Security difference between $\mathrm{Keccak}(k\mathbin\|x)$ and $\mathrm{Keccak}(x\mathbin\|k)$ $\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.
$\endgroup$
See chapter 5.11.2 of the paper “Cryptographic sponge functions” (the link can be found here):  


  Note that one can also define a MAC function by taking as input the message followed
  by the key: $t = \left\lfloor {F(M \mathbin\Vert K)} \right\rfloor _n$.  In this case, an adversary has the advantage that she can try
  to generate inner collisions offline, i.e., without having to query the keyed sponge function. Additionally, she can try to construct a path to a state that occurs in the absorbing of a target
  message, leading to a second message with the same MAC.