X25519 is a key exchange protocol that allows two parties to securely share a secret over an insecure channel. It is based on elliptic curve cryptography, specifically using the Curve25519 curve, which provides strong security and efficient performance. This makes it suitable for various applications, including secure messaging and encrypted communications. The protocol is designed to be fast and resistant to certain types of attacks, making it a popular choice for modern cryptographic systems. X25519 is widely used in protocols like TLS and SSH, ensuring that data exchanged between users remains confidential and secure.