$\require{begingroup}\begingroup \DeclareMathOperator{\keccak}{\mathrm{Keccak}} \DeclareMathOperator{\concat}{\|} $According to Keccak's website, $\keccak(k \concat x)$ is safe:
Unlike SHA-1 and SHA-2, Keccak does not have the length-extension weakness, hence does not need the HMAC nested construction. Instead, MAC computation can be performed by simply prepending the message with the key.
My question is, is $\keccak(x \concat k)$ safe?
I read Why is $H(k\mathbin\Vert x)$ not a secure MAC construction? but I guess it doesn't apply to Keccak. $\endgroup$