If I can compile audited code from source myself, without any backdoors, then I can be reasonably assured there aren't any backdoors (excluding perhaps hardware level backdoors--but that's why we do the encryption in software).
Implementing hardware backdoors that are opaque to end users is theoretically possible, but more difficult in practice. You could, for example, build a screen/monitor that just captures everything on the screen and forwards it to some other entity, but in practice it's not so easy because of bandwidth limitations, etc. I suppose it would be much easier to create a physical keyboard that phones home over a mobile network, although it would only give you half the conversation.
Implementing hardware backdoors that are opaque to end users is theoretically possible, but more difficult in practice. You could, for example, build a screen/monitor that just captures everything on the screen and forwards it to some other entity, but in practice it's not so easy because of bandwidth limitations, etc. I suppose it would be much easier to create a physical keyboard that phones home over a mobile network, although it would only give you half the conversation.
*edit: added the word "audited".