Which use case best illustrates symmetric cryptography?

• A. Encrypting data at rest with AES.
• B. Encrypting TLS with RSA keys.
• C. Digital signatures with ECDSA.
• D. Key exchange with Diffie-Hellman.

Answer: (A)

Symmetric cryptography relies on the same secret key to encrypt and decrypt data, enabling fast protection of large data volumes. Encrypting data at rest with AES fits this perfectly because AES is a symmetric cipher that uses a single key to both hide and reveal the stored information. The other options rely on public-key techniques: RSA keys in TLS are part of the public-key handshake, not bulk encryption with a shared secret; digital signatures with ECDSA are about producing and verifying signatures, not encryption; Diffie-Hellman is a method to establish a shared secret (which can then seed symmetric keys) rather than encrypting data itself. So the data-at-rest encryption with a symmetric algorithm like AES is the clearest example of symmetric cryptography.