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Back doors

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Thought i'd share something I came across in the Jargon file - the entry for back door.

Though the compiler was never officially distributied, in an early version of UNIX Ken Thompson put code into the C compiler to recognize when the login command was being compiled - it then put in a back door which Thompson knew the password for.

What made it really clever is that the compiler would also recognize when it was compiling the compiler, and insert the code into the compiler to change login. Since the instructions were in the compiler's executable, you wouldn't be able to discover the back door by looking through the source code for either login or the compiler. Absolutely fiendish.
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Heard about this before, really funny stuff.
Just proves that you can never really trust someone elses code, but it's also impossible to do a code review of everything you do in your day to day life. (Your OS, your apps, your car, your cellphone)

So what's better being blissfully ignorant or parinodily aware? ;)

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