#942 1940 · Bletchley Park (Alan Turing, Government Code and Cypher School) · Cryptography / military intelligence
Bletchley Park broke Enigma by exploiting the Nazis' own predictable paperwork habits
问题
Enigma had roughly 10^114 possible daily settings, far too many for any brute-force search to break in useful time
背景
The German Enigma machine scrambled messages through a combination of rotor positions, ring settings and plugboard wiring that produced an astronomical number of possible daily configurations, and it never encrypted any letter as itself — every encoded character was guaranteed to differ from the original. Attacking the cipher mathematically, by trying to reverse-engineer the day's settings from the scrambled text alone with no other information, meant searching a space too large for any method available in the 1940s to finish before the settings changed again the next day.
Alan Turing and his colleagues at Bletchley Park needed a way to narrow that search radically before the Bombe machines even started running, and pure cryptanalysis of the ciphertext in isolation couldn't provide it — they needed some way to already know, or strongly guess, what a piece of the message said before decrypting it.
换别人会怎么做
The available approach was to attack Enigma's mathematics directly — try to reverse-engineer the day's rotor and plugboard settings from the scrambled ciphertext alone — which required searching a space of roughly 10^114 possibilities, far beyond what any method of the era could finish before the settings rotated again.
他们看到了什么
Turing did not need to guess the day's settings from scratch — German operators' routine headers and weather reports handed codebreakers the plaintext half of the puzzle for free.
那一手
Turing's team built their attack around 'cribs' — guessed fragments of German plaintext that operators could count on appearing in real messages, because German military communications followed rigid, repetitive habits: routine weather reports used the same standard vocabulary at the same times each day, and message preambles opened with predictable call signs, headers and sign-offs. Matching a crib against its ciphertext, while exploiting the rule that no letter ever encrypted to itself, let the Bombe machines eliminate impossible rotor settings and converge on the day's actual key.
为什么管用
Enigma's mathematical strength assumed an attacker had only ciphertext to work with, but German operational discipline quietly violated that assumption every single day by generating messages with highly predictable content and structure. Once Turing's team had even a short, confidently guessed plaintext fragment to pair against its ciphertext, the fact that Enigma never encrypts a letter as itself let them instantly discard any candidate rotor alignment where that constraint was violated, collapsing an astronomically large search space down to a small number of candidates the Bombe could check mechanically. The vulnerability wasn't in the cipher; it was in the humans following a script the cipher couldn't hide.
值了多少
Routine weather-report phrasing gave the Bombe known plaintext, letting it rule out impossible settings and find the daily key.
什么时候会失灵
The technique only works when the underlying content is genuinely predictable enough to guess reliably — a system where communications vary freely in content, timing and structure offers no crib to anchor onto, no matter how weak its underlying cipher might otherwise be. It also stops working the moment the operators become aware of it and deliberately vary their routine language, which is exactly why modern cryptographic protocol design treats predictable headers and formulaic content as a vulnerability to be engineered out.
后来呢
Crib-based attacks remained Bletchley Park's core method throughout the war and became a foundational technique in the emerging discipline of cryptanalysis, later formalized in cryptography as the 'known-plaintext attack' — a category of vulnerability system designers still explicitly guard against by eliminating predictable, formulaic content from anything meant to stay unreadable.
资料来源
- [1]Menus and CribsThe National Museum of Computing, 2023tnmoc.org