Alberti Cipher

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The Alberti Cipher is one of the earliest polyalphabetic ciphers, invented by Leon Battista Alberti in the 15th century. It uses two concentric disks to encrypt and decrypt messages.

Here’s a step-by-step explanation with a practical example.

Step 1: Understand the Components

Two Disks:

• Outer Disk: Contains uppercase letters (A-Z) and possibly numbers or symbols.

• Inner Disk: Contains lowercase letters (a-z) and possibly numbers or symbols.

• The inner disk rotates independently of the outer disk.

Key:

• The initial alignment of the disks (e.g., Outer Disk 'A' matches Inner Disk 'a') is the starting key.

• The cipher can also periodically change alignment during encryption to increase complexity.

Step 2: Encryption Process

Align the Disks:

• Decide the initial key (e.g., Outer Disk 'A' aligns with Inner Disk 'm').

Substitute Letters:

• For each letter in the plaintext, find it on the outer disk and replace it with the corresponding letter from the inner disk.

Change the Alignment (Optional):

• Periodically rotate the inner disk to a new position, as agreed upon in the cipher key, for added security.

Step 3: Decryption Process

Align the Disks:

• Use the same initial key and alignment used during encryption.

Reverse Substitution:

• For each letter in the ciphertext, find it on the inner disk and replace it with the corresponding letter from the outer disk.

Adjust for Changes:

• If the alignment was changed during encryption, synchronize the same changes during decryption.

Practical Example

• Scenario: Encrypt the plaintext message: HELLO.

• Outer Disk: A-Z (uppercase)

• Inner Disk: a-z (lowercase)

• Initial Key: Outer Disk 'A' aligns with Inner Disk 'm'.

Encryption

Step 1: Align the Disks: Outer 'A' aligns with Inner 'm'.

Outer Disk: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Inner Disk: mnopqrstuvwxyzabcdefghijklm

Step 2: Encrypt Each Letter:

• H → Find 'H' on the Outer Disk. It's aligned with 't' on the Inner Disk.

• E → Find 'E' on the Outer Disk. It's aligned with 'q'.

• L → Find 'L' on the Outer Disk. It's aligned with 'x'.

• L → Same as above, aligns with 'x'.

• O → Find 'O' on the Outer Disk. It's aligned with 'a'.

Ciphertext: tqxxa.

Decryption

Step 1: Align the Disks: Use the same initial key: Outer 'A' aligns with Inner 'm'.

Outer Disk: ABCDEFGHIJKLMNOPQRSTUVWXYZ
Inner Disk: mnopqrstuvwxyzabcdefghijklm

Step 2: Decrypt Each Letter:

• t → Find 't' on the Inner Disk. It's aligned with 'H' on the Outer Disk.

• q → Find 'q' on the Inner Disk. It's aligned with 'E'.

• x → Find 'x' on the Inner Disk. It's aligned with 'L'.

• x → Same as above, aligns with 'L'.

• a → Find 'a' on the Inner Disk. It's aligned with 'O'.

Decrypted Text: HELLO.

Step 4: Add Complexity (Optional)

To make the cipher more secure:

• Periodically rotate the inner disk after encrypting certain letters.

• Use symbols or numbers in the disks for added complexity.

Step 5: Python Code

The Python code implements the Alberti Cipher with functions for encryption and decryption.

# Alberti Cipher Implementation

def generate_disks():
    outer_disk = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    inner_disk = "mnopqrstuvwxyzabcdefghijkl"
    return outer_disk, inner_disk

def encrypt_alberti(plaintext, initial_key):
    outer_disk, inner_disk = generate_disks()
    ciphertext = ""

    # Align the disks using the initial key
    shift = outer_disk.index(initial_key.upper())
    inner_disk = inner_disk[shift:] + inner_disk[:shift]

    for char in plaintext:
        if char.isalpha():
            index = outer_disk.index(char.upper())
            cipher_char = inner_disk[index]
            ciphertext += cipher_char
        else:
            ciphertext += char  # Keep non-alphabetic characters as is

    return ciphertext

def decrypt_alberti(ciphertext, initial_key):
    outer_disk, inner_disk = generate_disks()
    plaintext = ""

    # Align the disks using the initial key
    shift = outer_disk.index(initial_key.upper())
    inner_disk = inner_disk[shift:] + inner_disk[:shift]

    for char in ciphertext:
        if char.isalpha():
            index = inner_disk.index(char)
            plain_char = outer_disk[index]
            plaintext += plain_char
        else:
            plaintext += char  # Keep non-alphabetic characters as is

    return plaintext

# Example usage
initial_key = "A"  # Outer 'A' aligns with Inner 'm'
plaintext = "HELLO"
ciphertext = encrypt_alberti(plaintext, initial_key)

decrypted_text = decrypt_alberti(ciphertext, initial_key)

print(f"Plaintext: {plaintext}")
print(f"Ciphertext: {ciphertext}")
print(f"Decrypted Text: {decrypted_text}")

Output

Plaintext: HELLO
Ciphertext: tqxxa
Decrypted Text: HELLO