Program to encrypt a string using Vertical Cipher in Python

The Vertical Cipher is an encryption technique that rearranges a string by distributing characters into columns and reading them vertically. Given a string and number of rows, we arrange the string into a grid and extract each column to form encrypted segments.

For example, with string "ilovepythonprogramming" and n = 5 rows, we arrange characters into 5 rows and read each column from top to bottom, left to right.

How Vertical Cipher Works

The algorithm distributes characters across n rows in a round-robin fashion ?

Implementation Using List Comprehension

We use Python's slicing notation s[i::n] to extract every nth character starting from position i ?

def vertical_cipher(s, n):
    return [s[i::n] for i in range(n)]

# Test the function
text = "ilovepythonprogramming"
rows = 5
result = vertical_cipher(text, rows)
print(f"Original text: {text}")
print(f"Number of rows: {rows}")
print(f"Encrypted segments: {result}")

Original text: ilovepythonprogramming
Number of rows: 5
Encrypted segments: ['ipnrn', 'lypag', 'otrm', 'vhom', 'eogi']

Step-by-Step Process

Let's trace through the algorithm to understand how it works ?

def vertical_cipher_detailed(s, n):
    print(f"Input string: '{s}'")
    print(f"Number of rows: {n}")
    print("\nExtracting characters for each row:")
    
    result = []
    for i in range(n):
        segment = s[i::n]
        print(f"Row {i}: s[{i}::{n}] = '{segment}'")
        result.append(segment)
    
    return result

# Demonstrate the process
text = "ilovepythonprogramming"
encrypted = vertical_cipher_detailed(text, 5)
print(f"\nFinal result: {encrypted}")

Input string: 'ilovepythonprogramming'
Number of rows: 5

Extracting characters for each row:
Row 0: s[0::5] = 'ipnrn'
Row 1: s[1::5] = 'lypag'
Row 2: s[2::5] = 'otrm'
Row 3: s[3::5] = 'vhom'
Row 4: s[4::5] = 'eogi'

Final result: ['ipnrn', 'lypag', 'otrm', 'vhom', 'eogi']

Alternative Implementation

Using a more explicit approach with loops ?

def vertical_cipher_loops(s, n):
    segments = [''] * n
    
    for i, char in enumerate(s):
        row = i % n
        segments[row] += char
    
    return segments

# Test both approaches
text = "hello world"
rows = 3

method1 = vertical_cipher(text, rows)
method2 = vertical_cipher_loops(text, rows)

print(f"Text: '{text}' with {rows} rows")
print(f"List comprehension: {method1}")
print(f"Loop method: {method2}")
print(f"Results match: {method1 == method2}")

Text: 'hello world' with 3 rows
List comprehension: ['hlwr', 'eool', 'l d']
Loop method: ['hlwr', 'eool', 'l d']
Results match: True

Comparison

Conclusion

The vertical cipher encrypts text by arranging characters in rows and reading columns vertically. Python's slicing notation s[i::n] provides an elegant solution for extracting every nth character starting from position i.