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    <p>You should always use a key stretching algorithm when taking user input for a key such as a typed password. The key stretching does a few nice things. First it redistributes the entropy of your key (SHA1 does this as well) making the key appear more random (it isn't actually more random, the entropy remains the same), and second it makes brute forcing the key more computationally intensive (increasing with iterations obviously). The use of a random salt also precludes using precomputed lookup tables.</p> <p>You should absolutely use a standard algorithm for this such as <a href="http://en.wikipedia.org/wiki/PBKDF2" rel="noreferrer">PBKDF2</a>. In Java you can get a key factory for this via <code>SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1");</code></p> <p>If your storing encrypted data in an environment you do not control you should also generate a MAC on your IV+Ciphertext and store it with your ciphertext. You can prepend it in the clear similar to the way you are storing the IV. Verify the MAC before decryption, you should verify indirectly by hashing the MAC first (a simple SHA1 works here) so as not to create a timing attack vector. </p> <p>A MAC algorithm such as HMACSHA1 requires a secret key similar to a cipher. You should not use the same key for encrypting and generating the MAC. You can use the key stretching algorithm to generate a long enough key that you can use part for your cipher and part for your MAC.</p> <p>ADDENDUM: If you are using Java 7 (or an external JCA provider that supports it) include a MAC with your AES cipher by using GCM mode. AES in GCM mode is a form of authenticated encryption that validates integrity as part of cipher. Implementing MAC generation and validation has various pitfalls that need to be avoided (such as the timing attack I mentioned or using separate keys) and rolling it in to the cipher is one less thing to screw up.</p> <p>Creating secure crypto systems is not a trivial task, there are lots of ways to screw it up and make the entire process insecure. Instead of creating your own crypto system by putting together various crypto primitives it is generally a better idea to use a higher level library to handle things like cookie encryption and data storage or SSL/TLS for data in transit.</p>
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    1. POAES with password based SecretKeySpec vs PBE
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    1. COThe MAC is critically important for cookie encryption. You could also consider an authenticated encryption mode such as [GCM](http://en.wikipedia.org/wiki/Galois/Counter_Mode) instead of HMAC, as it's faster and easier.
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    2. CO@ntoskrnl Excellent point, I added it to my answer. AEAD modes becoming a standard part of the Java library is an excellent advancement. I also forgot to include my usual warning about rolling ones own crytpo systems.
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    3. COThanks for the feedback, there is one point though that I don't quite understand yet: how to verify MAC indirectly before decryption? (unfortunately this code needs to work with Java 6, so I'm not sure I can use GCM yet, nonetheless I will read up on it as well, thanks for the pointer!)
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