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We're using OpenSSLs EVP_aes_265_xts encryption on data (with unique, random, IVs for every new message encrypted) - which is working well.

But, I've been asked; what is the forward cipher function of this algorithm? I was going to say it's AES256, but am having doubts as to whether that's what the actual forward cipher function is defined as and/or encompasses!

Can someone give a little more of a precise answer as to what the forward cipher function is defined as for AES (256 bit) XTS?


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I dont know about the openssl implementation, but XTS splits the input key into 2 equal halves, so if you supply it a 256-bit key, the cipher function would be AES-128 – Richie Frame Feb 27 '14 at 5:17
Thanks Richie, you're correct there ... I've also learnt a lot since posting this question! – mrtimdog Feb 12 '15 at 22:32
up vote 1 down vote accepted

With OpenSSL the forward cipher for EVP_aes_265_xts is AES 256. The key being 512 bits, internally split into two 256 bit keys for each of the AES 256 ciphers used within the XTS mode of operation.

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