Nicholas Merrill famously fought a 11 year legal battle (and finally won) the right to reveal all aspects of a National Security Letter (NSL) served to him. Almost all such letters are accompanied by a complete gag order.
https://www.calyxinstitute.org/news/federal-court-invalidate...
EDITED / CORRECTIONS - Thanks commenters - The battle was won by Nicholas Merrill not Ladar Levison of Lavabit fame as I originally posted.)