AES-CBC with zero IV and a unique first block Is it safe to generate the IV for AES-CBC as the following?


It's like the real IV is all zeros.
The first block of data will be a low-entropy but unique data.
Once the first block is encrypted, the ciphertext will look like a random sequence of bytes that will automatically serve as IV of the second block


This would allow to save space since we don't have to store a "real" IV in addition to the low-entropy data of the first block that we need to store anyway.

Is it safe to to this thing?
If the IV is all zeroes, then you basically have ECB for the first block.  Basically you're proposing to use this first block's encryption as the IV for the second block.  You're implying that the first block will always be unique, but low entropy, which sounds like a counter or time stamp.  There are attacks when the IV is predictable, and while this is a step away from that, it may still be useful information for an attacker.