Enigma's reflector prevents a letter from encrypting as itself. Codebreakers use that constraint to reject impossible message guesses before more demanding machine tests.
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What the large number includes
The Short gives about 159 quintillion combinations. This example assumes three ordered rotors from five, ten plugboard pairs, and fixed ring settings. It does not count every possible Enigma configuration.
A large combination count makes exhaustive checking difficult. It does not prove that each combination must receive a separate full test. The attack uses constraints shared by many possibilities.
Following the signal
A key press sends a signal through the plugboard and three rotors. A reflector connects it to a different return contact. The signal then passes back through the rotors and plugboard to light a letter.
The rotors change position as the operator types. Repeated input letters can therefore produce different outputs. They can also repeat an output, so movement does not guarantee a new letter every time.
At a matching rotor state, encryption is reciprocal. If A maps to G, G maps to A. Matching initial settings let the recipient reverse the message through the same stepping sequence.
Why A cannot become A
The reflector has paired contacts, with no contact paired to itself. The rest of the signal path is reversible. Together, these properties prevent the final output from matching the input letter.
This is a rule for the described machine, not a statement about every encryption system. It creates useful information for an attacker who knows part of the original message.
A likely word becomes a test
Codebreakers use expected words or phrases as cribs. Routine weather reports provide one possible source. WETTERBERICHT means weather report in German.
They slide a guessed word along the ciphertext. If an E in the guess matches an E in the ciphertext, that alignment cannot work. Any identical-letter match gives the same rejection.
This test can remove many candidate placements using only the visible letters. It does not yet reveal the correct rotor positions or plugboard connections.
What remains for the bombe
The Short compresses surviving alignments into “a few settings”. More precisely, a few crib placements may survive while many complete keys remain possible.
Turing's bombe then tests consistency among rotor transformations and plugboard relationships. Contradictions reject groups of possibilities. Gordon Welchman's improvements use the reciprocal nature of plugboard pairs to strengthen those checks.
Candidates that survive still require verification. The process combines a likely message fragment, a placement test, machine constraints, and human checking.
What this means
The setting count is only part of a security analysis. Enigma's structure supplies information that codebreakers can use to avoid a simple exhaustive search.
The wider history also includes essential Polish work before the British wartime effort. The Short isolates one mechanism within that larger achievement.
FAQ
Does the no-self rule decrypt a message by itself?
No. It rejects impossible crib placements and contributes to a larger attack.
Is a crib known to be correct?
No. It is a hypothesis that needs testing.
Does one surviving placement identify the key?
No. Many machine settings can remain for further checks.