The SSH (Secure Shell) protocol, developed by Tatu Ylonen in 1995 at the Helsinki University of Technology, was created to replace insecure remote access protocols like Telnet, rsh, and rlogin that transmitted data, including passwords, in plaintext. SSH provides encrypted communication channels over unsecured networks, and its key-based authentication mechanism eliminates the security vulnerabilities inherent in password-based login. The protocol has become the standard for remote server administration, secure file transfer, and Git repository access.

Asymmetric cryptography forms the foundation of SSH key authentication. A key pair consists of a private key (kept secret on the client machine) and a mathematically related public key (placed on servers in the ~/.ssh/authorized_keys file). During authentication, the server sends a random challenge encrypted with the user's public key. Only the holder of the corresponding private key can decrypt this challenge and prove their identity. This mechanism is immune to password brute-force attacks, credential stuffing, and keylogging because the private key never leaves the client machine and is never transmitted over the network.

Ed25519, introduced in OpenSSH 6.5 (2014), uses elliptic curve cryptography based on Curve25519, designed by Daniel J. Bernstein. Ed25519 keys are only 256 bits (32 bytes) yet provide security equivalent to approximately 3000-bit RSA keys. The algorithm is designed to be resistant to side-channel attacks, produces deterministic signatures (eliminating the catastrophic failure mode where a compromised random number generator leaks the private key), and is significantly faster than RSA for both key generation and signing operations. RSA keys, while older and more widely compatible, require much larger key sizes (2048-4096 bits) to achieve comparable security and are slower in both generation and authentication operations.

The key exchange process during SSH connection establishment uses a separate mechanism from user authentication. The Diffie-Hellman key exchange (or its elliptic curve variant, ECDH) allows client and server to derive a shared secret over an insecure channel without any prior shared keys. This shared secret is used to derive symmetric session keys for encrypting the connection. The server's host key, verified against the client's known_hosts file, authenticates the server's identity and prevents man-in-the-middle attacks. When connecting to a server for the first time, the client displays the server's key fingerprint (a SHA-256 hash of the public key) for manual verification, which is why managing and verifying key fingerprints is an important operational security practice.