Project 1: The Enigma


This programming assignment is intended to exercise a few useful data structures and an object-based view of a programming problem. There is some background reading, but the necessary program is not (or rather need not be) terribly big. The video walkthrough is located here.

We will be grading largely on whether you manage to get your program to work (according to our tests). In addition, we will be looking at your own tests (which you should be sure to turn in as well). While we have supplied a few unit tests and some simple integration tests and testing utilities, the tests in skeleton are entirely inadequate for testing your program. There is also a stylistic component: the submission and grading machinery require that your program pass a mechanized style check (style61b), which mainly checks for formatting and the presence of comments in the proper places. See the course website for a brief description of the style rules. You may change any of the code we've provided, as long as the resulting program works according to the specifications here.

To obtain the skeleton files (and set up an initial entry for your project in the repository), you can use the command sequence

git fetch shared
git merge shared/proj1 -m "Get proj1 skeleton"

from your Git working directory. Should we update the skeleton, you can use exactly the same sequence to update your project with the same changes.


You may have heard of the Enigma machines that Germany used during World War II to encrypt its military communications. If you have not, I recommend you read the wikipedia page on them, or similar resource, especially the part about design and operation. This project involves building a simulator for a generalized version of this machine (which itself had several different versions.) Your program will take descriptions of possible initial configurations of the machine and messages to encode or decode (the Enigma algorithms were reciprocal, meaning that encryption is its own inverse operation.)

The Enigmas effect a substitution cipher on the letters of a message. That is, at any given time, the machine performs a permutation—a one-to-one mapping—of the alphabet onto itself. The alphabet consists solely of the 26 letters in one case (there were various conventions for spaces and punctuation).

Plain substitution ciphers are easy to break (you've probably seen puzzles in newspapers that consist of breaking such ciphers). The Enigma, however, implements a progressive substitution, different for each subsequent letter of the message. This made decryption considerably more difficult.

The device consists of a simple mechanical system of (partially) interchangeable rotors (Walzen) that sit side-by-side on a shaft and make electrical contact with each other. Most of these rotors have 26 contacts on both sides, which are wired together internally so as to effect a permutation of signals coming in from one side onto the contacts on the other (and the inverse permutation when going in the reverse direction). To the left of the rotors, one could select one of a set of reflectors (Umkehrwalzen), with contacts on their right sides only, and wired to connect half of those contacts to the other half. A signal starting from the rightmost rotor enters through one of the 26 possible contacts, flows through wires in the rotors, "bounces" off the reflector, and then comes back through the same rotors (in reverse) by a different route, always ending up being permuted to a letter position different from where it started. (This was a significant cryptographic weakness, as it turned out. It doesn't really do a would-be code-breaker any good to know that some letters in an encrypted message might be the same as the those in the plaintext if he doesn't know which ones. But it does a great deal of good to be able to eliminate possible decryptions because some of their letters are the same as in the plaintext.)

Each rotor and each reflector implements a different permutation, and the overall effect depends on their configuration: which rotors and reflector are used, what order they are placed in the machine, and which rotational position they are initially set to. This configuration is the first part of the secret key used to encrypt or decrypt a message. In what follows, we'll refer to the selected rotors in a machine's configuration as 1 through N, with 1 being the reflector, and N the rightmost rotor. In our simulator, N will be a configuration parameter. In actual Enigma machines, it was fixed for any given model (the Navy used four and the Wehrmacht used three.)

The overall permutation changes with each successive letter because some of the rotors rotate after encrypting a letter. Each rotor has a circular ratchet on its right side and an "alphabet ring" (Ringstellung) on its left side that fits over the ratchet of the rotor to its left. Before a letter of a message is translated, a spring-loaded pawl (lever)—one to the right of each rotating rotor—tries to engage the ratchet on the right side of its rotor and thus rotate its rotor by one position, changing the permutation performed by the rotor. Thus, pawls always try to engage with the ratchet of their own rotor. The lever on the rightmost rotor (N) always succeeds, so that rotor N (the "fast" rotor) rotates one position before each character. The pawls pushing the other rotors, however, are normally blocked from engaging their rotors by the alphabet ring on the left side of the rotor to their right.

This ring usually holds the pawl away from its own ratchet, preventing the rotor wheel to its left from moving. However, the rings have notches in them (either one or two in the original Enigma machines), and when the pawl is positioned over a notch in the ring for the rotor to its right, it slips through to its own rotor and pushes it forward. A "feature" of the design called "double stepping" (corrected in other versions of the Enigma, since it reduced the period of the cipher) is that when a pawl is in a notch, it also moves the notch itself and the rotor the notch is connected to. Since the notch for rotor $i$ is connected to rotor $i$, when the pawl of rotor $i - 1$ slips through into the notch for rotor $i$, rotors on both sides of the pawl move (so rotor $i-1$ and rotor $i$ move).

Let's illustrate with a much simplified version. Suppose our alphabet has only the letters A-C and we have four rotors (numbered 1-4) each of which has one notch on its ring at the C position. Suppose also that there are 3 pawls, one for each of rotors 2-4. We will still refer to these as pawls 2-4, to maintain that pawl $i$ belongs to rotor $i$. There is no pawl for rotor 1, which will therefore not rotate. We'll start with the rotors set at AAAA. The next 19 positions are as follows:


As you can see,

Each rotor can only advance at most one position per keypress.

So the advancement of the rotors, while similar to that of the wheels of an odometer, is not quite the same. If it were, then the next position after AACA would be AACB, rather than ABAB.

The effect of advancing a wheel is to change where on the wheel any given signal enters or leaves. When a wheel is in its 'A' setting in the machine, then a signal that arrives from the right at, say, the 'C' position, goes into the 'C' contact on the wheel. Likewise, a signal that leaves the wheel from its left 'C' contact exits at the 'C' position. When the wheel is rotated by one to its 'B' setting, a signal that arrives at the 'C' position goes instead into the 'D' contact on the wheel, and a signal that leaves through the 'D' contact does so at the 'C' position. It's easier to calculate if we use numbers 0--25 rather than letters ('A' is 0, 'B' is 1, etc.). Then, when the wheel is in its $k$ setting, a signal entering at the $p$ position enters the $p+k \bmod 26$ contact on the wheel, and a signal exiting through the $c$ contact does so at the $c - k \bmod 26$ position. For example, Figure 1 shows one of the rotors from the real Enigma machines (called rotor "I") and the effect of moving from its 'A' to its 'B' setting.

Rotor Example

Figure 1. Permutations performed by a rotor in its 'A' and 'B' settings. The italicized alphabets at the top and bottom indicate the letters corresponding to the positions around the rotor. The inner alphabets indicate the positions along the rotor itself. The rotor depicted was designated 'I' in the original Enigma machine used by the German military.

The contacts on the rightmost rotor's right side connect with stationary input and output contacts, which run to keys that, when pressed, direct current to the contact from a battery or, when not pressed, direct current back from the contact to a light bulb indicating a letter of the alphabet. Since a letter never encrypts or decrypts to itself after going back and forth through the rotors, the to and from directions never conflict.

The German Navy used a machine with 12 rotors and five slots for them:

Given just this equipment, there are 614,175,744 possible configurations (or keys):

Especially by today's standards, this is not a large key size (less than 30 bits). To make things more difficult for code-breakers, therefore, the Enigma incorporated a plugboard (Steckerbrett) between the keyboard and the rightmost wheel. It acted as a non-moving, configurable rotor. The operator could choose any set of disjoint pairs of letters by means of cables placed between them on the plugboard. Each selected pair would then be swapped going into the machine from the keyboard and coming out into the indicator lights. Thus, if the operator connected ("steckered") the letters A and P, then P would be substituted for each A typed and vice versa. Likewise, if an ingoing letter was encrypted to P by the other rotors, it would display as A, and letters decrypted as A would display as P.

Describing Permutations

Since the rotors and the plugboard implement permutations, we'll need a standard way to describe them. We could simply have a table showing each letter and what it maps to, but we'll use a more compact notation known as cycle representation. The idea is that any permutation of a set may be described as a set of cyclic permutations. For example, the notation


describes the permutation in Figure 1. It describes seven cycles:

The inverse permutation just reverses these cycles:

Each letter appears in one and only one cycle, so the mapping is unambiguous. As a shorthand, we'll say that if a letter is left out of all cycles, it maps to itself (so that we could have left off "(S)" In the example above.)

Figure 2 shows the permutations corresponding to the rotors used in the German Navy's Enigma machine.

Rotor Permutation (as cycles) Notch
Rotor II (FIXVYOMW) (CDKLHUP) (ESZ) (BJ) (GR) (NT) (A) (Q) E
Reflector B (AE) (BN) (CK) (DQ) (FU) (GY) (HW) (IJ) (LO) (MP) (RX) (SZ) (TV)
Reflector C (AR) (BD) (CO) (EJ) (FN) (GT) (HK) (IV) (LM) (PW) (QZ) (SX) (UY)

Figure 2. The rotors used in the Naval Enigma, showing the permutations they> implement and the point(s) at which there are notches that allow their left neighbor rotor to advance. Thus, when Rotor I is at 'Q' and the machine advances, the rotor to the left of Rotor I will advance. The Beta and Gamma rotors and the reflectors do not rotate. The reflectors shown here are the "thin" versions of the reflectors used in the naval M4 Enigma machine. The B reflector together with the Beta rotor in the 'A' position had the same effect as the usual ("thick") B reflector in the older three-rotor M3 Enigma machine (likewise the C reflector with the Gamma rotor). This allowed the M4 to encrypt and decrypt messages to and from M3 Enigmas. Source: Tony Sale's pages.


As an example of a translation, consider the set of rotors from Figure 2, and suppose that

We input the letter 'Y', which causes the following steps:

  1. The pawls all move. This causes rotor 5 to advance from E to F. The other two pawls are not over notches, so rotors 3 and 4 do not move.
  2. The letter 'Y' enters the plugboard and is converted to 'F'.
  3. The letter 'F' enters the right side of rotor 5 (an I rotor) at position 5, since 'F' is the 5th letter of the alphabet numbering from 0. Since the current setting of rotor 5 is 'F', the signal enters the rotor at position $5$, but hits contact $5+5=10$, or 'K'.
  4. According to Figure 2, rotor I converts 'K' to 'N' (letter number 13). Because the setting of rotor I is 'F', however, the signal actually comes out at letter position $13-5=8$ ('I').
  5. The 'I' signal from rotor 5 now goes into the right side of rotor 4. Since rotor 4 is a IV rotor and is in the 'L' (or 11) setting, the 'I' enters at contact $8+11=19$ ('T'), and is translated to 'G' (6), which comes out at position $6-11=-5 = 21 \bmod 26$, the fifth letter from the end of the alphabet ('V').
  6. The 'V' from rotor 4 goes now to the right side of rotor 3, a III rotor in setting 'X' (23). The signal enters the rotor at contact $21+23 = 44 = 18 \bmod 26$ ('S'), is translated to 'G' (6), which exits at position $6 - 23 = -17 = 9 \bmod 26$ ('J').
  7. Rotor 2 (Beta) is in position 'A', and thus translates 'J' to 'W'.
  8. Rotor 1 (B) converts the 'W' to 'H' and bounces it back to the left side of rotor 2.
  9. Rotor 2 (Beta in the 'A' position) converts 'H' on its left to 'X' (23) on its right.
  10. The 'X' from rotor 2 now goes into the $23 + 23 = 46 = 20 \bmod 26$ ('U') contact on the left side of rotor 3 (III in setting'X'), and is converted to 'W' (22), which comes out at position $22-23=-1=25 \bmod 26$ ('Z') on the right side of rotor 3.
  11. 'Z' now enters the left side of rotor 4 (IV in setting 'L') at $25+11=36=10 \bmod 26$ ('K'), and is translated to 'U' (20), which comes out at position $20 - 11=9$ ('J') on the right side of rotor 4.
  12. The 'J' from rotor 4 enters the left side of rotor 5 (I at setting 'F') at contact $9 + 5= 14$ ('O'), is translated to 'M' (12), and comes out at position $12 - 5 = 7$ ('H') on the right side of rotor 5.
  13. Finally,the letter 'H' is converted to 'Z' by the plugboard.

Therefore, 'Y' is converted to 'Z'.

After a total of 12 characters are converted, the rotor settings will have become 'AXLQ'. Just before the next character is converted, rotor 5 will advance to 'R' and, since its notch is at position 'Q', rotor 4 will advance to 'M', so that the rotor configuration will be 'AXMR' before the 13th character is converted. After an additional 597 characters have been converted, the configuration will be 'AXIQ'. The character after that will advance rotor 5 to 'R' and rotor 4 to 'J', giving 'AXJR'. The next character will advance 5 to 'S', and, since rotor IV's notch is at 'J', rotor 3 will advance to 'Y'. Also, as rotor 3's pawl advances rotor 3, it also moves the notch on rotor 4, advancing rotor 4 to 'K', so that the configuration goes from 'AXJR' to 'AYKS'. Rotor 3 in this case has a notch at 'V', but since rotor 2 has no pawl, rotor 3's notch never has any effect.

Input and Output

To run your program on the command line, first compile all of your files with

    javac -g -Xlint:unchecked enigma/*.java

or, if you have it, just use


After compiling, you can use the command

    java -ea enigma.Main [configuration file] [input file] [output file]

to run your program. The configuration file contains descriptions of the machine and the available rotors. The data are in free format. That is, they consist of strings of non-whitespace characters separated by arbitrary whitespace (spaces, tabs, and newlines), so that indentation, spacing, and line breaks are irrelevant. Each file has the following contents:

For example, the German Naval Enigma machine might be described with this configuration file (see Figure 2):

          5 3
          I MQ      (AELTPHQXRU) (BKNW) (CMOY) (DFG) (IV) (JZ) (S)
          II ME     (FIXVYOMW) (CDKLHUP) (ESZ) (BJ) (GR) (NT) (A) (Q)
          B R       (AE) (BN) (CK) (DQ) (FU) (GY) (HW) (IJ) (LO) (MP)
                    (RX) (SZ) (TV)
          C R       (AR) (BD) (CO) (EJ) (FN) (GT) (HK) (IV) (LM) (PW)
                    (QZ) (SX) (UY)

The input file to your program will consist of a sequence of messages to decode, each preceded by a line giving the initial settings. Given the configuration file above, a settings line looks like this:


(all upper case.) This particular example means that the rotors used are reflector B, and rotors Beta, III, IV, and I, with rotor I in the rightmost, or fast, slot. The remaining parenthesized items indicate that the letter pair Y and F and the pair Z and M are steckered (swapped going in from the keyboard and going out to the lights).

In general for this particular configuration, rotor 1 is always the reflector; rotor 2 is Beta or Gamma, and each of 3-5 is one of rotors I-VIII. A rotor may not be repeated. The four letters of the following word (AXLE in the example) give the initial positions of rotors 2-5, respectively (i.e., not including the reflector). Any number of steckered pairs may follow (including none).

After each settings line comes a message on any number of lines. Each line of a message consists only of letters, spaces, and tabs (0 or more). The program should ignore the blanks and tabs and convert all letters to upper case. The end of message is indicated either by the end of the input or by a new configuration line (distinguished by its leading asterisk). The machine is not reset between lines, but continues stepping from where it left off on the previous message line. Because the Enigma is a reciprocal cipher, a given translation may either be a decryption or encryption; you don't have to worry about which, since the process is the same in any case.

Output the translation for each message line in groups of five upper-case letters, separated by a space (the last group may have fewer characters, depending on the message length). Figure 3 contains an example that shows an encryption followed by a decryption of the encrypted message. Since we have yet to cover the details of File I/O, you will be provided the File IO machinery for this project.

             Input                              |         Output
FROM his shoulder Hiawatha                      | BHCNS CXNUO AATZX SRCFY DGU
Took the camera of rosewood                     | FLPNX GXIXT YJUJR CAUGE UNCFM KUF
Made of sliding folding rosewood                | WJFGK CIIRG XODJG VCGPQ OH
Neatly put it all together                      | ALWEB UHTZM OXIIV XUEFP RPR
In its case it lay compactly                    | KCGVP FPYKI KITLB URVGT SFU
Folded into nearly nothing                      | SMBNK FRIIM PDOFJ VTTUG RZM
But he opened out the hinges                    | UVCYL FDZPG IBXRE WXUEB ZQJO
Pushed and pulled the joints                    | YMHIP GRRE
   and hinges                                   | GOHET UXDTW LCMMW AVNVJ VH
Till it looked all squares                      | OUFAN TQACK
   and oblongs                                  | KTOZZ RDABQ NNVPO IEFQA FS
Like a complicated figure                       | VVICV UDUER EYNPF FMNBJ VGQ
in the Second Book of Euclid                    |
                                                | FROMH ISSHO ULDER HIAWA THA
YMHIP GRRE                                      | ANDOB LONGS
OUFAN TQACK                                     | INTHE SECON DBOOK OFEUC LID
KTOZZ RDABQ NNVPO IEFQA FS                      |
VVICV UDUER EYNPF FMNBJ VGQ                     |
Figure 3. Sample input and output (using the Naval cipher).

Handling Errors

You can see a number of opportunities for input errors:

A significant amount of a program will typically be devoted to detecting such errors, and taking corrective action. In our case, the main program is set up in such a way that the only corrective action needed is to throw an EnigmaException with an explanatory message. The existing code in the main program will catch this exception, print its message, and exit with a standard Unix error indication. The skeleton supplies a simple static utility method, error, which provides a convenient way to create error exceptions. There are examples of its use in the skeleton.


The directory testing contains the scripts test-correct and test-error for testing the execution of enigma.Main.

This needs to be run in your proj1 directory, with paths to test-correct and input files. For example:

bash testing/test-correct testing/correct/trivial1.inp

The tests we've supplied are nowhere near adequate to test your program, so you will need to generate your own integration tests as well as unit tests (we will check to see that you make an effort to test).

Extra Credit

If you feel up to it, consider extending your program to work on more general alphabets. To specify an arbitrary alphabet, the first string in the configuration will be allowed to have the form $C_1C_2C_3\dots C_n$, where the $C_i$ are non-whitespace characters, not including lower-case letters, or the special characters "(", ")", "-", or "*". The standard $C_1$-$C_2$ notation should continue to work, so that for example, A-Z is equivalent to ABCDEFGHIJKLMNOPQRSTUVWXYZ.

The effect of the more general specification is to specify the character set and the ordering of the characters along the rotors. For example, an alphabet specification of ACEDB means that the only characters in the alphabet are A-E, with A at the 0 position on each rotor, C at the 1 position, etc.

Office Hours

For office hours debugging, we will stick to the following norms.


By Wednesday, October 3, (10/3) you must have completed the following in order to receive 2 points of extra credit.

  1. Completed this conceptual quiz. Note - you do not need to answer all questions correctly to receive full points, but you should understand all of the correct answers by the time you start coding. Feel free to ask any questions you have about the quiz in office hours, but please refrain from revealing answers on Piazza.
  2. Have made some changes to the Permutations and Rotors parts of the project. This involves significant edits being made in the following files: - - - - -
  3. All of your code compiles.

What To Turn In

  1. You should commit and push your completed project with the commit you want graded tagged and tags pushed. Your submission should contain no debugging print statements upon submission.
  2. Your program must pass the style checker with 0 errors. This includes adequately commenting all methods.
  3. You should have added adequate unit tests, both for the provided methods and any additional methods you write. We say "adequate" because the amount may change depending on how many additional methods you write or how rigorous each test is. You will know if your test suite is adequate.
  4. You should have added additional integration tests beyond those provided in the skeleton. Again, the exact number is not specified, but we recommend having enough that you feel confident that passing them means your program is correct.


First, get started immediately, of course. Don't just jump in and code, though. Make sure you understand the specifications (which have their subtleties) and plan out how you're going to meet them. Read the skeleton code and understand the design. Figure out how to break this problem down into small pieces, and how to implement and test them one piece at a time. Know in detail how you're going to do something before writing a line of Java code for it. In particular, take some time to understand how the rotors work, and especially how the rotor position modifies the permutation.

Don't allow things to remain mysterious to you, or they'll surely bite you at some point. You don't have to use any of the skeleton code we've provided as long as the command java enigma.Main works as specified. However, if you find yourself throwing something away because you don't understand it, STOP! You're allowing yourself to be mystified by something that is intended to be simple.

There is a fair amount of string-hacking involved. The Java library can help you. Look at the documentation of String, HashMap, ArrayList, and Scanner in the on-line Java library documentation. We particularly invite you to consider the constructors and the methods

From String From ArrayList From HashMap From Scanner
charAt add get next
replaceAll get put nextInt
split size hasNext()
startsWith hasNext(...)
substring nextLine

Be creatively lazy. Feel free to browse the Java library for useful stuff, even if you haven't seen it in class. If you find yourself writing:

if (rotor.equals("I") {
    if (c == 'A')
        e = 'E';
    else if (c == 'B')
        e = 'K';

or something equally hideous, STOP! you are doing something wrong!

You can write JUnit tests and integration tests (see the testing directory) before you write a line of code. Be particularly careful to include tests that at one point caused your program to fail (regression tests) so that you can be sure you don't backslide when you make further changes. There's no reason you can't have tests that you fail, by the way; they'll serve to remind you of things you still need to do.

Use the git repository. Frequently commit your work so that you'll never have to reconstruct too much if your files somehow vanish mysteriously.

Above all, it is always fair to ask for help and advice. We don't ever want to hear about how you've been beating your head against the wall over some problem for hours. If you can't make progress, don't waste your time guessing or bleeding: ask. If nobody's available to ask, do something else (or get some sleep).

A Plan of Attack some TAs found useful was

Common Bugs

  1. % in Java is remainder, and has different behavior than you might expect with negative numbers. Please see the provided method wrap for modular conversions, or see what the body of wrap does and replicate this if you prefer to use your own methods.