Comment by uncle-betty

More evidence for an ECC, obtained by looking at how the 10 output bits of the function depend on its 256 input bits:

Each of the 10 parity bits output by the function is the xor of exactly 104 of the 256 input bits.

Each of the 256 input bits contributes to (= is xor-ed into) either 3 or 5 of the 10 parity bits.

This is in line with the SEC-DED (single error correction, double error detection) ECC construction from the following paper:

https://people.eecs.berkeley.edu/~culler/cs252-s02/papers/hs...

Translating the above observations about the function into properties of the H matrix in the paper:

Each row of the matrix contains an identical number of ones (104).

Each column of the matrix contains an odd number of ones (3 or 5).