Lyft CS Foundation Interview: TLS Deployment & Debugging

This question tests your ability to deploy and troubleshoot secure web communication using TLS/SSL in production-like environments. You’ll be asked to configure web servers (Nginx/Apache), provision and manage certificates, and diagnose handshake or compatibility failures using networking and crypto tools.

You should be ready to walk through end-to-end tasks: obtaining/renewing certificates (ACME/Let's Encrypt or private CA), installing the certificate chain, enabling appropriate TLS versions and cipher suites, and optionally implementing mutual TLS (mTLS) for client authentication. Expect to explain trade-offs between strict security (TLS 1.3, forward secrecy, strong ciphers) and client compatibility (older TLS versions, legacy clients, SNI issues).

The interview flow typically moves from high-level architecture to hands-on debugging. First, outline how you would terminate TLS (load balancer vs app server), where keys live, and how rotation/automation works. Next, demonstrate troubleshooting steps when connections fail: check time and DNS, inspect cert chain and SANs, run openssl s_client -connect, curl -v --tlsv1.2, and capture packets with tcpdump/Wireshark (use filters for TLS). Be prepared to read server logs and interpret SSL handshake errors, certificate expiration, incomplete chains, ALPN/SNI mismatches, and revocation (OCSP/CRL) issues.

Skills to demonstrate: practical PKI knowledge, TLS protocol internals (handshake, cipher negotiation), server configuration, command-line diagnostic proficiency (OpenSSL, tcpdump, Wireshark), and operational thinking for rotation, monitoring, and balancing security with interoperability.