Question

The prompt is using Python:  Write a 3 rail transposition encryption algorithm, and a corresponding decryption algorithm. Implement these two algorithms in their own function. Now write a testing function that demonstrates your algorithms work for all interesting cases!

Answer 1

# 3Rail Cipher Encryption

# and Decryption

  

# function to encrypt a message

def encryptRail(text, key):

  

    # create the matrix to cipher

    # plain text key = rows ,

    # length(text) = columns

    # filling the rail matrix

    # to distinguish filled

    # spaces from blank ones

    rail = [['\n' for i in range(len(text))]

                  for j in range(key)]

      

    # to find the direction

    dir_down = False

    row, col = 0, 0

      

    for i in range(len(text)):

          

        # check the direction of flow

        # reverse the direction if we've just

        # filled the top or bottom rail

        if (row == 0) or (row == key - 1):

            dir_down = not dir_down

          

        # fill the corresponding alphabet

        rail[row][col] = text[i]

        col += 1

          

        # find the next row using

        # direction flag

        if dir_down:

            row += 1

        else:

            row -= 1

    # now we can construct the cipher

    # using the rail matrix

    result = []

    for i in range(key):

        for j in range(len(text)):

            if rail[i][j] != '\n':

                result.append(rail[i][j])

    return("" . join(result))

      

# This function receives cipher-text

# and key and returns the original

# text after decryption

def decryptRail(cipher, key):

  

    # create the matrix to cipher

    # plain text key = rows ,

    # length(text) = columns

    # filling the rail matrix to

    # distinguish filled spaces

    # from blank ones

    rail = [['\n' for i in range(len(cipher))]

                  for j in range(key)]

      

    # to find the direction

    dir_down = None

    row, col = 0, 0

      

    # mark the places with '*'

    for i in range(len(cipher)):

        if row == 0:

            dir_down = True

        if row == key - 1:

            dir_down = False

          

        # place the marker

        rail[row][col] = '*'

        col += 1

          

        # find the next row

        # using direction flag

        if dir_down:

            row += 1

        else:

            row -= 1

              

    # now we can construct the

    # fill the rail matrix

    index = 0

    for i in range(key):

        for j in range(len(cipher)):

            if ((rail[i][j] == '*') and

               (index < len(cipher))):

                rail[i][j] = cipher[index]

                index += 1

          

    # now read the matrix in

    # zig-zag manner to construct

    # the resultant text

    result = []

    row, col = 0, 0

    for i in range(len(cipher)):

          

        # check the direction of flow

        if row == 0:

            dir_down = True

        if row == key-1:

            dir_down = False

              

        # place the marker

        if (rail[row][col] != '*'):

            result.append(rail[row][col])

            col += 1

              

        # find the next row using

        # direction flag

        if dir_down:

            row += 1

        else:

            row -= 1

    return("".join(result))

  

# Driver code

if __name__ == "__main__":

    print(encryptRail("attack at once, 2))

    print(encryptRail("defend the east wall",3))

      

    # Now decryption of the

    # same cipher-text

    print(decryptRail("dnhaweedtees alf tl", 3))

    print(decryptRail("atc toctaka ne", 2))

#output:-

#Encryption output:-

atc toctaka ne

dnhaweedtees alf  tl

#Decryption output:-
attack at once
​​​​​
delendfthe east wal