What does the term "symmetric" refer to in symmetric cryptography?

In symmetric cryptography, the term "symmetric" specifically refers to the identical nature of the keys used for both encryption and decryption processes. This means that the same key is used to encrypt the plaintext into ciphertext and to decrypt the ciphertext back into plaintext. The strength and security of symmetric cryptography rely significantly on keeping this key secret; if someone obtains the key, they can easily decrypt any data that has been encrypted using it.

The other choices refer to different concepts not specific to the definition of "symmetric" in this context. The symmetry of data structure does not relate directly to the encryption processes. While it is true that symmetric algorithms often have similar performance characteristics for encryption and decryption, stating that they take equal time does not define what symmetry means in cryptography. Similarly, mentioning the balance of security and speed does not capture the essence of the term as it encompasses various other aspects beyond just the identity of the keys. Hence, the focus on the identical nature of the keys is what makes this answer accurate and relevant within the field of cryptography.