trusting trust considered harmful

http://cm.bell-labs.com/who/ken/trust.html
Ken Thompson, creator of UNIX
"You can't trust code that you did not totally create yourself."
C compiler could put an invisible backdoor in the Unix login command when it noticed that the login program was being compiled
C compiler could also add this feature undetectably to future compiler versions upon their compilation as well.
"I picked on the C compiler. I could have picked on any program-handling program such as an assembler, a loader, or even hardware microcode. As the level of program gets lower, these bugs will be harder and harder to detect. A well installed microcode bug will be almost impossible to detect."
"No amount of source-level verification or scrutiny will protect you from using untrusted code."

>>14
It's easy to reason the logic of IC. You put in some test input and the output should be as expected. If there is any discrepancy, that particular function is broken.