TLS: how RSA key exchange works, how DH key exchange works, what a cipher suite includes, the purpose of session resumption, how client authentication works, the advantages of forward secrecy, how TLS 1.3 differs from TLS 1.2
Passwords: how UNIX passwords are stored, how to salt and hash a password, how to crack a password, what a rainbow table attack is, the purpose of a PAKE protocol, advice from An Administrator's Guide to Internet Password Research
Buffer Overflow: how the stack is organized, what gets pushed on the stack when a function is called, how buffer overflow leads to vulnerabilities, what shell code is and what it does, why a NOP sled is needed, what the defenese against buffer overflow attacks do, how the different types of canaries operate, how to examine the stack using gdb
Secure Email: how SMTP works, why and how email is insecure, defenses that email providers are taking and what they do, differences between S/MIME and PGP, how secure webmail works, familiarity with viewpoints in the debate over end-to-end-encryption and exceptional (or backdoor) access
Signal: the purpose of X3DH (Extended Triple Diffie-Hellman), how X3DH works, the purpose of the double ratchet, how the KDF chain works, what the KDF chains are used for