Hilchos Lashon Hara, chapter 1, section 8, 9
The prohibition of lashon hara applies whether one actually speaks of the person with one’s mouth, or writes it in a letter. Also, it makes no difference if one says the lashon hara explicitly, or if one relates through a hint. All these cases are lashon hara.
Know also, that even if one denigrates oneself while denigrating one’s fellow, it is still considered lashon hara.