Function aes_xtime_kernel 

pub fn aes_xtime_kernel(value: b8) -> b8

AES field multiplication by x, reduced modulo x^8 + x^4 + x^3 + x + 1.

“Multiply by x” is “multiply by 2” in GF(2^8): shift left one bit, and if bit 7 was set, XOR in 0x1b. In hardware that is wiring plus three XOR gates, which is why MixColumns is cheap compared with an S-box.