It is known as Britain's listening post, operating secretly to detect threats against national security. But now GCHQ has emerged from the shadows of cyber space to become the first of the country's spy agencies on Twitter.
Its debut post, sent at 11.02am from the account @GCHQ, simply said: "Hello, world."
Twitter may be more than a decade old and firmly embedded in the lives of hundreds of millions, but joining the site represents a departure for an organisation whose ventures on to the public stage have traditionally been rare.
The move is the latest step in a drive towards greater transparency after the service came under intense scrutiny in the wake of revelations by Edward Snowden in 2013.
A GCHQ spokesman said: "We know that some will say we're joining the Twitter party slightly late but we're the first intelligence agency in the UK to do this and it's a big step for the organisation as we become more open about the work we do to keep Britain safe."
Andrew Pike, director of communications at GCHQ, added: "In joining social media, GCHQ can use its own voice to talk directly about the important work we do in keeping Britain safe."
However, the agency's followers should not expect real-time updates on its activities supporting efforts to combat terrorism or cyber crime, or any clues about its techniques.
"Some things have to stay secret - sorry - so we won't be providing intelligence updates or giving away tradecraft but we will be tweeting about our history, mission outcomes, languages, maths, cyber security, technology and innovation, information about GCHQ in general, job opportunities and yes, there will be puzzles!" the spokesman said.
More than 600,000 people entered a Christmas card cryptography challenge issued by GCHQ in December.
The Twitter account - operated by the service's media team - will also be used to highlight events, publications, news blogs and opinion pieces.
GCHQ - which stands for Government Communications Headquarters - has some way to go if it is to catch up with its US equivalent the National Security Agency, which joined Twitter in December 2013 and has more than 200,000 followers.
After about an hour, GCHQ had attracted more than 4,000 followers, while its post was retweeted more than 500 times.
The official James Bond account was among the first to be followed by the agency, as well as more prosaic selections relating to government, law enforcement and the Royal Family.
It was welcomed onto the platform by the US Central Intelligence Agency, which said: " Welcome to Twitter @GCHQ!"
Inevitably, its arrival also drew some teasing. One user wrote: " @GCHQ is now following you' is going to really freak some people out." Another said: " GCHQ joins Twitter - as if they weren't already here."
Others pointed out that Edward Snowden's account was not included in the early flurry of follows, while the development also prompted speculation over whether Britain's two other intelligence agencies - MI5 and MI6 - may follow suit.
GCHQ's first foray on the microblogging site doubles as an introduction and an in-joke for computer programming circles.
The spokesman explained: "For those just starting to program, getting 'Hello, world' to appear on a screen is a gentle introduction to coding because you can see the output of your code represented visually on the screen.
"As a technical organisation with computing at its core, 'Hello, world' resonated with us because this is how many of our staff, who are amongst some of the world's most proficient programmers, started."
Dutch Sim card maker Gemalto was hit by a huge hack by the British and American intelligence agencies, but they were probably unable to steal the encryption keys that they were after, the company has claimed.