Explain symmetric versus asymmetric encryption and when each is used.

This question tests whether you understand why two families of cryptography exist, not just their names. The honest answer is that each solves a different problem, and modern systems use them together.

Symmetric encryption

A single secret key both encrypts and decrypts the data. Algorithms like AES are extremely fast and efficient, which makes symmetric crypto ideal for encrypting large amounts of data — files, disks, or the bulk of a network session.

The catch is key distribution: both sides need the same key, and getting that key to the other party safely over an untrusted network is hard. If an attacker intercepts the key, the encryption is worthless.

Asymmetric encryption

Each party has a mathematically linked key pair: a public key anyone can hold, and a private key kept secret. Data encrypted with the public key can only be decrypted with the matching private key (and signatures work the reverse way). Algorithms like RSA and elliptic-curve crypto fall here.

This solves key distribution — you can publish your public key openly — but it is much slower and unsuitable for encrypting large payloads.

The hybrid model

Real protocols combine the two. In TLS, the client and server use asymmetric cryptography to authenticate and agree on a fresh symmetric session key, then encrypt the actual traffic symmetrically. You get the trust and key-exchange benefits of asymmetric crypto with the speed of symmetric crypto.

Why this matters

Interviewers want to hear that you grasp the tradeoff — speed versus the key-distribution problem — and that you know production systems blend both. Saying "TLS uses asymmetric to exchange a symmetric key" signals real understanding rather than rote memorization.