Suppose the attacker knows $X, Z$ such that
$H(X || Y) = Z$
bit-length(Y) < 60 then a brute force attack is possible.
bit-length(Z) = 256 (such as in SHA-256),
bit-length(X) = 128, or
bit-length(Y) = 256? Are there any published paper with results/experiments with SHA-256/512 in full or reduced rounds? Are there specific known attack techniques?