An implementation of the two-square cipher in Python 3
This program encypts and decrypts messages using the two-square cipher, aka double Playfair. There are two variations of the cipher, vertical and horizontal, depending on how the two Playfair tables are arranged. This implementation uses the first variation with the two tables placed vertically.
System Requirements:
Python 3.8 or greater is required to run this program.
Dependencies:
No third-party packages are required.
This implementation has a main program with two modes: interactive and command-line. The interactive mode offers users a a full menu of options for encrypting and decrypting messages. The program can be run from the command-line to encrypt and decrypt messages as well, but with fewer options. Here is a guide to command-line usage:
Command-line usage:
- twosquare [OPTION] FILENAME KEY1 KEY2
or:
- twosquare [OPTION] "MESSAGE" KEY1 KEY2
OPTIONS:
- -e, --encrypt encrypt MESSAGE using KEY1 and KEY2
- -d, --decrypt decrypt MESSAGE using KEY1 and KEY2
- -z, --remove_z remove trailing Z from MESSAGE :: decryption only
- -j, --omit_j omit the letter J from MESSAGE :: decryption only
- -h, --help display this help and exit
- -v, --version output version information and exit
For interactive mode with a full in-program menu, run the program without adding any command-line arguments.
The functions in the global scope of this implementation can also be used as a module.
This implementation includes a custom test suite (test.py) and custom error classes (exceptions.py).
The Twosquare cipher uses two keys to encrypt and decrypt messages.
Each key can be a key word or phrase:
- Up to twenty-five letters in length
- ASCII letter characters only; no Unicode allowed
- Each letter may not be used more than once in a key
- A key may contain either 'I' or 'J', but not both
- Digits are not allowed so any numbers must be spelled out
- No white space, punctuation, or special characters
The Twosquare cipher uses two 5 x 5 Playfair tables to encrypt and decrypt messages for you. These two tables are each generated using one of the keys you create.
When encrypting a plaintext message, please note the following: All white space, punctuation, special characters, and digits will be removed from the plaintext during the encryption process. In basic terms, all non-alpha characters, while allowed, will be ignored and thus removed from the message. No data is stored about what was removed and therefore, when the ciphertext is later decrypted, the white space, digits, and punctuation will not be restored.
Another thing to keep in mind is all 'J' letter characters are replaced with an 'I' character by this cipher. This is done to allow the entire twenty-six letter alphabet to fit in a 5 x 5 Playfair table with only twenty-five cells. While this design choice solves the extra letter problem, it can result in some loss of information when the process is reversed and the ciphertext is decrypted back to a plaintext. In practicality, this makes little difference, as the decoded message is still typically easy to read and understand.
If the number of characters in the plaintext is odd after removing all white space, special characters, and digits, a 'Z' character is added to the end to make the number of characters even. This is necessary for the cipher to function properly, as the text is broken into digraphs (two-letter combinations) during the encoding or decoding process. This trailing character is, of course, easy enough to ignore when reading the decrypted message. That said, this implementation has an option to remove a trailing 'Z' character from the message automatically during the decryption process.
For these reasons, among others, the Twosquare cipher is not a tool with practical use for encrypting and decrypting files and documents where a loss of data would be unacceptable, or where high levels of data security and integrity are essential. It is best used as a relatively simple means of sending English alphabetic messages and is perhaps valuable in real terms mainly for its historical significance and for educational purposes.
This implementation enables loading and saving messages as .txt files. This is the only file type supported and files will not be encrypted in place, that is to say, they will not be overwritten. The reason for these restrictions is that the Twosquare cipher is not suitable for encrypting all types of data or file, but rather only works with alphabetic ASCII character sets. The encoding process omits white space, digits, punctuation, special characters, and other valuable data and thus is not suitable for general usage. The file capabilities are provided merely to facilitate the storage and transfer of messages at the personal discretion of the user. Professional use is discouraged. No guarantee is made that the data will be secure, and no warranty is made against the possibility of data loss or corruption. Use at your own risk.
For more information on the Twosquare cipher, check out the Wikipedia article at:
https://en.wikipedia.org/wiki/Two-square_cipher
Program Credits <<<
- Creator: Scott Milton
- Contributors: <- (add your name here)
- License: GPL-3.0
If you would like to contribute to this project, please feel free to visit the GitHub repo at:
https://github.com/scottmilton1/two-square-cipher
This implementation is based on the description of the Twosquare cipher provided in the book Modern Cryptography: Applied Mathematics for Encryption and Information Security (First Version) by Chuck Eastom, McGraw Hill Publishing.
Special thanks to Chuck for writing such a great book!
This implementation is primarily function-based and aside from custom error classes, it does not define and use classes. This was a design decision made principally to focus energy on procedural programming techniques and to develop greater skill at writing and using functions as the basic modular unit. The decision had nothing to do with an aversion to Object-Oriented Programming and should not be taken in any way as a reflection on the effectiveness or utility of the OOP paradigm itself. Certainly, OOP could just as easily be used to satisfy the requirements of this project.
I hope you enjoy the program and welcome your comments and contributions.
Happy encrypting!
- Optional: Make program work for either vertical or horizontal table alignment
- Optional: Add file viewer to load and save menu options (e.g. - os.listdir())
- Optional: Refactor display_title: add align arg with left, right, and center options
- Optional: Make program work for more than one cipher: (e.g. - Playfair, Four-square, etc.)