Security of Randomized Encryption Scheme
Ciphertext Indistinguishability means that if you encrypt a message many times, each time the ciphertexts must be different.

For example, think about a device that searches flaws in a dam. Every time, when this device traverses, it will send "Yes" or "No" signals that it means for a receiver that the device has found a flaw or not. If an eavesdropper sniffs these signals, because of the number of "no" signals are more Than "yes" signals, he could understand the meaning of these signals even though they are encrypted and could not break their encryption. This weakness happens because the encryption algorithm is DETERMINISTIC and it means that always it produces same encryptions for "YES" signal and same encryptions for "NO" signals, in other words, the attacker could distinguish these encrypted messages without knowing the nature of the encryptions.

The solution for making messages resistance against distinguishability is using Non-Deterministic encryption and use parameters like IV and so for make ciphertext Random and each time giving the same message, it produces different ciphertexts and the attacker could not distinguish them.