I've always felt the oft-repeated assertion "even at the end of World War 2, the Germans believed that the Enigma crypto system had theoretical weaknesses but remained unbroken in practice" sounded too good to be true. This was based on a number of concerns:
1) if they genuinely believed Enigma to be secure, which have another mechanical cipher system (Lorenz/Fish) for "high grade" traffic?
2) why have a continual process of refinement, both of procedures and hardware (e.g. adding additional plugs to the stecker), to improve security throughout the war if the base system was believed to be secure and unbroken?
3) from the start, why would the navy use a much stronger 4-rotor Enigma and better procedures on security grounds if the base 3-rotor system used by the Wehrmacht & Luftwaffe was generally considered to be adequately secure?
I'm pleased to see that an analyst at no lesser an authority than the NSA seems to agree with me, in this declassified paper I stumbled over recently http://www.nsa.gov/public_info/_files/tech_journals/Der_Fall_Wicher.pdf