A developer is looking for a solution that will provide confidentiality and check its integrity and authenticity for encrypted plaintext. Which solution should the developer use?

The solution that best meets the developer's requirement for providing confidentiality while also ensuring the integrity and authenticity of encrypted plaintext is Authenticated Encryption with Associated Data (AEAD).

AEAD is designed specifically for scenarios where both confidentiality and integrity of the data are essential. It leverages encryption to protect the plaintext from unauthorized access, while simultaneously incorporating a mechanism that generates a message authentication code (MAC). This MAC serves to verify that the data has not been altered in transit and confirms the identity of the sender. As a result, AEAD effectively guarantees the confidentiality of the data, and its integrity and authenticity through cryptographic methods.

In contrast, while Perfect Forward Secrecy (PFS) provides an additional layer of security for session keys, it does not address the integrity and authenticity of the encrypted data itself. Similarly, Elliptic Curve Cryptography (ECC) focuses on key generation and does not inherently provide secure transmission of data by verifying its integrity or authenticity. Finally, the Authentication Header (AH) protocol provides integrity and authenticity but does not ensure confidentiality, as it does not encrypt the payload.

Thus, AEAD is distinctly suitable for the requirement of simultaneously protecting confidentiality while confirming integrity and authenticity, making it the correct choice.