Manual "encryption" only outputs 0's to file - c

Question

My assignment (not homework, just a "try if you can do this" thing) is to use bit operations to encrypt and decrypt a .txt file.
This is the program. It successfully opens files for read/write but puts all 0's and spaces into the output.txt file instead of the expected "encrypted" text. I am guessing the issue comes from a fundamental misunderstanding of either data types or putc(). I understand that it outputs an unsigned char, but my professor said that an unsigned char is nothing but an unsigned int -- not sure if this is purely true or if it was a pedagogical simplification. Thanks a lot for any help.
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#define NUMARG 3
#define INFILEARG 1
#define OUTFILEARG 2
int main(int argc, char *argv[]){
/* Function prototypes */
unsigned int encryptDecrypt(unsigned int x, unsigned int ed);
const char *get_filename_ext(const char *filename);
FILE *finp;
FILE *foutp;
//ed for encryption/decryption choice
unsigned int ed, c;
const char *ext;
//Check for errors in argument number and file opening.
if(argc != NUMARG){
printf("You have to put the input and output files after the
program name.\n");
return(1);
}
if( (finp = fopen(argv[INFILEARG], "r")) == NULL ){
printf("Couldn't open %s for reading.\n", argv[INFILEARG]);
return(1);
}
if( (foutp = fopen(argv[OUTFILEARG], "w")) == NULL){
printf("Couldn't open %s for writing.\n", argv[OUTFILEARG]);
return(1);
}
//Get and check file extension.
ext = get_filename_ext(argv[INFILEARG]);
if(strcmp(ext, "txt")){
printf("Input file is not a .txt file.\n");
return(1);
}
ext = get_filename_ext(argv[OUTFILEARG]);
if(strcmp(ext, "txt")){
printf("Output file is not a .txt file.\n");
return(1);
}
//Get command to encrypt or decrypt.
do{
printf("Enter e to encrypt, d to decrypt: ");
ed = getchar();
} while(ed != 'e' && ed != 'd');
//Send characters to output file.
while((c = getc(finp)) != EOF ){
putc(encryptDecrypt(c, ed), foutp);
}
// Close files.
if (fclose(finp) == EOF){
printf("Error closing input file.\n");
}
if (fclose(foutp) == EOF){
printf("Error closing output file.\n");
}
if ( ed == 'e'){
printf("Encrypted data written.\n");
} else {
printf("Data decrypted.\n");
}
return 0;
}
const char *get_filename_ext(const char *filename) {
const char *dot = strrchr(filename, '.');
if(!dot || dot == filename) return "";
return dot + 1;
}
unsigned int encryptDecrypt(unsigned int c, unsigned int ed){
if( ed == 'e' ){
printf("%d before operated on.\n", c);
c &= 134;
printf("%d after &134.\n", c);
c ^= 6;
printf("%d after ^6. \n", c);
c <<= 3;
printf("%d after <<3\n", c);
}
else {
c >>= 3;
c ^= 6;
c &= 134;
}
return c;
}
Output:
ZacBook:bitoperations $ cat input1.txt
Please encrypt this message.
ZacBook:bitoperations $ ./encrypt.o input1.txt output.txt
Enter e to encrypt, d to decrypt: e
80 before operated on.
0 after &134.
6 after ^6.
48 after <<3
108 before operated on.
4 after &134.
2 after ^6.
16 after <<3
[...Many more of these debug lines]
2 after &134.
4 after ^6.
32 after <<3
Encrypted data written.
ZacBook:bitoperations $ cat output.txt
00 0 00000 0 0
As you can see, the unsigned int is being operated on successfully. I believe the problem is with putc() but I have tried changing the type of c to char and int and neither have worked.

Answer 1

Your main problem is that &= is a lossy transformation: that is you lose data.
Ditto <<= and >>=, as both cause extreme 1 bits to be lost.
You'll have more luck sticking to XOR; at first at least. That's because x ^ y ^ y is x.
You can eliminate putc &c. by isolating the encryption / decryption processes from the data acquisition stages, and by hardcoding the inputs whilst your getting things working.