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Rejecting replays is the duty of a higher level protocol. Simple authenticated encryption will accept any message with a valid MAC, even if you receive it several times. Decryption is a stateless process, but you need state to keep track of messages you already received. For example you could associate an increasing counter for each message you send. The ...


AES CBC usually requires padding, such as PKCS#7 padding. This padding is 1 to 16 bytes, 16 being the block size of AES. The HMAC will add 256 / 8 = 32 bytes to the total. Usually you will need to store the randomized IV as well with ciphertext, to allow for reuse of the key, adding another 16 bytes (the block size again). So the total overhead will be about ...


You should get the IV of CBC as the first ciphertext block. Wikipedia: CBC

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