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

technical-conceptual · Junior level · cloud-devops-security

What the interviewer is really asking

Test baseline cryptography literacy that underpins TLS, secrets management, and cloud KMS usage.

What to say

• Symmetric encryption uses the same key to encrypt and decrypt (e.g., AES-256). It's fast and suitable for bulk data at rest — the per-object data keys that AWS KMS generates for S3/EBS/RDS encryption are symmetric.
• Asymmetric encryption uses a public/private key pair (e.g., RSA-2048, ECDSA). The public key encrypts or verifies; the private key decrypts or signs. It's used for TLS server authentication and key agreement, SSH login, and code/document signing.
• In practice you combine them: TLS uses asymmetric crypto to authenticate the server and agree on a session key (ephemeral ECDHE in TLS 1.3), then switches to symmetric (AES-GCM) for the bulk data — best of both worlds.

What to avoid

• Saying asymmetric is 'more secure' than symmetric categorically — AES-256 is computationally very strong; the trade-off is about key distribution, not raw strength.
• Confusing hashing (one-way, no key) with encryption.
• Claiming SSE-KMS uses an RSA/asymmetric KMS key to wrap the data key — S3/EBS/RDS only accept symmetric KMS keys, and the wrapping is symmetric envelope encryption.

Example answers

Strong: When I set up an S3 bucket with SSE-KMS, AWS generates a per-object AES-256 data key (symmetric), then wraps that data key with my symmetric KMS key — envelope encryption — and stores only the encrypted data key alongside the object. The whole at-rest flow is symmetric; KMS asymmetric/RSA keys are for separate use cases like signing or client-side encryption, not for wrapping SSE-KMS data keys.

Weak: Asymmetric is better because it uses two keys so it's twice as safe.