Q:

How to map a string property to a binary column in the database? 

I have a class for a user entity.  One of the properties is the user's password (a hash, actually).  I made it a string (streamlined code):
public class User
{
    public virtual int Id { get; set; }
    public virtual string Password { get; set; }
}

There's also a Fluent NHibernate mapping (streamlined code):
public class UserMap : ClassMap<User>
{
    public UserMap()
    {
        Table("users");
        Id(x => x.Id).GeneratedBy. Sequence("users_id_seq");
        Map(x => x.Password); // what do I put here??? 
    }
}

The database column is of bytea data type on PostgreSQL.  The above mapping doesn't work, because the property is string (text).  What should I do? 

A:

you can Make Password a public property, which is only used to reference to the underlying private property HashedPassword. 
something like so:

protected virtual byte[] /*or whatever the type is*/ HashedPassword {get; set;}
public virtual string Password
  get
  {
     return (string)(HashedPassword); //or however you want to cast it to string...
  }
  set
  //... 
you can then tell fluent nHib to ignore your Password property. 