Answer to Question #164675 in Python for Ray

Question

The transposition cipher can be generalized to any number of rails. Write a function to implement a three-rail fence cipher that takes every third character and puts it on one of the three rails. Test your algorithm with two sample input as shown below.

Sample:

PlainText: ABCDEFGHI

CipherText: CFIBEHADG

Answer 1

def encryptRailFence(text, key):
    rail = [['\n' for i in range(len(text))]
            for j in range(key)]

    dir_down = False
    row, col = 0, 0

    for i in range(len(text)):
        if (row == 0) or (row == key - 1):
            dir_down = not dir_down


        rail[row][col] = text[i]
        col += 1

        if dir_down:
            row += 1
        else:
            row -= 1
    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))


def decryptRailFence(cipher, key):
    rail = [['\n' for i in range(len(cipher))]
            for j in range(key)]

    dir_down = None
    row, col = 0, 0

    for i in range(len(cipher)):
        if row == 0:
            dir_down = True
        if row == key - 1:
            dir_down = False

        rail[row][col] = '*'
        col += 1
        if dir_down:
            row += 1
        else:
            row -= 1
    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

    result = []
    row, col = 0, 0
    for i in range(len(cipher)):


        if row == 0:
            dir_down = True
        if row == key - 1:
            dir_down = False

        if (rail[row][col] != '*'):
            result.append(rail[row][col])
            col += 1

        if dir_down:
            row += 1
        else:
            row -= 1
    return ("".join(result))


if __name__ == "__main__":
    print(encryptRailFence(str(input("Enter the plain text: ")), int(input("Enter the key: "))))
    print(decryptRailFence(str(input("Enter the cipher text: ")), int(input("Enter the key: "))))