What is the difference between symmetric and asymmetric encryption, and when would you use each?

Encryption schemes split into two families based on how keys relate to each other, and understanding that split is foundational to almost every secure protocol.

Symmetric encryption

In symmetric cryptography, the same secret key both encrypts and decrypts. AES is the workhorse here. It is extremely fast — modern CPUs have dedicated AES instructions — so it is what actually protects the bulk of your data, whether that is disk volumes or TLS session traffic.

The catch is key distribution. Both parties need the same key, and getting that key to the other side over an untrusted network without an eavesdropper grabbing it is the hard part.

Asymmetric encryption

Asymmetric cryptography uses a key pair: a public key anyone can hold and a private key the owner keeps secret. Data encrypted with the public key can only be decrypted with the private key (confidentiality), and data signed with the private key can be verified with the public key (authenticity). RSA and elliptic-curve schemes are the common examples.

This elegantly solves key distribution — you can hand your public key to the world. The cost is speed: asymmetric operations are orders of magnitude slower and have size limits, so they are unsuitable for encrypting large payloads directly.

Why real systems combine them

Practical protocols use a hybrid approach. Asymmetric crypto authenticates the parties and securely establishes a shared symmetric session key; the fast symmetric cipher then encrypts the actual data. TLS does exactly this.

What interviewers look for

A junior candidate should clearly map keys to operations, state that symmetric is fast but has a distribution problem, and — the differentiator — explain that we don't pick one, we combine them in a hybrid scheme.