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I want to encrypt and decrypt some wordpress posts. Only the content field i care about.

Is it safe to use the same key and iv for the process?

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No. To which degree it is unsafe, depends on many factors. The reason that you choose to use an algorithm that provides you with the facility to use both an IV and a key should be a clue that they should not, in fact, be the same. The entire purpose of an IV is, in fact, to allow you to safely reuse a single key, by changing the IV.

However, another nice property of an IV is that it need not be a secret. Only the key must remain secret. This is why IVs are often sent in the clear along with a ciphertext. So, depending on the constraints of your system, hopefully you can find a way to use a unique IV which will help you to maintain the expected security properties of the algorithm and mode you choose.

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