When designing an encryption algorithm what are the key factors that determine what the algorithm would do? [closed]

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I'm looking into a little project of designing my own symmetric crypt algorithm. It will be for mainly file encryption. What is the design process for such a project?

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First lesson - don't.

Second lesson - learn cryptanalysis. If you aren't aware of how crypto schemes are already being broken, how will you assess how "strong" or "weak" your own algorithms are.

The self study paper I linked to above is aging a bit however. There are newer attacks (such as the recent bi-clique attack against AES) which won't be covered there.

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There's also a fledgling crypto stack exchange site, which may be a better place to ask questions, if you actually intend to study crypto seriously (rather than "I've invented the best crypto system ever, nobody will ever be able to crack it").

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Build an algorithm that is resistant to...

Known plain-text attacks: http://en.wikipedia.org/wiki/Known-plaintext_attack

Chosen plain-text attacks: http://en.wikipedia.org/wiki/Chosen_plaintext_attack

Differential cryptoanalysis: http://en.wikipedia.org/wiki/Differential_cryptanalysis

Linear cryptoanalysis: http://en.wikipedia.org/wiki/Linear_cryptanalysis

My specific suggestion is that you start with a cryptographically secure random number generator (http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator), seed said generator with your symmetric encryption key and work from there avoiding the attack vectors above.