I saw /etc/passwd and asked my boss how to decrypt the passwords. He told me it was a one-way encryption, so the login program would just encrypt the password you provided and compare to the encrypted value. He went on explaining the old crypt algorithm and even made a bet I could not guess his password. He said it was related to a movie.
So at 17 I was hooked and started studying the sources. In the end I just patched and recompiled the passwd binary to store clean text passwords in a hidden file. Later I learned this was called a trojan horse.
And even now, 30 years later, I remember his face when I told the movie was Citizen Kane and his password was "rosebud".
Thank you Miguel and Gorgonio for teaching me about C and Unix! This knowledge paid my rent for 3 decades and I still love the job.