[openssl-dev] Question on private_AES_set_decrypt_key "invert the order of the round keys"

[Rod Falck]

Hi,

In the function private_AES_set_decrypt_key, in aes_core.c, it inverts the order of the round keys before applying the MixColumn transform from low to high (excluding low and high, order doesn't really matter as each transformation is dependent only on the item in question). Then AES_decrypt applies the round keys from low to high on the input.

It seems to me that you will get the same result if you do away with the inversion of the round keys, apply the MixColumn transform (excluding low and high) and then AES_decrypt applies the round keys from high to low on the input.

Using this method would therefore save the amount (pretty small) of cpu time used to invert the round keys, with no apparent penalty, assuming looping backwards costs the same as looping forwards.

Is there something functionally wrong with my hypothesis? If not, is there something wrong security-wise, e.g. will the different cpu time cause a vulnerability (it seems that currently one can theoretically tell the difference between an encryption and a decryption based on cpu usage, but I don't know if that is a real problem)?

Rod.
--
Rod Falck. Software Architect. comForte Pty Ltd.

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