Timing difference between AES encryption and decryption about arduinolibs

[rweather]

The Wikipedia description is sufficient to implement it, but there is also the NIST specification which should be linked somewhere in the References.

The key performance difference between encryption and decryption is in the MixColumns step where it multiplies the state by a matrix of constants. The inverse of this matrix is more complex so it takes longer to perform the step in reverse for decryption. See https://en.wikipedia.org/wiki/Rijndael_MixColumns

On 32-bit systems it is possible to combine the SubBytes, ShiftRows, and MixColumns/InverseMixColumns steps into lookup tables. This makes the speed about the same for both directions. But 8-bit platforms don't have enough memory for that so I didn't bother doing that.

In practice the slowness of decryption isn't an issue because you should be using a mode like CTR or GCM that doesn't require the block decrypt operation. Only CBC requires decryption and you shouldn't use that due to timing attacks against the padding algorithms.

[HEMANT8712]

Thanks Sir, For the help and Explanation. Regards, Hemant

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-- Thanks and Regards Hemant Gupta