the public/private keypairs are only used to negotiate a session key, the actual encryption is done by a symmetric cipher.
The SSH2 spec lists the following ciphers:
3des-cbc REQUIRED blowfish-cbc OPTIONAL twofish256-cbc OPTIONAL twofish-cbc OPTIONAL twofish192-cbc OPTIONAL twofish128-cbc OPTIONAL aes256-cbc OPTIONAL aes192-cbc OPTIONAL aes128-cbc RECOMMENDED serpent256-cbc OPTIONAL serpent192-cbc OPTIONAL serpent128-cbc OPTIONAL arcfour OPTIONAL idea-cbc OPTIONAL cast128-cbc OPTIONA none OPTIONAL
It does appear the spec allows for no encryption at all but you would have to manually specify this, by default ssh is encrypted by one of the ciphers supported on both ends.
[URL="http://www.ietf.org/rfc/rfc4253.txt"] Here[/URL] is the rest of the spec for the transport layer which is where the encryption occurs.