What is the forward cipher function of AES-XTS? 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?

Thanks!
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.