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AES has the following steps in encryption: SubBytes, ShiftRows, MixColumns, and AddRoundKey and the following steps in decryption: InvSubBytes, InvShiftRows, InvMixColumns, and Inverse of AddRoundKey.

My question is: in which step of encryption and decryption is the key used to encrypt and decrypt data?

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    $\begingroup$ How was that not explicitly clear by the names of the steps? $\endgroup$ – Richie Frame Jul 28 '15 at 19:37
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AddRoundKey.

That step takes 16 bytes from the expanded key schedule, and exclusive-or's ("adds" in $GF(256)$ terminology) it to the intermediate block state.

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