Reference manual

The Py-Enigma simulation is made up of several Python classes as described below.


The EnigmaMachine class represents an assembled Enigma machine that consists of rotors, a plugboard, a keyboard, and indicator lamps. The keyboard and lamps act as input and outputs. The other components are represented by Python classes.

EnigmaMachine class reference

The top-level EnigmaMachine class represents an assembled Enigma machine. The EnigmaMachine class resides in the enigma.machine module.

class enigma.machine.EnigmaMachine(rotors, reflector, plugboard)

Top-level class that represents Enigma machines.

  • rotors – A list containing 3 or 4 (for the Kriegsmarine M4 version) Rotor objects. The order of the list is important. The first rotor is the left-most rotor, and the last rotor is the right-most (from the operator’s perspective sitting at the machine).
  • reflector – A Rotor object that represents the reflector (UKW).
  • plugboard – A Plugboard object that represents the state of the plugboard (Steckerbrett).
classmethod from_key_sheet([rotors='I II III'[, ring_settings=None[, reflector='B'[, plugboard_settings=None]]]])

Convenience function to build an EnigmaMachine from the data as you might find it on a monthly key sheet (code book).

  • rotors – Either a list of strings naming the rotors from left to right or a single string: e.g. ["I", "III", "IV"] or "I III IV".
  • ring_settings – Either a list/tuple of integers, a string, or None to represent the ring settings to be applied to the rotors in the rotors list (see below).
  • reflector – A string that names the reflector to use.
  • plugboard_settings – A string of plugboard settings as you might find on a key sheet (see below).

The ring_settings parameter can accept either:

  • A list/tuple of integers with values between 0-25.
  • A string; either space separated letters or numbers, e.g. 'B U L' or '1 20 11'. Note that if numbers are used, they should be between 1-26 to match historical key sheet data.
  • None means all ring settings are 0.

The plugboard_settings parameter can accept either:

  • A string of space separated letter pairs; e.g. 'PO ML IU KJ NH YT GB VF RE DC'.
  • A string of slash separated number pairs; e.g. '18/26 17/4 21/6 3/16 19/14 22/7 8/1 12/25 5/9 10/15'.
  • A value of None means no plugboard connections are made.
classmethod from_key_file(fp[, day=None])

Convenience function to build an EnigmaMachine by reading key parameters from a file.

  • fp – A file-like object that contains daily key settings, one day’s settings per line.
  • day – The line in the file labeled with the day number (1-31) will be used for the settings. If day is None, the day number will be determined from today’s date.

For more information on the file format, see Key File Format.


Sets the simulated rotor operator windows to val. This establishes a new starting position for a subsequent encrypt or decrypt operation. See also get_display().

Parameters:val – Must be a string or iterable containing uppercase letter values, one for each window from left to right. For example, a valid value for a 3 rotor machine would be 'ABC'.

This method returns the current position of the rotors as a string. See also set_display().

Returns:a string of uppercase letters, one for each rotor (left to right)
Return type:string

Returns a list of integers that represent the rotation counts for each rotor. The rotation counts are reset to 0 every time set_display() is called.


Simulate a front panel key press. First the rotors are stepped by simulating the mechanical action of the machine. Next a simulated current is run through the machine. The lamp that is lit by this key press is returned as a string (a single uppercase letter A-Z).

Parameters:key (string) – the letter pressed (A-Z)
Returns:the lamp that is lit (A-Z)
Return type:string
process_text(text[, replace_char='X'])

This is a convenience function for processing a string of text. For each character in the input text, key_press() is called. The output text is returned as a string.

This function performs some pre-processing of the input text, unlike key_press(). First, all input is converted to uppercase. Secondly, the parameter replace_char controls what is done to input characters that are not A-Z. If the input text contains a character not on the keyboard, it is replaced with replace_char. If replace_char is None the character is dropped from the input. replace_char defaults to X.

  • text (string) – the text to process
  • replace_char – invalid input is replaced with this string or dropped if it is None

EnigmaMachine exceptions

EnigmaMachine operations may raise enigma.machine.EnigmaError under error conditions. The two classmethod constructors, from_key_sheet and from_key_file assemble an EnigmaMachine from parts, and those parts may raise these exceptions themselves:

  • rotor.rotors.RotorError
  • plugboard.PlugboardError

Rotors & Reflectors

The Rotor class represents the Enigma rotors, also known as the wheels or Walzen in German. They are the most important parts of the machine.

Rotors have little use on their own. They are placed inside an EnigmaMachine object, which then calls the public Rotor methods.

Rotor class reference

class enigma.rotors.rotor.Rotor(model_name, wiring[, ring_setting=0[, stepping=None]])

A rotor has 26 circularly arranged pins on the right (entry) side and 26 contacts on the left side. Each pin is connected to a single contact by internal wiring, thus establishing a substitution cipher. We represent this wiring by establishing a mapping from a pin to a contact (and vice versa for the return path). Internally we number the pins and contacts from 0-25 in a clockwise manner with 0 being the “top”.

An alphabetic or numeric ring is fastened to the rotor by the operator. The labels of this ring are displayed to the operator through a small window on the top panel. The ring can be fixed to the rotor in one of 26 different positions; this is called the ring setting (Ringstellung). We will number the ring settings from 0-25 where 0 means no offset (e.g. the letter “A” is mapped to pin 0 on an alphabetic ring). A ring setting of 1 means the letter “B” is mapped to pin 0.

Each rotor can be in one of 26 positions on the spindle, with position 0 where pin/contact 0 is being indicated in the operator window. The rotor rotates towards the operator by mechanical means during normal operation as keys are being pressed during data entry. Position 1 is thus defined to be one step from position 0. Likewise, position 25 is the last position before another step returns it to position 0, completing 1 trip around the spindle.

Finally, a rotor has a “stepping” or “turnover” parameter. Physically this is implemented by putting a notch on the alphabet ring and it controls when the rotor will “kick” the rotor to its left, causing the neighbor rotor to rotate. Most rotors had one notch, but some Kriegsmarine rotors had 2 notches and thus rotated twice as fast.

Note that we allow the stepping parameter to be None. This indicates the rotor does not rotate. This allows us to model the entry wheel and reflectors as stationary rotors. The fourth rotor on the Kriegsmarine M4 models (Beta or Gamma) did not rotate.

The rotor constructor establishes the rotor characteristics.

  • model_name (string) – e.g. “I”, “II”, “III”, “Beta”, “Gamma”
  • wiring (string) – This should be a string of 26 uppercase characters A-Z that represent the internal wiring transformation of the signal as it enters from the right side. This is the format used in various online resources. For example, for the Wehrmacht Enigma type I rotor the mapping is "EKMFLGDQVZNTOWYHXUSPAIBRCJ".
  • ring_setting (integer) – This should be an integer from 0-25, inclusive, which indicates the Ringstellung. A value of 0 means there is no offset; e.g. the letter A is fixed to pin 0. A value of 1 means B is mapped to pin 0.
  • stepping – This is the stepping or turnover parameter. When it is an iterable, for example a string such as “Q”, this indicates that when the rotor transitions from “Q” to “R” (by observing the operator window), the rotor will “kick” the rotor to its left, causing it to rotate. If the rotor has more than one notch, a string of length 2 could be used, e.g. “ZM”. Another way to think of this parameter is that when a character in the stepping string is visible in the operator window, a notch is lined up with the pawl on the left side of the rotor. This will allow the pawl to push up on the rotor and the rotor to the left when the next key is depressed. A value of None means this rotor does not rotate.

RotorError – when an invalid parameter is supplied

Note that for purposes of simulation, our rotors will always use alphabetic labels A-Z. In reality, the Heer & Luftwaffe devices used numbers 01-26, and Kriegsmarine devices used A-Z. Our usage of A-Z is simply for simulation convenience. In the future we may allow either display.


Spin the rotor such that the string val appears in the operator window. This sets the internal position of the rotor on the axle and thus rotates the pins and contacts accordingly.

A value of ‘A’ for example puts the rotor in position 0, assuming an internal ring setting of 0.

Parameters:val (string) – rotor position which must be in the range A-Z
Raises:RotorError – when an invalid position value is supplied
Returns:current rotor position in the range A-Z
Return type:string

Simulate a signal entering the rotor from the right at a given pin position n.

Parameters:n (integer) – pin number between 0 and 25
Returns:the contact number of the output signal (0-25)
Return type:integer

Simulate a signal entering the rotor from the left at a given contact position n.

Parameters:n (integer) – contact number between 0 and 25
Returns:the pin number of the output signal (0-25)
Return type:integer

Returns True if this rotor has a notch in the stepping position and False otherwise.

Return type:Boolean

Rotates the rotor forward.

A note on the entry wheel and reflectors

The entry wheel (ETW) is a special non-movable rotor that sits on the far right of the rotor array. It connects the rotor array with the plugboard wiring. On Wehrmacht Enigmas, the entry wheel performs a straight-through mapping. In other words, the wire from the ‘A’ key is passed to pin position 0, ‘B’ to pin position 1, etc. Thus there is no need to simulate the entry wheel given our current scope to model only military Enigmas.

The reflector, or Umkehrwalze (UKW), sits at the far left of the rotor array. It simply reflects the incoming signal coming from the right back through the left side of the rotors. We can thus model the reflector as a special non-movable rotor.

If you decide to create your own reflector, and you desire to maintain reciprocal encryption & decryption, your connections must be made in pairs. Thus if you wire ‘A’ to ‘G’, you must also wire ‘G’ to ‘A’, and so on.

Rotor & reflector factory functions

While it is possible to create your own rotor type, for convenience two factory functions have been created to return rotors and reflectors used by the Wehrmacht. These factory functions let you refer to the rotors and reflectors by name instead of providing their internal wiring every time you need one (which would be both tedious and error prone).

The following table lists the names of the rotors we currently simulate.

Simulated rotor models
Rotor names Enigma Models
I, II, III, IV, V All Wehrmacht models
VI, VII, VIII Kriegsmarine M3 & M4
Beta, Gamma Kriegsmarine M4 (with thin reflectors)

Any of the names in the first column of the above table can be used by the factory function enigma.rotors.factory.create_rotor(), described below.

Likewise there exists a factory function to create reflectors by name. The following table lists the names of the supported reflectors.

Simulated reflector types
Reflector names Enigma Models
B, C All Wehrmacht models
B-Thin, C-Thin Kriegsmarine M4 (with Beta & Gamma rotors)

The two factory functions are described next:

enigma.rotors.factory.create_rotor(model[, ring_setting=0])

Create and return a Rotor object with the given ring setting.

  • model (string) – the model name to create; see the Simulated rotor models table
  • ring_setting (integer) – the ring setting (0-25) to use

the newly created Rotor


RotorError – when an unknown model name is provided


Create and return a Rotor object that is meant to be used in the reflector role.

Parameters:model (string) – the model name to create; see the Simulated reflector types table
Returns:the newly created reflector, which is actually of type Rotor
Raises:RotorError – when an unknown model name is provided

Rotor exceptions

Rotor objects may raise enigma.rotors.RotorError when an invalid constructor argument is given, or if the rotor object is given an invalid parameter during a set_display operation.


The plugboard, or Steckerbrett in German, allows the operator to swap up to 10 keys and indicator lamps for increased key strength.

Plugboards have little use on their own. They are placed inside an EnigmaMachine object, which then calls the public Plugboard methods.

Plugboard class reference

class enigma.plugboard.Plugboard([wiring_pairs=None])

The plugboard allows the operator to swap letters before and after the entry wheel. This is accomplished by connecting cables between pairs of plugs that are marked with letters (Heer & Luftwaffe models) or numbers (Kriegsmarine). Ten cables were issued with each machine; thus up to 10 of these swappings could be used as part of a machine setup.

Each cable swaps both the input and output signals. Thus if A is connected to B, A crosses to B in the keyboard to entry wheel direction and also in the reverse entry wheel to lamp direction.

The constructor configures the plugboard according to a list or tuple of integer pairs, or None.

Parameters:wiring_pairs – A value of None or an empty list/tuple indicates no plugboard connections are to be used (i.e. a straight mapping). Otherwise wiring_pairs must be an iterable of integer pairs, where each integer is between 0-25, inclusive. At most 10 such pairs can be specified. Each value represents an input/output path through the plugboard. It is invalid to specify the same path more than once in the list.
Raises:PlugboardError – If an invalid wiring_pairs parameter is given.
classmethod from_key_sheet([settings=None])

This is a convenience function to build a plugboard according to a settings string as you may find on a key sheet.

Two syntaxes are supported, the Heer/Luftwaffe and Kriegsmarine styles:

In the Heer syntax, the settings are given as a string of alphabetic pairs. For example: 'PO ML IU KJ NH YT GB VF RE DC'.

In the Kriegsmarine syntax, the settings are given as a string of number pairs, separated by a ‘/’. Note that the numbering uses 1-26, inclusive. For example: '18/26 17/4 21/6 3/16 19/14 22/7 8/1 12/25 5/9 10/15'.

To specify no plugboard connections, settings can be None or an empty string.

Parameters:settings – A settings string as described above, or None.
Raises:PlugboardError – If the settings string is invalid, or if it contains more than 10 pairs. Each plug should be present at most once in the settings string.

Simulate a signal entering the plugboard on wire n, where n must be an integer between 0 and 25.

Parameters:n (integer) – The wire number the input signal is on (0-25).
Returns:The wire number of the output signal (0-25).
Return type:integer

Note that since the plugboard always crosses pairs of wires, it doesn’t matter what direction (keyboard -> entry wheel or vice versa) the signal is coming from.

Plugboard exceptions

Plugboard objects may raise enigma.plugboard.PlugboardError when an invalid constructor argument is given.