we use 6 bytes of data and initialization vector is kept secret and fixed and a secret key is used. what is the probability of hacking? the possibilities 2^128 increase or decrease and to what extend?

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    $\begingroup$ 100%? All you'd have to do is XOR the ciphertext together to break it. Reusing the nonce:key tuple ever is absolutely fatal for CTR mode (and by extension GCM, which uses CTR internally). $\endgroup$ – forest May 6 '19 at 6:35

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