in symmetric key cryptography both encryption and decryption is done using shared transmission key.
Raju encrypts message using shared transmission key i.e)ka(m).
and transmit the shared key on transmission medium.
any intruder receives Raju's shared key and send message using his(trudy) key i.e)kt(m)
but Rani doesn't know this message is guarantly delivered by Raju.
the encrypted message can be readable by intruder trudy.in symetric key cryptography man in middle attacks occur