affine-cipher.R

# normalise function 
# - remove whitespace 
# - make all lower case
normalise<-function(text){
  return(tolower(gsub(" ", "", text)))
}

#lookup index of letter
lookupIndex<-function(normalisedtext){
  # list of letters
  letterslist <- strsplit(normalisedtext, "")[[1]]
  # index of letters
  return(match(letterslist, letters) - 1)
}

#find gcd 
gcd <- function(x, y) {
  r <- x %% y
  return(ifelse(r, gcd(y, r), y))
}

# computes the modulo multiplicative inverse of a^m
# a^-1 = mmi( a mod m)=mmi(a,m)
mmi <- function(a, m) {
  a <- a %% m
  for (x in 1:m) {
    if ((a * x) %% m == 1) {
      return(x)
    }
  }
  return(1)
}

# encrypts plaintext
encrypt <- function(plaintext, a, b) {
  m <- 26
  
  # must check a and m are coprime
  if (gcd(a, m) != 1) {
    stop(paste('a=',a,' and m=',m,'is coprime'))
  }
  
  #normalise text input
  normalisedplaintext<-normalise(plaintext) 
  x<-lookupIndex(normalisedplaintext)
  
  
  # E(x) = (ax + b) mod m
  return(paste(letters[ ((a * x + b) %% m) + 1], collapse = ""))
}

# decrypts encryption
decrypt <- function(encryption, a, b) {
  m <- 26
  
  # must check a and m are coprime
  if (gcd(a, m) != 1) {
    stop("a and 26 must be co-prime")
  }
  
  normalisedencryption<-normalise(encryption)
  y<-lookupIndex(normalisedencryption)
  
  # D(y) = a^-1(y - b) mod m
  return(paste(letters[((mmi(a, m) * (y - b)) %% m) + 1], collapse = ""))
}