C Decryption -> [] as output

I'm new to the site and to C! I'm trying to write an encryption/decryption program, but I've ran into a problem - my decrypted files aren't showing as characters but instead "[]". Perhaps someone could suggest where I've gone wrong - I think my decryption section is missing something! The program is meant to read commands from arguments(-e encrypt, -d decrypt, -i input file name, -o output file name, -a algorithm, -k key). Here follows my code (go easy!):

#include <stdio.h>

int main(int argc, char *argv[]) 
{
    if (strcmp(argv[1], "-e") == 0)
    {
        printf("\tEncrypting...\n");
        //File pointer input, 4th arg as filename, read
        FILE *fpi = fopen(argv[3],"r");
            if (fpi == NULL)
            {
                printf("\tError opening file!\n");
                exit(0);
            }
        //Find size of file
        int sz; 
        fseek(fpi, 0, SEEK_END);    
        sz = ftell(fpi);
        printf("\tLength of file is: %i\n", sz);
        fseek(fpi, 0, SEEK_SET);

        //10th arg is key
        int key = atoi(argv[9]);
        //I want to make a default key if not supplied in args
        //Not sure how to write, this crashes :(  
            if(!argv[9])
            {
                key = 200;
            }
        printf("\tYour key is: %d\n", key);
        // Encryption; does the job
        char input[512];
        while(fgets(input, 512, fpi))
        {
            FILE *fpo = fopen(argv[5], "w+");
            int i=0;    
            while (input[i] != 0) 
            {
                fprintf(fpo, "%04x ",input[i]+key << 2);
                i++;    
            }   
        }   
        printf("\tEncryption complete, exiting...\n");  
        fclose(fpi);
        //When I try to fclose(fpo) here it crashes     
    }
    else if (strcmp(argv[1], "-d") == 0)
    {
        printf("\tDecrypting...\n");
        FILE *fpi = fopen(argv[3],"r");
            if (fpi == NULL)
            {
                printf("\tError opening file!\n");
                exit(0);
            }   
        int key = atoi(argv[9]);
        printf("\tUsing key: %d\n", key);

        char input[512];
        while( fgets(input, 512, fpi))
        {
            FILE *fpo = fopen(argv[5], "w+");
            int i=0;    
            while (input[i] != 0) 
            {           //tried the reverse - returns [][][][]
                fprintf(fpo, "%c ",input[i] - key >> 2);
                i++;    
            }   
        }
        printf("\tDecryption complete, exiting...\n");  
        fclose(fpi);    
    }
else 
    {
        printf("\tUsage: -e/-d, -i input file name, -o output file name, -a alg to use, -k key value.\n\tAlgorithms supported: basic\n");
    }
    return 0;
}   

Thanks in advance for any help!