London (CNN) To mark the centenary of British intelligence agency GCHQ, Queen Elizabeth II visited the original London headquarters to unveil a coded plaque. On a tour of Watergate House, where the agency was first established in 1919, the Queen also met former wartime codebreakers.

GCHQ is perhaps best known for its operations during World War II, when it relocated to Bletchley Park in Buckinghamshire. There, code-breakers including Alan Turing famously decrypted coded German messages sent using the Enigma cipher machine.

The centenary plaque at Watergate House contains a coded message.

The plaque unveiled by the Queen appears fairly conventional -- before you notice the series of lines and dots under selected letters. Admittedly, it's not the hardest code to crack: combining the marked letters reveals the message, "1 HUNDRED YEARS."

At Watergate House, the Queen met Ruth Bourne, a Bletchley Park codebreaker who worked with famed Alan Turing. In 2018, Bourne was awarded the Legion d'Honneur, France's highest order of merit.

The monarch also met the winners of GCHQ's CyberFirst Girls competition, a contest aimed at introducing high-school girls to cyber security, as well as and the grandchildren of GCHQ's first head, Alastair Denniston.

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