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Since the block size of AES 256 is 256, how would we calculate the bytes of data AES 256 encrypts at one time?

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AES has always 128-bit block size with 128,192 and 256-bit keyspaces. Therefore, you can encrypt 16-byte at a time if you are using ECB and CBC modes. By using CTR mode you can encrypt 1-bit to 128-bit.

Some people confuse AES with Rijndael where AES is a variant of the Rijndael. Some old libraries use the Rijndael (see RijndaelManaged). Rijndael can have 128, 160, 192, 224, and 256-bit block size. Note that these are not standardized, see FIPS 197.

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    $\begingroup$ So both AES 128 and AES 256 encrypt 16 bytes at a time? $\endgroup$ – joshkmartinez Mar 9 at 19:14
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    $\begingroup$ @joshkmartinez Yes. 128 & 256 refer to the key size, not the block size. The block always remains at 128 bits/16 bytes. One way to look at it, is that the AES block (rectangular) gets deeper, rather than wider. $\endgroup$ – Paul Uszak Mar 9 at 22:05

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