cryptographer

Table of contents

• Description
• Roadmap
• Limitations
  • Affine cipher
  • Gamma cipher
  • Feistel network cipher
• Tech stack
• How to use
  • CLI args
    • Required
    • Optionally required for different ciphers
    • Optional
  • Example

Description

This project is an encryption/decryption app that can process the source using following ciphers:

• Caesar
• Vigenere
• Affine
• Gamma
• Feistel network

Note: All source characters will be converted to the upper case before further processing.\
Each source character which is not English letter will be skipped.\
These two statements are true for everything excluding Feistel network cipher.

Also, this app can try to break Vigenere cipher using the frequency cryptanalysis.

Note: the probability of the successful breaking is directly proportional to the similarity of the source to the
“normal” English text (“normal” means “written using a common English language”).

Roadmap

• add tests for all ciphers

Limitations

Affine cipher

An Affine cipher’s a variable is hardcoded in the current version of app.\
It’s value is:

• 3 for encryption
• 9 for decryption

Gamma cipher

Gamma cipher’s a, b, m variables are hardcoded in the current version of app.\
Their values are:

• a = 3
• b = 2
• m = 40692

Feistel network cipher

Feistel network’s keys and the round key calculation algorithm are hardcoded in the current version of app.\
Their values are:

• keys: 5, 2, 7, 6, 1, 8, 3, 9, 4, 6, 2, 3, 8, 5, 1, 4
• the round key calculation algorithm: key * 7 / 2 (an integer division)

The program works with texts which are encoded using UTF-8, so Feistel network’s block size equals 4.

Note: The program theoretically can fail while processing a text which contains some UTF-8 characters.\
At the moment there’s no workaround or fix for this issue.

Tech stack

• Kotlin
• Gradle

How to run

1. Install OpenJDK (≥ 17), GNU Make
2. Run make
3. Run java -jar ./build/cryptographer.jar with --help or some another CLI args

CLI args

Required

• --encrypt, --decrypt, --break - the name of an action to do
• --caesar, --vigenere, --affine, --gamma, --feistelNetwork - the name of the cipher to use
• --in IN, --input IN - an input file path
• --out OUT, --output OUT - an output file path

Optionally required for different ciphers

• --shift SHIFT - the shift required for the Caesar cipher encryption/decryption
• --key KEY - the key required for the Vigenere cipher encryption/decryption
  (must consist of English letters only)
• --b B - b variable required for the Affine cipher encryption/decryption

Optional

• --startKey START_KEY - the start key for the gamma cipher encryption/decryption (defaults to 1)
• -h, --help - show this help message and exit

Example

java -jar ./build/cryptographer.jar --encrypt --gamma --startKey 25 --in src.txt --out dest.txt