Battles have been lost because ciphers were cracked. There is a kind that is uncrackable. It’s called a One-time Pad.
Make two identical copies of a book of random numbers – the one-time pad. Each number is a key to use to encrypt one letter.
To share secrets, Alice and Bob each take a copy of the book. To encrypt a letter Alice takes the next unused number from the one-time pad. She counts forward that many letters in the alphabet from her letter (jumping back to A from Z). The letter she ends on is the next letter of the ciphertext – the secret message.
British spies used one-time pads in World War II. So did those fighting racism in South Africa in the 1980s.
For example, to encrypt “LOVE” when the next four numbers in the one time pad are 5 2 6 10. We move 5 letters on from L to get Q, 2 letters on from O to get Q (again), 6 letters on from V to get B (wrapping round from Z to A) and 10 letters on from E to get O. The ciphertext is QQBO.
To decrypt the message, Bob starts at the same place in the one-time pad. Unlike Alice, he counts back through the alphabet from the letter in the message instead of forwards. The letter he ends on is the next letter of the plaintext – the original message.
To decrypt “QQBO” when the next four numbers in Bob’s one time pad are 5 2 6 10. We move 5 letters back from Q to get L, 2 letters back from Q to get O, 6 letters back from B to get V (wrapping round from A back to Z) and 10 letters back from O to get E. The original message was LOVE.
It is uncrackable because it is random and you only ever use each key once, so there are no patterns to spot.