Open Whisper Systems is behind Signal and redacted documents showed it received a subpoena from an assistant attorney in Virginia demanding email addresses, history logs, browser cookie data and other information associated with two phone numbers as part of a grand jury probe.
The problem was that the man behind OWS, well-respected security researcher Moxie Marlinspike, could not hand over such information even if he wanted to; Signal does not store such details.
The app provides end-to-end encryption and a court order accompanying the subpoena meant that Marlinspike was not allowed to tell anyone about it for a year.
Moxie Marlinspike at the RSA conference in San Francisco in March this year.
OWS said in a statement: "We've designed the Signal service to minimise the data we retain about Signal users, so the only information we can produce in response to a request like this is the date and time a user registered with Signal and the last date of a user's connectivity to the Signal service.
"Notably, things we don't have stored include anything about a user's contacts (such as the contacts themselves, a hash of the contacts, any other derivative contact information), anything about a user's groups (such as how many groups a user is in, which groups a user is in, the membership lists of a user's groups), or any records of who a user has been communicating with."
The ACLU has now put up a full set of documents associated with the case on its website.
In an accompanying blog post, ACLU staff attorney Brett Max Kaufman wrote: "The First Amendment requires that to close courtrooms or seal evidence — and especially to prohibit a party from speaking publicly on a matter of public concern — the government (must) demonstrate a compelling interest in secrecy, and it must apply that secrecy in the narrowest possible way.
"But instead, the government appears to seek blanket gag orders by default, without considering precisely what information can be disclosed without harm to its interests."
He added: "To its credit, the government quickly agreed with us that most of the information under seal could be publicly disclosed. But the fact that the government didn't put up too much of a fight suggests that secrecy — and not transparency — has become a governmental default when it comes to demands for our electronic information, and critically, not everyone has the resources or the ability to work with the ACLU to challenge it."
Signal is available for both iOS and Android; a desktop client can be used by those who have the app on their smartphones.