One-Time Pads
Why are One-Time Pads perfectly secure? If the key is random, one-time pad is perfectly secret against ciphertext-only cryptanalysis(if the key is distributed reliably), even against brute force search. So how can it beat brute force attack. Think about below example: You have plain text: "Hello" Your OTP key: 0x146EDC3425 Thus, your ciphered text: 0x5C0BB0584A The attacker only knows ciphered text 0x5C0BB0584A in this scenario. From here he can try to decipher the message using brute force. But because the key is random, the deciphered text can be anything with equal possibility. So it can be "Think", "about", "Below", "force"... Anything meaningful words. So the attacker still cannot decide what is the true plain text, which the plain text set he gets from ciphered text is meaningless.