Rail Fence

The Rail Fence cipher is a transposition cipher that writes the plaintext in a zigzag pattern along a number of rails, then reads off the ciphertext row by row.

1. The encrypted message is: 3bd4af06-c958-c418-1528-4e0df7801e02
2. Remove the dashes to get: 3bd4af06c958c41815284e0df7801e02
3. This is 32 characters long, which matches a UUID without dashes (32 hex characters)

1. Since this is a daily challenge, try common rail numbers: 2, 3, 4, 5
2. Start with 3 rails as it's a common choice
3. If 3 doesn't work, try other numbers systematically

1. Create a 3-rail fence pattern
2. Write the encrypted text row by row into the fence
3. Read the plaintext in zigzag pattern
4. The result is: 3cf9b578dc844108a115f2e8040e602d

1. Add dashes to make it a proper UUID format
2. The flag is: 3cf9b578-dc84-4108-a115-f2e8040e602d
3. We can verify this is correct because it starts with "3" and ends with "d" as expected from the flag format

def rail_fence_decrypt(encrypted, rails):
    # Calculate the pattern
    pattern = []
    row, col = 0, 0
    direction = 1
    
    for i in range(len(encrypted)):
        pattern.append((row, col))
        col += 1
        
        if row == 0:
            direction = 1
        elif row == rails - 1:
            direction = -1
            
        row += direction
    
    # Sort pattern by position
    pattern.sort(key=lambda x: (x[0], x[1]))
    
    # Create the fence
    fence = [[''] * len(encrypted) for _ in range(rails)]
    
    # Fill the fence with encrypted text
    for i, (row, col) in enumerate(pattern):
        fence[row][col] = encrypted[i]
    
    # Read the decrypted text
    decrypted = ''
    row, col = 0, 0
    direction = 1
    
    for _ in range(len(encrypted)):
        decrypted += fence[row][col]
        col += 1
        
        if row == 0:
            direction = 1
        elif row == rails - 1:
            direction = -1
            
        row += direction
    
    return decrypted

# The encrypted message
encrypted = "3bd4af06c958c41815284e0df7801e02"

# Decrypt with 3 rails
flag = rail_fence_decrypt(encrypted, 3)
print(f"Flag: {flag[:8]}-{flag[8:12]}-{flag[12:16]}-{flag[16:20]}-{flag[20:]}")

• Transposition Ciphers: Rail Fence is a classic transposition cipher that rearranges characters without changing them, making it vulnerable to frequency analysis.
• Cryptanalysis: Rail Fence can be broken by trying different rail numbers and analyzing the resulting plaintext patterns.
• Security Implications: While Rail Fence provides basic obfuscation, it's not suitable for modern cryptographic applications due to its vulnerability to brute force attacks.
• Historical Context: Transposition ciphers like Rail Fence were used in early cryptography and military communications before the development of more sophisticated encryption methods.
• Modern Applications: Understanding basic ciphers helps in recognizing weak encryption in legacy systems and developing stronger cryptographic solutions.