In this project you’ll use Ruby to build a tool for cracking an encryption algorithm.
Learning Goals / Areas of Focus
- Practice breaking a program into logical components
- Testing components in isolation and in combination
- Applying Enumerable techniques in a real context
- Reading text from and writing text to files
You are to build an encryption engine for encrypting, decrypting, and cracking messages.
Additionally, your program will need to read messages from and output them to the file system.
The encryption is based on rotation. The character map is made up of all the lowercase letters, then the numbers, then space, then period, then comma. New lines will not appear in the message nor character map.
- Each message uses a unique encryption key
- The key is five digits, like 41521
- The first two digits of the key are the “A” rotation (41)
- The second and third digits of the key are the “B” rotation (15)
- The third and fourth digits of the key are the “C” rotation (52)
- The fourth and fifth digits of the key are the “D” rotation (21)
- The date of message transmission is also factored into the encryption
- Consider the date in the format DDMMYY, like 020315
- Square the numeric form (412699225) and find the last four digits (9225)
- The first digit is the “A offset” (9)
- The second digit is the “B offset” (2)
- The third digit is the “C offset” (2)
- The fourth digit is the “D offset” (5)
Encrypting a Message
- Four characters are encrypted at a time.
- The first character is rotated forward by the “A” rotation plus the “A offset”
- The second character is rotated forward by the “B” rotation plus the “B offset”
- The third character is rotated forward by the “C” rotation plus the “C offset”
- The fourth character is rotated forward by the “D” rotation plus the “D offset”
Decrypting a Message
The offsets and keys can be calculated by the same methods above. Then each character is rotated backwards instead of forwards.
Cracking a Key
When the key is not known, the offsets can still be calculated from the message
date. We believe that each enemy message ends with the characters
that to determine when you’ve correctly guessed the key.
Then we’ll exercise the functionality from a Pry session:
> require './lib/enigma' > e = Enigma.new > my_message = "this is so secret ..end.." > output = e.encrypt(my_message) => # encrypted message here > output = e.encrypt(my_message, "12345", Date.today) #key and date are optional (gen random key and use today's date) => # encrypted message here > e.decrypt(output, "12345", Date.today) => "this is so secret ..end.." > e.decrypt(output, "12345") # Date is optional (use today's date) => "this is so secret ..end.." > e.crack(output, Date.today) => "this is so secret ..end.." > e.crack(output) # Date is optional, use today's date => "this is so secret ..end.."
Working with Files
In addition to the pry form above, we’ll want to use the tool from the command line like so:
$ ruby ./lib/encrypt.rb message.txt encrypted.txt Created 'encrypted.txt' with the key 82648 and date 030415
That will take the plaintext file
message.txt and create an encrypted file
Then, if we know the key, we can decrypt:
$ ruby ./lib/decrypt.rb encrypted.txt decrypted.txt 82648 030415 Created 'decrypted.txt' with the key 82648 and date 030415
But if we don’t know the key, we can try to crack it with just the date:
$ ruby ./lib/crack.rb encrypted.txt cracked.txt 030415 Created 'cracked.txt' with the cracked key 82648 and date 030415
As you work to implement the project here are ideas for some of your first iterations:
1. Key Generator
It’d be great if instead of random numbers we could generate a legitimate key. Starting from your runner:
- Create an instance of a key generator
- Figure out what, if anything, you’d need to pass in to the object
- Start writing tests for the key generator object based on the specs above
- Go through building the implementation
- Use it from your runner to generate and output a valid key
Before we can start encrypting we probably need to calculate the offsets.
- From your runner, create an instance of an offset calculator
- Pass the current date and the generated key into the offset calculator
- Write tests and implementation around the idea of being able to pass in the date and key, then query the A, B, C, and D final rotations
Now that you have all the components you’re ready to encrypt a message.
- Create an encryptor object in your runner
- What information would the encryptor need to be “setup” and ready to encrypt messages? Pass that in.
- Call an encrypt method and pass in a string message. Get back the encrypted version.
4. Next Steps
Now you should have all the components in place such that your command-line encryption is working! Next up:
- Follow a similar flow to develop the decrypt script and functionality
- Swap some encrypted messages with a classmate and see if each other can decrypt them correctly
- Start experimenting with the cracking functionality
Improve your system so it supports all of the following:
- all capital letters
- all lowercase letters
- all numbers
- these symbols:
Please make sure that, before your evaluation, your project has the following:
- SimpleCov reporting accurate test coverage statistics
The project will be assessed with the following guidelines:
- 4: Above expectations
- 3: Meets expectations
- 2: Below expectations
- 1: Well-below expectations
1. Ruby Syntax & Style
- Applies appropriate attribute encapsulation
- Developer creates instance and local variables appropriately
- Naming follows convention (is idiomatic)
- Ruby methods used are logical and readable
- Developer implements appropriate enumerable methods (#each is used sparingly)
- Code is indented properly
- Code does not exceed 80 characters per line
- Each class has correctly-named files and corresponding test files in the proper directories
2. Breaking Logic into Components
- Code is effectively broken into methods & classes
- Developer writes methods less than 8 lines
- No more than 3 methods break the principle of SRP
3. Test-Driven Development
- Each method is tested
- Functionality is accurately covered
- Tests implement Ruby syntax & style
- Balances unit and integration tests
- Evidence of edge cases testing
- Test Coverage metrics are present (SimpleCov)
- Application meets all requirements (extension not req’d)