Memfrob

November 10, 2020

We give a function that performs encryption, and leave it to the reader to wrap that in a function that handles files:

(define (memfrob str)
   (define (frob c)
     (integer->char (bitwise-xor (char->integer c) 42)))
   (list->string (map frob (string->list str))))

 > (memfrob (memfrob "Programming Praxis"))
   "Programming Praxis"

You can run the program at http://ideone.com/RnJDWB.

Pages: 1 2

Posted by programmingpraxis
Filed in Exercises
3 Comments »

3 Responses to “Memfrob”

  1. chaw said
    November 10, 2020 at 11:08 AM

    A quick solution using standard Scheme:

    (import (scheme base)
        (scheme write)
        (only (srfi 60)
              bitwise-xor)
        (only (org eip10 okmij myenv)
              assert))

(define (memfrob-byte byt)
  (bitwise-xor byt 42))

(assert (equal? '(42 0 43 41)
                (map memfrob-byte '(0 42 1 3))))

(define (memfrob-bytevector bytvec)
  (let* ((n (bytevector-length bytvec))
         (r (make-bytevector n)))
    (do ((i 0 (+ i 1)))
        ((>= i n) r)
      (bytevector-u8-set! r i (memfrob-byte (bytevector-u8-ref bytvec i))))))

(assert (equal? #u8(42 0 43 41)
                (memfrob-bytevector #u8(0 42 1 3))))

(define (memfrob-string str)
  (utf8->string (memfrob-bytevector (string->utf8 str))))

(assert (string=? "Hello, World!"
                  (memfrob-string (memfrob-string "Hello, World!"))))

  2. Alex B said
    November 19, 2020 at 6:53 AM

    In Python:

    def memfrob(msg):
    return bytes(c ^ 42 for c in msg)

    Input argument must be a bytes-like object.

  3. Daniel said
    November 25, 2020 at 11:32 PM

    Here are a few solutions in C. The first is a plain vanilla implementation, followed by an implementation that uses memfrob from the GNU C library, and then an implementation that uses AVX2 intrinsics (the other solutions—compiled with optimizations—are still faster in my tests on a large file).

    #include <stdio.h>

int main(void) {
  char buffer[BUFSIZ];
  while (1) {
    size_t n = fread(buffer, 1, BUFSIZ, stdin);
    if (!n) break;
    for (int i = 0; i < n; ++i)
      buffer[i] ^= 42;
    fwrite(buffer, 1, n, stdout);
  }
}

    #define _GNU_SOURCE
#include <stdio.h>
#include <string.h>

int main(void) {
  char buffer[BUFSIZ];
  while (1) {
    size_t n = fread(buffer, 1, BUFSIZ, stdin);
    if (!n) break;
    memfrob(buffer, n);
    fwrite(buffer, 1, n, stdout);
  }
}

    // Requires -mavx2 compiler flag.

#include <immintrin.h>
#include <stdio.h>

int main(void) {
  int bufsize = 8192;  // Must be divisible by 32.
  char buffer[bufsize];
  __m256i mask = _mm256_set1_epi8(42);
  while (1) {
    size_t n = fread(buffer, 1, bufsize, stdin);
    if (!n) break;
    for (int i = 0; i < bufsize; i += 32) {
      char* p = (char*)buffer + i;
      __m256i x = _mm256_loadu_si256((__m256i*)p);
      __m256i y = _mm256_xor_si256(x, mask);
      *((__m256i*)p) = y;
    }
    fwrite(buffer, 1, n, stdout);
  }
  return 0;
}

    Example usage:

    $ echo "Programming Praxis" | ./a.out
zXEMXKGGCDM
zXKRCY

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