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I'm not sure I even understand this site, but based on what I see, how can one tell if a cipher is a good one-at what level is it considered good?

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The question seems to be rather broad. Especially since there's no "one fits all" solution when it comes to cryptography. Maybe you could edit your question and describe which exact scenario/situation you might have in mind? Meaning: what do you want to secure, and under what situation? That way, answers could help you understand what makes a good cipher or protocol under such conditions, and why. –  e-sushi 14 hours ago
    
Only trust a cipher if enough cryptographers tell you it's good. Winning a NIST competition (AES/Rijdnael or SHA-3/Keccac) or at least being among the finalists is a good sign. –  CodesInChaos 51 mins ago

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By cipher you could mean many things. What comes to mind is substitution (every a is __, every b is __...) but I don't think that's what you mean. I think you mean encryption algorithm (eg: AES, RSA, etc.) the unit of measure for security there is entropy.

And if you mean substitution, that is not at all secure and should almost never be used unless it is a PART of another algorithm.

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I mean some sort of algorithm although I have no clue what you are talking about with RSA and AES so how can I tell if the encryption algorithm I have is good? –  JediPythonClone 18 hours ago
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read this... stackoverflow.com/questions/2979174/… –  stealth9799 17 hours ago
    
but I didn't exactly understand..... Btw Do you know of any sight where people exchange large coded messages and try to crack them? –  JediPythonClone 17 hours ago
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@JediPythonClone Any encryption scheme you, or anyone without significant cryptographic background, comes up with is not worth the electrons used to implement it. Cryptography is not a field in which you want to be innovative -- there are standard algorithms that have resisted attacks for years, and you should use one of them. –  cpast 14 hours ago
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@Jedi No. If everyone uses it, that doesn't affect how hard it is to crack; if it's cracked, it was bad no matter how many or few people used it. The only way to be reasonably sure something is secure is that it is in wide use, has been for a while, and hasn't been broken, OR that for whatever reason large numbers of top cryptographers looked at it for a while and found no issues. –  cpast 7 hours ago

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