If you believe you are the only one who can break the cipher, then it doesn't really matter if your allies are using them - after all, spying happens even among

If you believe you are the only one who can break the cipher, then it doesn't really matter if your allies are using them - after all, spying happens even among ostensibly allied or friendly countries. I think most people's source of proof is the Snowden leaks, but I haven't actually read it or corroborated, and most backdoors should be deniable anyway - it'd be real dumb if they weren't. I think strong circumstantial evidence is really the only thing one can go on.