Which of the following is a limitation of using symmetric encryption for securing communications?

Correct — C. Symmetric encryption uses one secret key to encrypt and to decrypt. Both parties must hold that key before they talk, so it has to reach them securely, and every pair of parties needs its own.

NIST's example: 1,000 parties talking pairwise need at least 499,500 keys. Distributing and managing that many secrets is the limitation → option (c).

Why the others are wrong

• (a)
  It requires more computational resources than asymmetric encryption — It is the reverse. NIST says symmetric-key algorithms are generally significantly faster than asymmetric ones and use shorter keys for the same security strength.
• (b)
  The encryption and decryption process is too slow — Speed is symmetric encryption's advantage. Asymmetric algorithms are the slow ones, which is why NIST notes they are not used to process large amounts of data.
• (d)
  It cannot be used for data at rest — Symmetric encryption does protect data at rest: NIST approves the XTS-AES mode of AES, a symmetric cipher, for data on storage devices.

Symmetric (secret-key) algorithms such as AES use a single key for encryption and decryption, so the key must stay secret and be shared in advance. Asymmetric (public-key) algorithms use a key pair: a public key that can be published and a private key its owner keeps.

Symmetric algorithms are fast and asymmetric ones need fewer keys, so NIST describes a common hybrid: an asymmetric method establishes a symmetric key, and that key then encrypts the data.

Key facts

• Symmetric encryption uses the same key to encrypt and to decrypt.
• For n parties talking pairwise, symmetric encryption needs n(n − 1)⁄2 keys, which is 499,500 for 1,000 parties.
• NIST: symmetric-key algorithms are generally significantly faster than asymmetric-key algorithms.
• AES is a symmetric block cipher, and RSA is a public-key algorithm.

Common traps

• Remembering 'symmetric is weaker' and picking 'slower' or 'more resources', when symmetric is the fast one.
• Assuming symmetric keys suit only data in transit, when storage encryption uses them too.

No directly related past PYQ was found.