Help with a program

i am trying to understand how to encrypt a string.
i think i could use a for loop like


for(x=0; x < strlen(input); x++)
{
input[x] = (------);

}
where input[] is my string that the user inputs
i dont know what formula to put in (-------).
every formula i try creates a crazy output.
i just want a formula that will output all the letters shifted
two positions or something
any help would be greatly appreciated

Have a look at the Rot13 encryption:
http://tools.geht.net/rot13.html
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