pub fn pick_secret_kernel(low: [b8; 64], high: [b8; 64], offset: b6) -> [b8; 18]Expand description
Pick a chain’s 18 secret bytes from the two-block window.
Inputs: low (the block holding the secret’s first byte), high (the
block after it) and offset (even byte offset 0..62 within low).
Output: the 18 bytes starting at offset in low ‖ high. Combinational.
Why written as an OR over 32 cases: RHDL turns a dynamic array index into
whole-array copies, which is very large. Comparing offset against each
of the 32 possible even values with constant indices instead gives a
32-way select per output byte (see the RHDL notes in crate::packer).