there are 3 types of encryption( Ciphers ) used by ssh idea, 3des & blowfish.

unless explicitly mentioned ssh by default uses idea cipher to encrypt & decrypt.

Bruce Schneier’s block cipher blowfish was designed to be fast & secure, it uses a 128 bit key, although the algorithm allows anything from 32 to 448 bits.

We can explicitly mention the ssh client to use a certain type of encryption by using the following command

# slogin -2 -c blowfish user@some_remote_machine.com

the above command also explicitly mentions the ssh client to use ssh protocol version 2

The protocol version being used depends on the version enabled on the ssh server(sshd) you are connecting to. Older versions of the ssh daemon(sshd) dont support version2 of the ssh protocol.

# ssh -c blowfish user@some_remote_machine.com

the above command also does the same thing.