# Why should I use an Initialization Vector (IV) when I have unique keys?

I took a look at this question. My question is not the same.

I've unique keys encrypting (in CBC mode, AES-256) each plaintext, i.e. I do not use a key to encrypt more than one plaintext. Is it insecure to use null IVs in this case? Why?

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–  Jeremy Banks Jun 7 '13 at 20:51

Note: The single message per key setup leaves the total message length per key limit unchanged for CBC, PCBC, CFB: that's $2^{(b+1-k)/2}$ blocks for less than $2^{-k}$ odds of collision (implying some level of insecurity) using a $b$-bit block cipher. That limit is dramatically raised for OFB and CTR in the single message per key setup.