Editing Enigma machine (section)

=== Indicator ===
{{See also|Cryptanalysis#Indicator}}
Most of the key was kept constant for a set time period, typically a day. A different initial rotor position was used for each message, a concept similar to an [[Initialization vector|initialisation vector]] in modern cryptography. The reason is that encrypting many messages with identical or near-identical settings (termed in cryptanalysis as being ''in [[Cryptanalysis#Depth|depth]]''), would enable an attack using a statistical procedure such as [[William F. Friedman|Friedman's]] [[Index of coincidence]].<ref>{{cite book|last=Friedman| first=W.F.|author-link=William F. Friedman|title=The index of coincidence and its applications in cryptology|series=Department of Ciphers. Publ 22|publisher=Riverbank Laboratories|location=Geneva, Illinois, USA|oclc=55786052|year=1922}}</ref> The starting position for the rotors was transmitted just before the ciphertext, usually after having been enciphered. The exact method used was termed the ''indicator procedure''. Design weakness and operator sloppiness in these indicator procedures were two of the main weaknesses that made cracking Enigma possible.

[[File:Enigma-rotor-windows.jpg|left|thumb|With the inner lid down, the Enigma was ready for use. The finger wheels of the rotors protruded through the lid, allowing the operator to set the rotors, and their current position, here ''RDKP'', was visible to the operator through a set of windows.]]
One of the earliest ''indicator procedures'' for the Enigma was cryptographically flawed and allowed Polish cryptanalysts to make the initial breaks into the plugboard Enigma. The procedure had the operator set his machine in accordance with the secret settings that all operators on the net shared. The settings included an initial position for the rotors (the ''Grundstellung''), say, ''AOH''. The operator turned his rotors until ''AOH'' was visible through the rotor windows. At that point, the operator chose his own arbitrary starting position for the message he would send. An operator might select ''EIN'', and that became the ''message setting'' for that encryption session. The operator then typed ''EIN'' into the machine twice, this producing the encrypted indicator, for example ''XHTLOA''. This was then transmitted, at which point the operator would turn the rotors to his message settings, ''EIN'' in this example, and then type the plaintext of the message.

At the receiving end, the operator set the machine to the initial settings (''AOH'') and typed in the first six letters of the message (''XHTLOA''). In this example, ''EINEIN'' emerged on the lamps, so the operator would learn the ''message setting'' that the sender used to encrypt this message. The receiving operator would set his rotors to ''EIN'', type in the rest of the ciphertext, and get the deciphered message.

This indicator scheme had two weaknesses. First, the use of a global initial position (''Grundstellung'') meant all message keys used the same polyalphabetic substitution. In later indicator procedures, the operator selected his initial position for encrypting the indicator and sent that initial position in the clear. The second problem was the repetition of the indicator, which was a serious security flaw. The message setting was encoded twice, resulting in a relation between first and fourth, second and fifth, and third and sixth character. These security flaws enabled the Polish Cipher Bureau to break into the pre-war Enigma system as early as 1932. The early indicator procedure was subsequently described by German cryptanalysts as the "faulty indicator technique".{{sfn|Huttenhain|Fricke|1945|pp=4,5}}

During World War II, codebooks were only used each day to set up the rotors, their ring settings and the plugboard. For each message, the operator selected a random start position, let's say ''WZA'', and a random message key, perhaps ''SXT''. He moved the rotors to the ''WZA'' start position and encoded the message key ''SXT''. Assume the result was ''UHL''. He then set up the message key, ''SXT'', as the start position and encrypted the message. Next, he transmitted the start position, ''WZA'', the encoded message key, ''UHL'', and then the ciphertext. The receiver set up the start position according to the first trigram, ''WZA'', and decoded the second trigram, ''UHL'', to obtain the ''SXT'' message setting. Next, he used this ''SXT'' message setting as the start position to decrypt the message. This way, each ground setting was different and the new procedure avoided the security flaw of double encoded message settings.<ref>Rijmenants, Dirk; [https://www.ciphermachinesandcryptology.com/en/enigmaproc.htm Enigma message procedures] Cipher Machines & Cryptology</ref>

This procedure was used by ''Heer'' and ''Luftwaffe'' only. The ''Kriegsmarine'' procedures for sending messages with the Enigma were far more complex and elaborate. Prior to encryption the message was encoded using the ''[[Kurzsignale|Kurzsignalheft]]'' code book. The ''Kurzsignalheft'' contained tables to convert sentences into four-letter groups. A great many choices were included, for example, logistic matters such as refuelling and rendezvous with supply ships, positions and grid lists, harbour names, countries, weapons, weather conditions, enemy positions and ships, date and time tables. Another codebook contained the ''[[Discriminant Book|Kenngruppen]]'' and ''Spruchschlüssel'': the key identification and message key.<ref>Rijmenants, Dirk; [https://www.ciphermachinesandcryptology.com/en/kurzsignale.htm Kurzsignalen on German U-boats] Cipher Machines & Cryptology</ref>