There's a reason Germany believed its WW2 Enigma cipher machine was unbreakable: it was capable of up to 103 sextillion (1 03,000,000,000,000,000,000,000) encryption settings, which were changed daily. But at Bletchley Park, 21-year-old mathematician John Herivel realises the key to cracking Enigma lies not in the machine itself, but in taking advantage of human errors by its operators. He theorizes that, out of routine or laziness, some German operators might choose easily guessable settings for the daily Enigma key—such as aligning the rotors in alphabetical order or close to the previous day's setting. This insight, later known as the 'Herivel Tip,' provides Bletchley Park with crucial clues that help break the cipher."
Similarly, one-time pads used by the Soviets are unbreakable as long as they literally only used one time. But thats not the case, which allowed plenty of them to be cracked over time.
