broken US/UK naval ciphers

I've read dozens of the books on Ultra, etc., and I remember in one of
them, there was an account of one of the UK or US codebreaking
organizations discovering that the Germans were reading some of the
critical naval ciphers protecting convoy movement information.
However, said code breaking organization had a difficult time of
convincing the Royal and/or US Navy that their ciphers were indeed
weak and needed replacing.  My flakey memory is such that this might
have been either WWI or WWII, so it might have been a Room 40 story
rather than a Bletchley Park story.

Unfortunately, I can't remember which book had this story, and several
Google searches have turned up plenty of information about the
B-Dienst breaking the naval ciphers during WWII, and of Bletchley
discovering this, but none of the Google searches have turned up the
story about how hard it was to convince the Navy.  I've checked through
the books that I have easy access to, and I've failed to turn it up, yet
I'm sure I read it at one point (not recently).

Does anyone recall this story and where it was printed?  Or whether my
memory is just flakey or whether the story was later discredited?

I have information on the Black code and the US military attache in
Cairo.  This is a different story that was specifically about convoy
ciphers (and/or code books, of course).

I'm hoping to cite this in a paper that I'm writing, so, assuming that
my memory is correct, would anyone have a good citation to where this
was published?

Thanks in advance to anyone who could help on this!

       - Paul