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Encryption challenge

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I remember reading a post a week or two ago about XOR encryption and to prove that it wasn''t secure, someone decrypted a jpg sent to them. Well, since reading that I''ve had ideas on easy, yet secure forms of encryption, and so I want to see just how secure my encryption algorithm is. If you want to, try to decrypt this jpg: coded.jpg You''ll have to right click and save as, since it''s no longer a valid jpg. Thanks and good luck! P.S. - if you need any hints I''ll give a few, and if you crack it, I''ll be very impressed.

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encryption isn''t even close to my strong suit, but i believe that the strength of an encryption algorithm is partly based on how secure it is _even when the cracker has your algorithm_. if your encryption is easily broken when the algorithm is public then it''s not good encryption. the idea is that your algorithm _will_ get out. if it''s not secure under that condition = no good.

-me

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In a way the encryption algorithm is already public (that's a mild hint). But if someone were to get a hold on my code then it wouldn't be secure anymore, so your point is well taken.

[edited by - YoshiN on December 2, 2002 8:47:18 PM]

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obscurity is not security

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A good encryption algorithm must be that even if you know the decryption algorithm, you can''t decrypt the packet without the key.

Then, even if you did that, it must be that the key can''t be discovered by any mathematical way, and beleive me, when you know cryptography, mathematics can do miracles.

______________________________
Oooh, you found the horadric cube!

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Well the algorithm itself is really simple, I took the ascii value of every character and added a certain number to it (and that number fluctuated throughout the file), and then XORed it. It also uses a random number range and a random key each time (so the range and key are hidden in the file). If someone knows the algorithm, it''s useless without the range/key, but if you have a file and you know where they are hidden, then it''s easy from there, so I guess it''s not too secure.

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quote:
Original post by YoshiN
Well the algorithm itself is really simple...

A good encryption scheme is secure even when the implementation of the encryption (and, in some cases, decryption) is in the public domain.

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On the topic of encryption and security, I have to wonder what will happen when/if quantum computer come into being. Apparently the current encryption schemes will be easily cracked with the new computers. Though apparently there are quantum encryption schemes out there that may be uncrackable. Maybe I should learn more about encryption

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Ok the following is not meant to sound harsh, it is just some advice, but I really hope you're not going to think that your cryptosystem is secure because no one here breaks it.

Posting that encrypted pic here is going to demonstrate whether it is a safe encryption or not. I can tell you this much - Unless you disclose at least precisely what algorithm you used, no one here will break it. In the thread where XOR encryption was broken, we all knew the algorithms. Posting that thing without telling us what the algorithm is, is equivalent to posting a file full of random garbage. If you fail to see even that, I already seriously doubt that you know anything about cryptography at all.

Read here why "contests" like this will prove nothing.

I will take the liberty of quoting some relevant pieces.
quote:

Cryptanalysis assumes that the attacker knows everything except the secret. He has access to the algorithms and protocols, the source code, everything. He knows the ciphertext and the plaintext. He may even know something about the key.

quote:

Unfair contests aren't new. Back in the mid-1980s, the authors of an encryption algorithm called FEAL issued a contest. They provided a ciphertext file, and offered a prize to the first person to recover the plaintext. The algorithm has been repeatedly broken by cryptographers, through differential and then linear cryptanalysis and by other statistical attacks. Everyone agrees that the algorithm was badly flawed. Still, no one won the contest.

And now, I really don't intend to sound harsh, but the fact that you're admitting that if your algorithm can probably be broken if it is known indicates that you aren't even familiar with Kerckhoffs principle. It embodies the entire philosophy of modern cryptography:
quote:

The security of a cryptosystem must not depend on keeping secret the crypto algorithm. The security depends only on keeping secret the key.

[edited by - mr BiCEPS on December 2, 2002 12:23:49 AM]

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try contacting the folks at http://members.aol.com/jpeschel/ to see how easy it would be to crack your crypto. they have contests for such things and have cracked all sorts of apps. take a look at http://members.aol.com/jpeschel/crack.htm for a list. and this is just a few guys. there are hundreds of sites/newsgroups like this around. give up your notion that you can invent a safe algorithm. these people are smarter than you.

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