Explain why the cipher

ek$($m$)=$ k $$ m and dk$($c$)=$ k $$ c

defined by XOR of bit strings is not secure against a known plaintext attack. As usual,

ek and dk are encryption and decryption functions, respectively. In this problem, the

plaintext space, key space, and ciphertext space are all the same $($binary strings of

some fixed length n$).$