Thread: Encryption/Decryption Program

I am trying to build an encryption/decryption program for fun. I know it won't be strong, I know people would crack it in seconds but I don't care. This is just to get to grips with it. I have code that works with the XOR encryption method. It needs a bit of work but its operating and that was my goal. What I want to do is design a very simple algorithm to replace XOR in my code. I assume that I should write a function and replace

Code:

c^=*(key++);

with my algorithm e.g.

Code:

c = algo(c, *key++);

Can someone tell me how I might write a function to say increment every letter by 5, i.e. a = (a+5), b = (b+5), etc?

I should know this but I'm blanking!

Code so far:

Code:

#include <stdio.h>


int main(int argc, char*argv[])
{
FILE *fp1, *fp2;
char* key;
int c;


key = argv[1];


	if(*key != '\0')
	{
	fp1 = fopen(argv[2],"rb");
		if(fp1 != NULL)
		{
		fp2 = fopen(argv[3],"wb");
		if (fp2 != NULL)
		{
			while((c=getc(fp1))!=EOF)
			{
				if(!*key)key = argv[1];
				c ^=*(key++);
	
				putc(c, fp2);
			}
		fclose(fp2);
		}
	fclose(fp1);
	}
	}
return 0;
}