Question

In python write a function

Answer 1

How the Encryption is done:

In the program, first a grid is made, where number of rows is given and number of columns is such that it can store the whole 'plaintext' (There can be extra spaces left in the grid other than 'plaintext'). Number of columns is calculated using ceil function.

The plaintext is stored in the grid in following manner.

1st letter of plaintext	10th letter	11th letter
2nd letter	9th letter	last letter
3rd letter	8th letter
4th letter	7th letter
5th letter	6th letter

Table 1

The empty spaces are filled with '%' and then the whole grid is added from first row then second row ... to get the "encrypted word".

For example: 'america' , num_rows = 4

then, num_col = 2

a	%
m	a
e	c
r	i

Table 2

So the encrypted word is 'a%maecri'

So how to decrypt:

First we make the grid from the encrypted word. This can be easily just reverse as Table 2.

Now to get decryption, we move in the grid as in Table 1 and add letters to decrypted string(which is intially ""). We return decrypted string when we first get the '%' beacuse after that there is no need to go.

Points to note:

1. We have to run a initial loop as number of times as number of column.

2. Not in even columns, we have to move from up to down in the column(through rows) and in odd columns we have to move from down to up.

Here is the code:

import math
def sbu_decrypt(encrypted,num_rows):
    if num_rows <=0:                    #Return encrypted if num_rows<=0
        return encrypted
    if encrypted=="":                   #Return None id encrypted is empty string
        return None
    num_cols = len(encrypted)//num_rows    #Get num_cols. (Note that len(encrypted)=num_rowss*num_cols)
    k = 0
    grid = [['' for i in range(num_cols)] for j in range(num_rows)] #Make a grid
    '''Fill the grid from encrypted'''
    for i in range(num_rows):
        for j in range(num_cols):
            grid[i][j] = encrypted[k]
            k+=1
    decrypted = ""                     #Empty string to store decrypted letters
    '''In even columns move from top to bottom
    And in odd columns move from bottom to top'''
    for col in range(num_cols):
        if col%2 == 0:
            row = 0
        else:
            row = num_rows-1
        k = 0
        while(k<num_rows):       #Moving in a column(through rows)
            if grid[row][col]=="%":
                return decrypted       #Retirn whenever % is encountered
            decrypted+=grid[row][col]
            if col%2 == 0:
                row+=1
            else:
                row-=1
            k+=1
    return decrypted

print(sbu_decrypt('AMERICA',7))
print(sbu_decrypt('SOO%TRK%OBUVNYNI',4))
print(sbu_decrypt('CHICKENWINGS',-2))
print(sbu_decrypt('',5))
print(sbu_decrypt('Srs%tei%ovt%niy%ynN%BUekrkwrooYo',8))
print(sbu_decrypt('UtetasOmecanidStefAri%',2))

Screenshots:

Output: