Question

In what type of attack does the attacker have the ciphertext of several messages that were encrypted with the same encryption algorithm, but has no access to the plaintext so he or she must try to calculate the key used to encrypt the data

Answer 1

In ciphertext-only.

EXPLANATION: it is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts. The attack is completely successful if the corresponding plaintexts can be extracted or, even better, the key.