pgm 22.14, encrypt a character stream

Author: twcamper

.gitignore

@@ -177,3 +177,4 @@ spec/parts-list-io-spec
 22/22.11-format-dates
 22/22.13-find-departure
 22/22.13-find-departure-unbuffered
+22/22.14-ceasar-cipher

22/22.14-ceasar-cipher.c

@@ -0,0 +1,67 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+#define INPUT_FILENAME_SIZE FILENAME_MAX - 4
+#define OUTPUT_FILENAME_SIZE FILENAME_MAX
+
+int main(void)
+{
+  char input[INPUT_FILENAME_SIZE + 1];
+  char output[OUTPUT_FILENAME_SIZE + 1];
+  int ch, i, shift_amount;
+  int rc = 0;
+
+  printf("Enter name of file to be encrypted: ");
+
+  for (i = 0; i < INPUT_FILENAME_SIZE && (ch = getchar()) != '\n'; i++) {
+    input[i] = output[i] = ch;
+  }
+  input[i] = output[i] =  '\0';
+  strcat(output, ".enc");
+
+  printf("Enter shift amount (1-25): ");
+  scanf("%d", &shift_amount);
+
+  FILE *istream, *ostream;
+  if ((istream = fopen(input, "r")) == NULL) {
+    perror(input);
+    exit(EXIT_FAILURE);
+  }
+
+  if ((ostream = fopen(output, "w")) == NULL) {
+    perror(output);
+    if (fclose(istream) == EOF)
+      perror(input);
+    exit(EXIT_FAILURE);
+  }
+
+  while ((ch = fgetc(istream)) != EOF) {
+    if (islower(ch))
+      ch = (( ch - 'a') + shift_amount) % 26 + 'a';
+    if (isupper(ch))
+      ch = (( ch - 'A') + shift_amount) % 26 + 'A';
+    if (fputc(ch, ostream) == EOF) {
+      perror(output);
+      rc = -1;
+      break;
+    }
+  }
+  if (ferror(istream)) {
+    perror(input);
+    rc = -2;
+  }
+
+  if (fclose(istream) == EOF) {
+    perror(input);
+    rc = -3;
+  }
+
+  if (fclose(ostream) == EOF) {
+    perror(output);
+    rc = -4;
+  }
+
+  return rc;
+}