If its new plaster then put a mist coat of matt emulsion on to seal it. You then have a choice of paints that you could use. For durability why not try an oil based paint, satin or eggshell finish. This will with stand the moisture and steam more. You could use water based satin, or eggshell, vinyl matt,silk or softsheen aswell. If applied properly they should be able to withstand the moisture. Always make sure you have a vent in the bathroom to take away the bulk of the steam first.