Crypto

An encrypting / decrypting utility (Linux/Windows). This utility will take any file and encrypt it in such a way that its contents are not readable until they are decrypted by the utility.

Table of content

• Specification
• Text Scheme
• JPG Scheme
• License
• Acknowledgments

Specification

1. The utility is called crypto and is written in Python

   • The utility has 2 command-line switches
     • –e or --encrypt
     • –d or --decrypt
   • If none of these switches is specified, the file's extension is inferred.
   • The utility also takes the name of an ASCII and/or JPG input file to encrypt/decrypt as an argument.
   • For example:
     • py crypto –e/--encrypt myFile.txt will encrypt the contents of the myFile.txt file
     • py crypto –e/--encrypt aFile.jpg will encrypt the contents of the myFile.jpg file
     • py crypto myFile.txt aFile.jpg will encrypt the contents of the myFile.txt and aFile.jpg file
     • py crypto –d/--decrypt myFile.crp will decrypt the contents of the myFile.crp file
     • py crypto –d/--decrypt aFile.cip will decrypt the contents of the myFile.cip file

2. When the utility is asked –encrypt, it will take the input filename and produce the encrypted file with the same base filename and an encrypted file extension

   • py crypto –-encrypt SampleFile.txt will produce an encrypted file called SampleFile.crp
     
   • py crypto –-encrypt SampleFile.jpg will produce an encrypted file called SampleFile.cip
     

3. When the utility is asked –decrypt, it will take the input filename and produce the decrypted file with the same base filename, and it's corresponding file extension

   • py crypto –-decrypt SampleFile.crp will produce a decrypted file called SampleFile.txt
     
   • py crypto –-decrypt SampleFile.cip will produce a decrypted file called SampleFile.jpg
     

TXT

Encryption scheme

The encryption scheme is applied to each character in the line:

• If the character is a <tab> (ASCII value 9) it will simply transform it into the output character sequence TT.
• The carriage return characters are not encrypted – they are left as is in the resultant output file.
• NOTE: The term “carriage return” in this document does not apply to any specific ASCII code. It applies to the typical end-of-line character that exists in TEXT files within the Linux OS.
• If is not a tab or a carriage return character, then apply the encryption scheme in steps below
• Take the ASCII code for the input character and subtract a value of 16 from it.
• If the resulting outChar value is less than 32, then another step is taken: outChar = (outChar – 32) + 144
• Crypto will write the ASCII value of the new encrypted character (i.e. outChar) to the destination file as a 2 digit hexadecimal value.
  • For example

    • Hello There how are you?<CR\>
      My name is Sean Clarke.<tab\>I like software!\<CR\>
      

  • The encrypted file is:

    • 38555C5C5F80445855625580585F678051625580695F652F<CR\>
      3D69805E515D55805963804355515E80335C51625B558ETT39805C595B5580635F56646751625581<CR\>
      

Decryption scheme

The decryption scheme is applied to each pair of characters in the input line:

• If the pair of characters is the sequence TT it simply transforms into a <tab> character (ASCII value 9) in the output file.
• If the pair of characters is not the sequence TT, it translates it from hex to decimal
  • For example:
    • The pair of characters “38” from the encrypted file will translate into an outChar value of 56 decimal.
    • The pair of characters “5C” from the encrypted file will translate into an outChar value of 92 decimal.
• 16 is addedto outChar.
• If the resulting outChar value is greater than 127, then another step is taken: outChar = (outChar - 144) + 32
• The outChar value now contains the decrypted ASCII code for the character that you have just decoded. This decrypted character value (i.e. outChar) will be written to the destination file as a character.
• The carriage return characters are not decrypted. They are left as is in the resultant file.
  • For example – if the input (encrypted) file is:

    • 4458596380555E5362696064595F5E80635358555D55805963806062556464698067555962548E<CR\>
      39635E87648059642F812F<CR\>
      

  • Then the decrypted file is:

    • This encryption scheme is pretty weird. <CR\>
      Isn't it?!? <CR\>
      

JPG

Encryption scheme

The XOR Encryption algorithm is based on applying an XOR mask using the plaintext and a key:

• For example

    Plain:  00110101
    Key:    01000000
    Cipher: 01110101
  

Decryption scheme

Reapplying the same XOR mask (using the same key) to the cipher text outputs the original plain text.

• For example

      Plain:  01110101
      Key:    01000000
      Cipher: 00110101

Setup

Python Virtual Env

With python installed, run:

python -m venv env

Then:

# On Windows
.\env\Scripts\Activate.ps1
# On Unix
./env/Scripts/activate

Install python packages with

pip install -r requirements.txt

Formatting

This project uses flake8 for linting, and black for formatting.

With the virtual env activated, run:

black
flake8

License

This project is licensed under the MIT License - see the LICENSE.md file for details

Acknowledgments

• Sean Clarke
• Kai Prince