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Hopefully the question isn't too broad or off.

Basically, are there any practical, and by practical I mean secure enough that a ciphertext only attack is infeasible, key-less cryptosystems? For example, there's the Caesar Cipher (I'm aware of the fact that it can pass as a special case of the shift cipher with the key = 3, but let's assume that there is no general case of the hypothetical cipher), however, it's very impractical according to the aforementioned definition of practicality.

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Look up "security by obscurity" and "Kerckhoffs's Principle". –  Thomas Jan 28 '13 at 12:11
    
Are you assuming an attacker will never be able to look at the source code? –  mikeazo Jan 28 '13 at 12:20
    
That's a good place to start off, thanks a lot. Security through obscurity seems like a good candidate to answer the question. –  Khaled Nassar Jan 28 '13 at 12:22
    
@mikeazo Yes, I'm referring to a system that is un-crackable under ciphertext only attacks. –  Khaled Nassar Jan 28 '13 at 12:24
    
Is this an actual problem/question that you face or is this more of a thought exercise? The reason I ask, is if it is the former, perhaps a "keyless" cipher is not the only option and you might be better off posting your requirements or a problem description and seeing if there is a better solution than the "keyless" cipher. –  mikeazo Jan 28 '13 at 12:57

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up vote 7 down vote accepted

Sure, use AES with a fixed key. Since the key is fixed, it could be considered keyless in the traditional sense. Someone with only access to the ciphertext could not crack it as long as you use a good mode (say GCM, CCM, or EAX) using proper nonces, IVs, or whatever else is required.

This is security by obscurity as the plaintext is only as secure as the fixed key. Since the fixed key is on the devices, it is only as secure as the security of the devices. A physical security breach could leak the keys and thus destroy security. A "cyber" attack could do the same.

Note though, that this security will not continue forever. IV's will eventually repeat, counters will eventually repeat, etc.

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I think we can assume that IV's and counters will never feasibly repeat for a decent block size (128 or 256 bits), the only problem left being key compromise: how to recover if the key is leaked? do you lose backward and possibly forward secrecy? etc.. –  Thomas Jan 28 '13 at 12:48
    
@Thomas, IV's yes. Counters are not always the block length. It depends on how the counter is implemented. Recovery from key compromise is why I wouldn't touch a system like this with a thousand foot pole. It is hairy, but it was the assumption we were given to work with. –  mikeazo Jan 28 '13 at 12:56

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