SSH (Secure Shell), per RFC 4251, uses asymmetric cryptography (e.g., RSA, ECDSA) for secure authentication:

Key Pair:

Public Key:Freely shareable, used to encrypt or verify.

Private Key:Secret, used to decrypt or sign.

Process:

User generates a key pair (e.g., ssh-keygen -t rsa -b 4096).

Public Keyis uploaded to the server, appended to ~/.ssh/authorized_keys (e.g., via ssh-copy-id).

Private Key(e.g., ~/.ssh/id_rsa) stays on the user’s machine.

Authentication: Client signs a challenge with the private key; server verifies it with the public key.

Technical Details:

Protocol: SSH-2 (RFC 4253) uses a Diffie-Hellman key exchange, then public-key auth.

Files: authorized_keys (server, 0644 perms), private key (client, 0600 perms).

Security: Private key exposure compromises all systems trusting the public key.

Security Implications:CNSP likely stresses key management (e.g., passphrases, rotation) and server-side authorized_keys hardening (e.g., PermitRootLogin no).

Why other options are incorrect:

B:Uploading the private key reverses the model, breaking security—anyone with the server’s copy could authenticate as the user. Asymmetric crypto relies on the private key remaining secret.

Real-World Context:GitHub uses SSH public keys for repository access, with private keys on user devices.References:CNSP Official Documentation (SSH Security); RFC 4253 (SSH Authentication Protocol).