Consider the double Vigen$`$ere cipher, where we choose two keys k$1,$ k$2$ with different periods

t$1,$ t$2$ in $[$tmax$],$ and then encrypt a message in the following way: first encrypt the message

using the Vigen$`$ere cipher with k$1$ to obtain an intermediate ciphertext c$,$ and then encrypt

c$$ using the Vigen$`$ere cipher with k$2$ to obtain the final ciphertext c$.$ Show how to break the

double Vigen$`$ere cipher