Basic Text Encryption / Decryption

This code provides a simple and effective implementation of the Caesar cipher with support for both text and file encryption/decryption, as well as a frequency analysis function to examine the encrypted content.

# Line-by-Line Explanation of the Caesar Cipher Code

This section provides a detailed explanation of the code for the Caesar cipher implementation, file encryption, decryption, and frequency analysis.

## Code Breakdown

### 1. `encrypt(msg, n)`
This function encrypts a given message using a Caesar cipher with a shift value `n`.

```python
def encrypt(msg, n):
    encrypted_msg = ""

• Initializes an empty string encrypted_msg to store the resulting encrypted message.

    for char in msg:

• Loops through each character in the input message (msg).

        if char.isalpha():

• Checks if the character is an alphabetical letter (either uppercase or lowercase).

            base = ord('a') if char.islower() else ord('A')

• If the character is lowercase (char.islower()), it assigns the ASCII value of 'a' to base. If the character is uppercase, it assigns the ASCII value of 'A'. This ensures that the shift respects the case of the letter.

            shifted = chr((ord(char) - base + n) % 26 + base)

• Converts the character char to its corresponding ASCII value using ord(char), then subtracts the ASCII value of the base (ord('a') or ord('A')).
• Adds the shift value n to this result, applies modulo 26 (% 26) to wrap around if it goes past 'z' or 'Z', and then adds the base value back to ensure the result is within the correct ASCII range.
• Converts the new shifted value back to a character using chr().

            encrypted_msg += shifted

• Adds the shifted character to the encrypted_msg.

        else:
            encrypted_msg += char

• If the character is not an alphabetic letter (e.g., punctuation, spaces), it is added to encrypted_msg without modification.

    return encrypted_msg

• Returns the fully encrypted message after processing all characters.

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2. decrypt(encrypted_msg, n)

This function decrypts the message by reversing the encryption process.

def decrypt(encrypted_msg, n):
    return encrypt(encrypted_msg, -n)

• The decryption process is identical to encryption but with a negative shift (-n). It simply calls the encrypt() function with -n to reverse the encryption.

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3. encrypt_file(input_file, output_file, n)

This function encrypts the content of a file and saves the result to another file.

def encrypt_file(input_file, output_file, n):
    try:
        with open(input_file, 'r') as file:
            content = file.read()

• Tries to open the input file in read mode ('r'). If the file is found, it reads its content into the variable content.

            encrypted_content = encrypt(content, n)

• Encrypts the file content using the encrypt() function with the provided shift value n.

        with open(output_file, 'w') as file:
            file.write(encrypted_content)

• Opens the output file in write mode ('w') and writes the encrypted content into it.

        print(f"Encryption successful! Encrypted content saved to {output_file}.")

• Prints a success message indicating that the encryption was successful and saved to the output file.

    except FileNotFoundError:
        print(f"File {input_file} not found!")
    except PermissionError:
        print(f"Permission denied for {input_file}!")

• Catches and handles two potential errors:
  • FileNotFoundError: If the input file does not exist.
  • PermissionError: If there are insufficient permissions to read the file.

---

4. decrypt_file(input_file, output_file, n)

This function decrypts the content of a file and saves the result to another file.

def decrypt_file(input_file, output_file, n):
    try:
        with open(input_file, 'r') as file:
            content = file.read()

• Tries to open the input file in read mode ('r'). If the file is found, it reads its content into the variable content.

            decrypted_content = decrypt(content, n)

• Decrypts the file content using the decrypt() function with the provided shift value n.

        with open(output_file, 'w') as file:
            file.write(decrypted_content)

• Opens the output file in write mode ('w') and writes the decrypted content into it.

        print(f"Decryption successful! Decrypted content saved to {output_file}.")

• Prints a success message indicating that the decryption was successful and saved to the output file.

    except FileNotFoundError:
        print(f"File {input_file} not found!")
    except PermissionError:
        print(f"Permission denied for {input_file}!")

• Catches and handles two potential errors:
  • FileNotFoundError: If the input file does not exist.
  • PermissionError: If there are insufficient permissions to read the file.

---

5. letter_frequency(encrypted_msg)

This function analyzes the frequency of each alphabetic character in the encrypted message.

def letter_frequency(encrypted_msg):
    frequency = {}

• Initializes an empty dictionary frequency to store the frequency of each character.

    for char in encrypted_msg:
        if char.isalpha():
            frequency[char] = frequency.get(char, 0) + 1

• Loops through each character in the encrypted message.
• If the character is alphabetic, it adds it to the frequency dictionary, incrementing the count for that character. If the character does not exist in the dictionary, it starts with a count of 0.

    return frequency

• Returns the frequency dictionary containing the counts of each alphabetic character.

---

6. Example Execution

message = input("Enter the message to encrypt: ")
key = int(input("Enter the key (shift value): "))
encrypted_message = encrypt(message, key)
print(f"Encrypted message: {encrypted_message}")
decrypted_message = decrypt(encrypted_message, key)
print(f"Decrypted message: {decrypted_message}")

• Prompts the user to input a message and a shift value (key).
• Encrypts the message with the specified key and prints the encrypted message.
• Decrypts the encrypted message using the same key and prints the decrypted message.

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7. File Encryption and Decryption

input_file_name_encrypt = input("Enter the full path of the input file to encrypt: ")
output_file_name_encrypt = input("Enter the full path of the output file for encryption: ")
encrypt_file(input_file_name_encrypt, output_file_name_encrypt, key)

input_file_name_decrypt = input("Enter the full path of the input file to decrypt: ")
output_file_name_decrypt = input("Enter the full path of the output file for decryption: ")
decrypt_file(input_file_name_decrypt, output_file_name_decrypt, key)

• Prompts the user to provide the paths for the input and output files for both encryption and decryption.
• Calls the encrypt_file and decrypt_file functions to process the files.

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8. Frequency Analysis of Encrypted Message

freq = letter_frequency(encrypted_message)
print("Frequency of each letter in the encrypted message:")
for char, count in freq.items():
    print(f"{char}: {count}")

• Calls the letter_frequency() function to analyze the frequency of each letter in the encrypted message.
• Prints the frequency of each alphabetic character in the encrypted message.

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