R/scytale.R

In cryptography: Encrypts and Decrypts Text Ciphers

#' Scytale cipher
#'
#' @description This can be used to encrypt and decrypt a
#'   Scytale cipher. A Scytale cipher is an ancient form of cryptography that wraps a message (typically written
#'   on a long thing piece of paper) around a device to create a matrix with a fixed number of columns that transposes
#'   the text.
#'
#'
#' @param message A character vector
#' @param col A positive integer, this determines the number of columns in the encryption matrix. 1 column will have no effect
#' @param encrypt (Default: `TRUE`) TRUE will encrypt the message, while FALSE will decrypt the message.
#'
#' @return A character vector of either plaintext that has been encrypted or ciphertext that has been decrypted.
#' @export
#'
#' @examples
#' scytale("very super secret message!", col = 4, encrypt = TRUE)
#' scytale("v eetseesrc s!ru rmaypseeg", col = 4, encrypt = FALSE)
#'
scytale <- function(message, col, encrypt=TRUE) {

  # stop if message is not a character vector
  if (!is.character(message) || !is.vector(message) || length(message) != 1) {
    stop("message must be a character vector")
  }

  # stop if cols is not a positive integer
  if (!is.numeric(col) || length(col) != 1 || col < 1 || col %% 1 != 0) {
    stop("col must be an integer greater than or equal to 1")
  }

  # stop if encrypt input is not logical
  if (!is.logical(encrypt)) {
    stop("encrypt must be TRUE or FALSE")
  }

  # calling the encryption or decryption method
  if (encrypt == TRUE) {

    # splitting the message into individual characters
    substrings <- strsplit(message, "")[[1]]

    # if the length of the message is not divisible by the number of columns must append with empty values to fill matrix
    if (length(substrings) %% col != 0) {
      for (i in 1:(col-(length(substrings) %% col))) {
        substrings <- append(substrings, "", after = length(substrings))
      }
    }

    # filling the cipher matrix with the individual characters
    cipher.matrix <- matrix(substrings, ncol = col, byrow = TRUE)

    # paste encrypted message from cipher matrix
    message <- paste(cipher.matrix, collapse = "")
  }

  if (encrypt == FALSE) {

    # splitting the message into individual characters
    substrings <- strsplit(message, "")[[1]]

    # if the length of the message is not divisible by the number of columns must append with empty values in the correct
    # places to fill matrix
    if (length(substrings) %% col != 0) {
      for (i in (length(substrings) %% col):(col-1)) {
        substrings <- append(substrings, "", after = ((i+1)*(ceiling(length(substrings)/col)))-1)
      }
    }

    # filling the decryption matrix with the substrings
    decrypted.matrix <- matrix(substrings, ncol = col, byrow = FALSE)

    # paste decrypted message from decrypted matrix
    message <- paste(t(decrypted.matrix), collapse = "")
  }
  return(message)
}