Ingenuous: candid, frank, or open in character or quality; characterized by an inability to mask feelings, not devious.
Disingenuous: the dis- prefix establishes the negative; thus, not candid, not frank, not open in character or quality; insincere.
Note: The meaning of disingenuous has been shifting about lately, as if people were unsure of its proper meaning. Generally, it means "insincere" and often seems to be a synonym of cynical or calculating. Not surprisingly, the word is used often in political contexts, as in It is both insensitive and disingenuous for the White House to describe its aid package and the proposal to eliminate the federal payment as "tough love." This use of the word is accepted by 94 percent of the Usage Panel. Most Panelists also accept the extended meaning relating to less reproachable behavior. Fully 88 percent accept disingenuous with the meaning "playfully insincere, faux-naïf," as in the example "I don't have a clue about late Beethoven!" he said. The remark seemed disingenuous, coming from one of the world's foremost concert pianists. Sometimes disingenuous is used as a synonym for naive, as if the dis- prefix functioned as an intensive (as it does in certain words like disannul) rather than as a negative element. This usage does not find much admiration among Panelists, however. Seventy-five percent do not accept it in the phrase a disingenuous tourist who falls prey to stereotypical con artists.