At HMC code level 3.2.5 and above, the .ssh subdirectory for an HMC user is owned by root, and difficult to write keys into by the method above. Instead, do the following as aixuser on the aix system:
mykey=`cat $HOME/.ssh/id_rsa.pub`
ssh hscroot@hmc.domain.com mkauthkeys -a \”$mykey\”