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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?