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POEncrypting a file with a weak password, bcrypt or SHA-256 + AES-256?
 

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  1. POEncrypting a file with a weak password, bcrypt or SHA-256 + AES-256?
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    <p>I start with a weak password (8 lower case characters for ex) and a file. I need to encrypt that file using that password. Result has to be secure against known attacks.</p> <p>Approach 1: I could hash the password using SHA-256 and then use the resulting hash and file as inputs to AES-256, giving me an encrypted file. I understand that both SHA-256 and AES-256 are very fast. Wouldn't this make the file vulnerable to a brute force attack? </p> <p>For example, could one grab a rainbow table of pre-computed SHA-256 hashes and, assuming its a really small file and a really weak password, try to AES-256 decrypt using each hash from that table in a reasonable time (a few months with specialized hardware).</p> <p>Approach 2: Use bcrypt. If I understand correctly, bcrypt is better suited for encrypting files than SHA-256 + AES-256, since it's key generation scheme has a work factor resulting in a stronger key. Or am I wrong?</p> <p>The Ruby and Python implementations (wrappers?) that I've seen focus on using bcrypt as a hashing scheme for passwords, not a cipher per se. Can I even use bcrypt to hash a weak pass AND encrypt the file in "one step"?</p> <p>Approach 3: Use bcrypt to hash the pass, use that hash and file as inputs into AES-256, giving me the encrypted file. This takes care of the "key is too fast to generate" problem. (Assuming its a problem.) However, bcrypt hashes are 448-bits long and AES-256 wants a 256-bit key. Naive solution is to simply drop the trailing bits of the hash and use that as the key for AES-256. I would NOT go this route because I don't know enough about cryptography to know what the consequences are.</p> <p>EDIT: <strike>I can't salt the pass, since this is for an offline application. ie. there is no reasonable place to store the salt.</strike> I can salt the pass and store the salt unencrypted along with the encrypted file. Salts are almost inherently public/visible if say a database is compromised. Purpose of a salt is to prevent a rainbow table attack. Thanks to Nemo, bellow.</p>
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    1. USNick Zalutskiy
    1. USNick Zalutskiy
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    1. COAll of them sound like a bad idea. If you start with a weak password, any transformations you apply are still going to make a weak password as they will probably be known or rely on obscurity.
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      1. POEncrypting a file with a weak password, bcrypt or SHA-256 + AES-256?
      1. USJeff Foster
    2. COI guess the question is what is the most we can offer when the user, against all suggestions, decides to use a weak password. I don't want to require passwords with symbols and CAPS and all that because people forget them, as a rule.
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      1. USNick Zalutskiy
    3. COHow about leaving that decision to the user, instead of forcing them to use weaker passwords?
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      1. USAaron Klotz
 

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