Perfect Encryption

Explain the concept of "Perfect Encryption? Why is it not practical?

Perfect encryption is achieved when probability of cracking is no better than guessing the message, even if hacker knows the encrypted message. And even if the actual message is retrieved, no guarantee is that it is the actual message.

 In one-time pad, a true random (not reused) secret key (at least as long as the message) is used and combined with actual message in parts (bit by bit) using modular addition (XOR). So, theoretically, Perfect encryption is achieved here.

But requirement of secured exchange of the secret key makes it perfect in theory only. If my exchange is secured, I may rather want to send plain text securely. Second thing, I doubt is, how to claim true randomness of the secret key. Third thing, sending such a secured message itself is alarming.

Reference:

1. https://www.quora.com/Cryptography-What-is-a-perfect-cipher-and-why-is-the-one-time-pad-a-perfect-cipher#
2. Shon Harris Book - page 771

How can you show that OTP is perfect?

I do not advocate one-time pad is perfect in practical but, it may be considered theoretically perfect:-

As per, http://people.seas.harvard.edu/~salil/cs127/fall06/docs/lec3.pdf  [2 Perfect Secrecy], the definition of perfect encryption is :

Perfect encryption is achieved when probability of cracking is no better than guessing the message, even if hacker knows the encrypted message. And even if the actual message is retrieved, no guarantee is that it is the actual message.

In Proposition 4 is this paper, author mathematically proves one-time pad to be perfect encryption which satisfy above.