There's a very amusing epilogue in Steven Levy's "Crypto" [0] about the fact that either RSA or Diffie-Hellman (I've forgotten which) was in fact known to the intelligence services for 10+ years prior to its re-discovery among public researchers. These are people who are very good at keeping secrets.

[0] https://www.amazon.com/Crypto-Rebels-Government-Privacy-Digi...