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As title I have a password-like passphrase (8 chars at least) that is then hashed with argon2(with the salt is SHA256 of that passphrase).

Then use it as AES256 key along with a random IV generated by CryptoJS.lib.WordArray.random(256 / 8);

Is it okay?

Or should I use the IV as a key?

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    $\begingroup$ Did you search our site? We had tons of questions like this. Hash functions cannot increase the entropy. So, your input has 64-bit entropy ( assuming the ASCII encoding). This is a matter of time for huge adversaries like NSA, Summit, and collective bitcoin miners to reach your key in seconds. IV is not a key. Start searching from here password+AES $\endgroup$
    – kelalaka
    Commented Aug 17, 2022 at 15:06

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with the salt is SHA256 of that passphrase

Don't use the SHA-256 of the passphrase as the salt; randomly generate it so it's not the same for identical passwords.

CryptoJS.lib.WordArray.random(256 / 8);

Don't use CryptoJS. From memory, it generated random numbers incorrectly, the passphrase methods for the ciphers used/use some broken KDF, and it doesn't offer AEAD modes.

See if you can use libsodium.js instead. Alternatively, there's the Web Crypto API. Unfortunately, I know nothing about JavaScript.

Then use it as AES256 key

I presume you're currently using AES-CBC (the default for CryptoJS). It needs to be paired with HMAC-SHA-256 or HMAC-SHA-512 to be secure.

If you switch to a better library, you should just use an AEAD mode like AES-GCM or ChaCha20-Poly1305. You then don't need to apply HMAC because they authenticate the inputs for you. Importantly, with most AEAD modes, you should use a counter nonce (number used once), meaning you increment it after each encryption operation with the same key.

should I use the IV as a key?

No, the IV should only be used as the IV and is a different size to the key. The Argon2 output should be used as the key.

along with a random IV

With AES-CBC, the IV should indeed be random and unpredictable for each encryption operation. It should not be reused with the same key. You can prepend it to the ciphertext output.

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    $\begingroup$ CBC requires not only random also unpredictable in network communications. $\endgroup$
    – kelalaka
    Commented Aug 17, 2022 at 18:54
  • $\begingroup$ I originally wrote that but dumbed it down because random via a CSPRNG is unpredictable. I will add it back. $\endgroup$ Commented Aug 17, 2022 at 19:37
  • $\begingroup$ I'm not sure a counter nonce is a good suggestion. In many cases there is no good way to keep the state of the counter. And e.g. AES-GCM leaks the key if you accidentally re-use that nonce. $\endgroup$
    – Elias
    Commented Aug 18, 2022 at 10:40
  • $\begingroup$ @Elias It's not sensible to use a random 96-bit nonce either unless you're rotating the key frequently. $\endgroup$ Commented Aug 18, 2022 at 14:31
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    $\begingroup$ Agreed, just wanted to point out the danger. For a counter it's not being able to keep the state properly for random nonces it's not having good randomness or too few bits. $\endgroup$
    – Elias
    Commented Aug 23, 2022 at 7:20

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