- Revealing how the backdoor works would show methods the US agencies are also using.
- There are multiple backdoors, or there might be, and they don't want to tip their hand.
d. Admitting how they found the backdoor would reveal other issues (a source in the pipeline, or their own backdoor in something Huawei made use of).
And probably e-z too.
TR;DL Trump being president has not a thing to do with whether or not Huawei and the PRC are attempting to infiltrate the network and data infrastructure of the world at large.
A) They did show 'proof' ostensibly, to the specific agencies of interest i.e. UK/Germany.
B) Your 'a' and 'b' are missing the most important point, in that the revelation of their knowledge may compromise some ongoing intelligence program. The information may very well have been provided by spies who'd be put at risk.
Literally yesterday WaPo published a story on the most comprehensive US intelligence program ever and it quite nicely illustrated the challenges of how to reveal how much you know, and the consequences therein.
This is not 'open source land' where we like complete transparency, it's a different game.
C) There's also another issue, and that this may not be 'black and white'. For example, the ostensible 'back door' may not be clear cut: it may be that it doesn't yet exist, but there are elements in the design prepared for backdoors to be very easily added at a later date.
Could you (or anyone) link a source where the NSA, or other USA agencies discuss this with technical detail?
If they haven't done it, it means: a) there isn't any backdoor, or b) exposing their backdoors would expose their own backdoors.
I'm betting on b.