does that 'Vigenère variants' work also on non-alphabetic text ? I guess so, then it's maybe possible to do... |
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Quetzal
Registered: Jul 2002
Posts: 71 |
EOR encoded with a single byte would be no problem to break, encoded with a password gets tougher, but still very possible.
One game preview I cracked was encrypted with a password, which from memory was "GLORIOUS", but the programmer made a silly mistake in crunching both the viewable first part and the encrypted game with the same cruncher. Made my job so much easier having that known bit of data to work from.
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The Shadow
Registered: Oct 2007
Posts: 304 |
The good file coders I remember use an 8 byte system (The length of the password). If say someone were to 8 byte EOR code a file and remove the decoder, is there any possible way that a PC could crack it? |
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tlr
Registered: Sep 2003
Posts: 1790 |
Quote: The good file coders I remember use an 8 byte system (The length of the password). If say someone were to 8 byte EOR code a file and remove the decoder, is there any possible way that a PC could crack it?
Yes, probably. Especially if the file is in the normal ordering except for EOR:ing with a sequence of 8 bytes repeated.
De-Coder cracks those by assuming certain statistics of a particular byte value occuring.
I calculated those statistics from several thousands of files.
The length of the key is determined by an incidence check.
Key finding takes only ~15 seconds on a c64. :)
The algorithm used by J-Coder V1.0 and Encoder V1.0 is harder to break. It only does substitution per byte but in a fairly complex way.
Maybe someone with more insight into crypt analysis can break it?
@Marauder: yes, non alphabetic text works too. There is a disc of examples in the release you can try. A few have ctrl-chars in the password.
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AlexC
Registered: Jan 2008
Posts: 299 |
Quote: The good file coders I remember use an 8 byte system (The length of the password). If say someone were to 8 byte EOR code a file and remove the decoder, is there any possible way that a PC could crack it?
This is actually one of most basic and simple attacks on ciphers. Take a look here for some info: http://en.wikipedia.org/wiki/XOR_cipher |
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