I am developer of an application that uses XTS with ciphers which use 128-bit blocks (AES, Serpent and Twofish).
I now want to use XTS with a cipher which uses 256-bit blocks (Shacal-2). AFAIK, I just have to extend the tweak to a 256-bit value. But what to do with the multiplication of the tweak? Should I keep the 0x87 constant, and use it depending on the carry of the 256-bit value (bit 255)? But in this case, should I do something about the bit 127?