Cryptography

Shared-key encryption

• c = Enc(key, msg) ----(c)---- msg = Dec(key, c)

Shared-key signature

• m,a = h(key, m) ----(m,a)---- m,a =? h(key, m) 

Public-key encryption

• c = Enc(P-recv, m) ----(c)---- m=Dec(S-recv,m)

Public-key signature

• m,a = f(S-send, m) ----(m,a)---- m,a =? g(P-send,m)

Certificate is basically a message signatured by a Certificate Authority (CA)

• m,a = f(S-CA, m) ----(m,a)---- m,a =? g(P-CA, m)

SSH see a good summary at http://aperiodic.net/phil/ssh/

• Host sends its public key to client for authentication
• Key authentication
• Client generate public-private key pair
• public key is sent to Host
• Client doesn't need password anymore (it needs to input passphrase for accessing the private key though)
• ssl-agent is used to remember the passphrase so that client can log into host without being asked anything
• Sequence for Key authentication
• Client connect to Server
• Server returns a 'challenge'
• Client create an 'authenticator' from the challenge and its private key, and send it back to Server
• Server check the 'authenticator' using the Client's public key
• SSH-Transport layer uses Key Exchange algorithm
• One example is here: http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange

Jargon of terms: http://identityaccessman.blogspot.com/