"The Reader of Gentlemen's Mail", by David Kahn

[John Young]

Kahn's is a quite interesting and entertaining book. Among other 
tales about Yardley and his admirable battles with the USG, Kahn 
tells how through hilarious Gonzales-grade legal shenanigans 
the only time a US law has been by enacted against revealing 
cryptological information, in 1933, to prevent Yardley from 
publishing a book, and the one-man-law it is still in effect.

Chapter 15 A Law Aimed at Yardley, pp. 158-72:

The law:

An Act For the Protection of Government records

Be it enacted by the Senate and House of Representatives of
the United States of America in Congress assembled, That
whoever, by virtue of his employment by the United States, shall
obtain from another or shall have custody of or acess to, or shall
have had custody of or access to, any official diplomatic code or any
matter prepared in such code, or which purports to have been
prepared in any such code, and shall willfully, without authorization
or competent authority, publish or furnish to another any such
code or matter, or any matter which was obtained while in the
process of transmission between any foreign government and
its diplomatic mission in the United States, shall be fined not more
than $10,000 or imprisoned not more than ten years, or both.

Approved June 10, 1933 Franklin D. Roosevelt

See: USC Title 18 Section 952


http://assembler.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00000952--
--000-.html

Note the orignal $10,000 amount for the fine has been removed.