Functional Programming: A PragPub Anthology

Our Native Code

For our native code, we’re going to create an implementation of the Caesar cipher.^[27] This isn’t something you would use for real encryption, but if you think about the implications of bugs in native crypto implementations, the impact is pretty huge.

Let’s start with our legacy code:

caesar.c
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

char *caesar(int shift, char *input)
{
  char *output = malloc(strlen(input));
  memset(output, '\0', strlen(input));

  for (int x = 0; x < strlen(input); x++) {
    if (isalpha(input[x])) {
      int c = toupper(input[x]);
      c = (((c - 65) + shift) % 26) + 65;
      output[x] = c;
    } else {
      output[x] = input[x];
    }
  }

  return output;
}

Like all substitution ciphers, ...