Ernst Fetterlein

Ernst Constantin Fetterlein (3 April 1873 – June 1944 ) was a Russian cryptographer who later defected to Britain.

Fetterlein was born in St Petersburg, the son of Karl Fedorovich Fetterlein, a German-language tutor, and Olga Fetterlein, née Meier. holding the rank of "General-Admiral," an honorary title in Tsarist Russia. During World War I, he was known for a time as Ernst Popov as his German-derived name could have drawn unwanted attention. Amongst others, he solved German, Austrian and British codes. He was thought well of by his colleagues, one of whom wrote, "He was a brilliant cryptographer. On book cipher and anything else where insight was vital he was quite the best. He was a fine linguist and would usually get an answer no matter the language."