Get the correct IV length and uniqueness rule for CBC, CTR, GCM or XTS, plus the random-IV collision risk at your volume.
Random IVs collide on the birthday schedule, so a 96-bit GCM nonce reaches a 2⁻³² collision chance at about 2^32.5 messages while a 128-bit CBC IV has effectively unlimited headroom. The uniqueness rule differs by mode: CBC needs unpredictability, CTR and GCM need non-repetition, and XTS wants the sector number. IV reuse is the single most common way real AES deployments fail — repeating a GCM nonce leaks the authentication subkey and lets an attacker forge arbitrary messages under that key.
Initialization Vector (IV)
collision probability ≈ 1 − e^(−k² ÷ 2N) ≈ k² ÷ 2^(b+1) for k messages and a b-bit random IV.
Birthday-bound message limit
safe message count at a 2⁻³² risk target = 2^((b + 1 − 32) ÷ 2).
collision probability ≈ 1 − e^(−k² ÷ 2N) ≈ k² ÷ 2^(b+1) for k messages and a b-bit random IV. Random IVs collide on the birthday schedule, so a 96-bit GCM nonce reaches a 2⁻³² collision chance at about 2^32.5 messages while a 128-bit CBC IV has effectively unlimited headroom. The uniqueness rule differs by mode: CBC needs unpredictability, CTR and GCM need non-repetition, and XTS wants the sector number.
IV reuse is the single most common way real AES deployments fail — repeating a GCM nonce leaks the authentication subkey and lets an attacker forge arbitrary messages under that key.
This calculator takes 2 inputs: Cipher mode, Messages encrypted under one key. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
No. CBC XORs the IV into the first block, so a predictable IV lets an attacker who can choose part of the plaintext confirm guesses about the rest — this is the BEAST attack. Use a fresh random 16-byte IV for every CBC message.