Perfect secrecy is a theoretical property of encryption that the ciphertext reveals no information about the source plaintext. More formally, for every possible plaintext
This means that even if an attacker obtains the ciphertext, their probability of correctly guessing the original plaintext remains unchanged.
One-time pad is an algorithm that achieves perfect secrecy. However, it is not widely used due to various practical limitations. Claude Shannon proved that no encryption scheme can achieve perfect secrecy unless the key has as much entropy as the the message, and is used only once. 1