Cesar cipher in C

cipher.c

/* 2013/3/4 by darkf - licensed under WTFPL - cesar cipher program */
#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]) {
    int key, i, len;
    char output[128];
    
    if(argc != 3) {
        fprintf(stderr, "usage: %s CIPHER KEY\n", argv[0]);
        return 1;
    }
    
    if(sscanf(argv[2], "%d", &key) != 1) {
        fprintf(stderr, "invalid key: %s\n", argv[2]);
        return 2;
    }
    
    if(strcmp(argv[1], "cesar") == 0) {
        int c;
        i = 0;
        while((c = fgetc(stdin)) != EOF)
            output[i++] = ((unsigned char) c) + key;
        len = i;
    }
    else {
        fprintf(stderr, "invalid cipher: %s\n", argv[1]);
        return 2;
    }
    
    printf("output: ");
    for(i = 0; i < len; i++)
        printf("%02X (%c) ", output[i], output[i]);
    printf("\n");
  return 0;
}