Winternitz Signature in standard model
If he gets the signature for the message 00000..00000, then the checksum will be $t_1 2^w$.  For any other message, the checksum will be smaller, and hence the there will be at least one digit $i$ within the checksum for which the $c_i$ digit with value $v$ for the signed message will be larger than the corresponding digit for the new message.  The attacker does not know the $\mathcal{H}^{v-1}(x_{i+t_1})$ preimage for that digit, and hence he is unable to forge the signature.

That is the idea behind Winternitz; for any two messages $M_0$ and $M_1$, then either there will be a digit within $M_1$ which is smaller than the corresponding digit within $M_0$, or there will be a digit within the checksum for $M_1$ which is smaller than the corresponding digit within the checksum of $M_0$