Understanding SSH Key Pairs

SSH key authentication relies on public-key cryptography, utilizing a mathematically related pair of keys: a private key that remains secret and a public key that can be freely shared. When a user attempts to connect, the SSH server uses the public key to create a challenge that only the corresponding private key can solve. This asymmetric encryption approach provides several advantages over passwords: keys are nearly impossible to brute-force, immune to shoulder-surfing, and can be centrally managed and revoked.

The strength of SSH key authentication lies in the mathematical relationship between the key pair. Modern SSH implementations support several key algorithms, each offering different security characteristics. RSA keys, the traditional standard, remain widely supported but require larger key sizes for adequate security. Ed25519 keys, based on elliptic curve cryptography, provide excellent security with smaller key sizes and faster performance. ECDSA keys offer a middle ground, though they've faced scrutiny due to potential weaknesses in their random number generation.