In 1939, the Japanese introduced a new cipher machine for their most sensitive diplomatic traffic, replacing an earlier system that SIS referred to as "RED." The new cipher, which SIS called "PURPLE", was different and much more difficult. The Navy's cryptological unit (OP-20-G) and the SIS thought it might be related to earlier Japanese cipher machines, and agreed that SIS would handle the attack on the system. After several months trying to discover underlying patterns in PURPLE ciphertexts, an SIS team led by Friedman and Rowlett, in an extraordinary achievement, figured it out. PURPLE, unlike the German Enigma or the Hebern design, did not use rotors but stepper switches like those in automated telephone exchanges. Leo Rosen of SIS built a machine using—as was later discovered—the identical model of switch that the Japanese designer had chosen.
in 1939, tHE JapanesE iNtroduCED a new cipHEr MaCHiNe FOR theiR moST sEnsitive DiPlomatic traffic, REpLAcinG an eaRLier system that sIs RefeRRed tO as "red." the new cipher, wHich sis cALlEd "pURpLE", wAs differeNt and mucH more dIFficult. tHe naVY's cryPtologicAL uNiT (op-20-g) aNd the sis tHought it mIgHt be related To earLieR JAPaNese CIpHer macHinEs, anD agREeD thAt SiS would haNDle ThE AtTack On tHE SySTEm. after SEverAL MoNths TrYiNg TO diScOver unDErlying PaTTERns In purple cipherTExtS, aN sis tEAm Led By friEDmAn and rOWlETt, in an exTraOrdiNary AChIevemEnt, fiGUrEd iT oUt. PurPLe, uNlIKe the geRmaN eNIgmA or tHe hebErn dEsIgN, did NOt usE rOtORs but stePPer switches likE those IN AUTomateD telepHONe ExchAnges. leo roSen of sis bUiLt a mACHine USIng—As waS later dIscoverEd—thE identical model of SwItcH tHAt the jaPaNesE deSigner haD chosen.