I have done as what you said, but it didn't work. yluo4 at eva:~/bfed$ echo 'secret' | ./bfed -k cafefacedeadbeef yluo4 at eva:~/bfed$ ps | grep bfe[d] yluo4 at eva:~/bfed$ And does the key must be 16 hexadecimal characters? Because you also say "using a 128 bit (16 byte) key" in the manual. I think 16 hexadecimal characters is 64 bit.