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I have a file that supposedly contains AES-256 encrypted data. The file was created using a proprietary program that claims to use AES-256.

I have tested decrypting the file with an open source program (openssl) and it works, I get my exact original file back.

Given this, is it possible that the proprietary program created a back door into the encrypted file?

In other words, could the program's developer be able to access my original file given the encrypted data (assuming he doesn't know the password I supplied). Does my ability to open it with OpenSSL prove that there isn't some intentional weakness in the way that the proprietary software encrypted it?

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I don't understand what the issue is? –  Lucas Kauffman Jul 29 '13 at 12:18
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With your information nobody is able to answer your question. The main question that is open for me: Do you have put a passphrase or key to openssl to decrypt you file? Where does this key come from? –  Uwe Plonus Jul 29 '13 at 12:18
    
He's basically asking if the program that encrypted his file is able to inject a backdoor in his file. Well basically that's possible depending on the file, backdoor and the environment (win/lin/mac). –  HamZa Jul 29 '13 at 12:22
    
I have updated the question –  user28825 Jul 29 '13 at 12:27
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@user28825 What ? The "developer's program" needs a password to encrypt your file (data). So the last sentence doesn't make sense. If the "developer's program" has access to your file, then what stops him from modifying the content and making it malicious before encrypting it ? –  HamZa Jul 29 '13 at 12:30
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There is no reason to assume that because another program can decrypt an AES encrypted file that it's backdoored. AES is a standardized form of encryption, so if your proprietary protocol implements this standard, then it's quite normal that you can decrypt this data with openSSL as well (which also follows this standard).

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Man, if you decrypted this file with openssl and decrypted data are exactly this what you expected then it means that backdoor has not been injected. If you have sensitive data then I suggest you to use good known tools, then you do not have to be afraid of such things.

Second question: can sbd change encrypted by AES data? Answer is no, nobody can. AES is just symetric cipher and you can't manipulate data if you don't know a password. To manipulate encrypted data you need use a homomorphic cipher. To do this you have tool like HElib: https://github.com/shaih/HElib

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Your second part isn't strictly true. AES itself doesn't protect against the data changing, unless you're using an authenticated mode. So without the key, you can change the data, you just won't know what you're changing it to. –  Brendan Long Jul 31 '13 at 16:05
    
yep, you right sir! thanks for correction –  n0-clues Aug 2 '13 at 9:16
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