networking - Confusion regarding SSH Security after Authentication -

i have been reading ssh , how uses public key crytography authenticate client. have understood concepts have doubt:

quoting archlinux wiki page:

"when ssh server has public key on file , sees requesting connection, uses public key construct , send challenge. challenge coded message , must met appropriate response before server grant access. makes coded message particularly secure can understood private key. while public key can used encrypt message, cannot used decrypt same message. you, holder of private key, able correctly understand challenge , produce correct response."

after authentication happens , server gives me access, how further messages encrypted? of commands run on server, how ensure response of of commands indeed valid/genuine?

short version: during key exchange phase symmetric cipher chosen , new symmetric key generated. communications after point encrypted and, due properties of (good) key exchange protocol, session key known particular client , server, meaning nobody else can read or modify traffic.

from high level point of view same scheme used in ssl/tls , other similar protocols: 2 parties start asymmetric crypto (slow), use key exchange protocol generate new symmetric key (sometimes called session key) , actual communications encrypted using symmetric crypto (fast).

rfc 4253 , related rfc's contain detailed explanation of ssh protocol.