Question

please solve the following in python:

write a program for the decryption of 2-rail fence cipher. For example, the input "Cmhmtmrooeoeoorw" should return "Comehometomorrow", and input "topaesw lyr" should return "two players".

Answer 1

def decryptRailFence(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(encryptRailFence("attack at once", 2))

    print(encryptRailFence("GeeksforGeeks ", 3))

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

     

    # Now decryption of the

    # same cipher-text

    print(decryptRailFence("GsGsekfrek eoe", 3))

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

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