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SHA-256 generates a 32-byte hash, Is it a safe practice to use the first 16 as an iv (nonce) and the second 16 as a key ?
What other things I should consider when using PBKDF2 in a scenario like this ?

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Yes, you can use part of the output of the PBKDF2 function as the encryption key, and part of the output as the IV. This is exactly how openssl enc does it.

PBKDF2 accepts a parameter for the number of bits in the output. If you are using 128-bit AES, then you can set PBKDF2 to generate a 256-bit output and use 128 bits for the key and 128 bits for the IV. If you are using 256-bit AES, then you can set PBKDF2 to generate a 384-bit output and use 256 bits for the key, and 128 bits for the IV.

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  • that was a mistake... I meant 32-byte not bit
    – Mohammed Rady
    Commented Jul 27, 2021 at 21:03
  • I know that I can specify the number of output bytes. It is a bad practice when the output size is larger than the hash alg output size. What about if the output size is smaller than the hash alg output size ? Isn't it the best to use the whole output result ?
    – Mohammed Rady
    Commented Jul 27, 2021 at 21:10
  • No, it's not bad practice if the PBKDF2 output is larger than the hash algorithm size. In that case, the PBKDF2 function just combines successive hash outputs to create a PBKDF2 output of the desired size.
    – mti2935
    Commented Jul 27, 2021 at 21:28
  • mti2935 but this will cost me more performance while not costing the attacker anything, or am I not understanding correctly ?
    – Mohammed Rady
    Commented Jul 27, 2021 at 21:33
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    Yes, if this is a concern for you, then you might want to consider using a newer key derivation function.
    – mti2935
    Commented Jul 28, 2021 at 0:39

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