Stack overflows are the most basic binary exploitation technique, but they take a lot of skill to master. If you already know some C, these problems can help acquaint you with stacks and binary exploitation in general.

Problem available on the shell machine in /problems/stack_overflow_2_44e63640e033ff2b , downloadable here[1] with source here[2].

If you solve the problem you will be able to read the key file by running

    cat /problems/stack_overflow_2_44e63640e033ff2b/key

    on the PicoCTF shell machine.

    Hint: A function's arguments live on top of its stack frame, above its saved ebp and return address. Make sure not to clobber those, though... 
    [1]https://2013.picoctf.com/problems/overflow2-44e63640e033ff2b
    [2]https://2013.picoctf.com/problems/overflow2-44e63640e033ff2b.c