In fact, the British code-breakers at Bletchley Park found the extra security of the keyword procedure to be simply of "nuisance value". "U-570"s crew had indeed destroyed their Enigma machine and code-books but the Germans were unaware of the Royal Navy's earlier capture of 's secret material, thanks to which the British had been breaking German naval cyphers since June 1941. British code-breaking was not seriously impeded until February 1942, when a new naval Enigma cypher remained unbroken for 10 months—the so-called "Shark Blackout".