Purpose and origins Colossus computer

the lorenz sz machines had 12 wheels, each different number of cams (or pins ).





cams on wheels 9 , 10 showing raised (active) , lowered (inactive) positions. active cam reversed value of bit (0→1 , 1→0).

the colossus computers used decipher intercepted radio teleprinter messages had been encrypted using unknown device. intelligence information revealed germans called wireless teleprinter transmission systems sägefisch (sawfish). led british call encrypted german teleprinter traffic fish , , unknown machine , intercepted messages tunny (tunafish).

before germans increased security of operating procedures, british cryptanalysts diagnosed how unseen machine functioned , built imitation of called british tunny .

it deduced machine had twelve wheels , used vernam ciphering technique on message characters in standard 5-bit ita2 telegraph code. did combining plaintext characters stream of key characters using xor boolean function produce ciphertext.

in august 1941, blunder german operators led transmission of 2 versions of same message identical machine settings. these intercepted , worked on @ bletchley park. first, john tiltman, talented gc&cs cryptanalyst, derived key stream of 4000 characters. bill tutte, newly arrived member of research section, used key stream work out logical structure of lorenz machine. deduced twelve wheels consisted of 2 groups of five, named χ (chi) , ψ (psi) wheels, remaining 2 called μ (mu) or motor wheels. chi wheels stepped regularly each letter encrypted, while psi wheels stepped irregularly, under control of motor wheels.

with sufficiently random key stream, vernam cipher removes natural language property of plaintext message of having uneven frequency distribution of different characters, produce uniform distribution in ciphertext. tunny machine did well. however, cryptanalysts worked out examining frequency distribution of character-to-character changes in ciphertext, instead of plain characters, there departure uniformity provided way system. achieved differencing in each bit or character xor-ed successor. after germany surrendered, allied forces captured tunny machine , discovered electromechanical lorenz sz (schlüsselzusatzgerät, cipher attachment) in-line cipher machine.

a lorenz sz40 machine on display @ national cryptologic museum, fort meade, maryland, usa.

in order decrypt transmitted messages, 2 tasks had performed. first wheel breaking , discovery of cam patterns wheels. these patterns set on lorenz machine , used fixed period of time succession of different messages. each transmission, contained more 1 message, enciphered different start position of wheels. alan turing invented method of wheel-breaking became known turingery. turing s technique further developed rectangling , colossus produce tables manual analysis. colossi 2, 4, 6, 7 , 9 had gadget aid process.

the second task wheel setting , worked out start positions of wheels particular message, , attempted once cam patterns known. task colossus designed. discover start position of chi wheels message, colossus compared 2 character streams, counting statistics evaluation of programmable boolean functions. 2 streams ciphertext, read @ high speed paper tape, , key stream, generated internally, in simulation of unknown german machine. after succession of different colossus runs discover chi-wheel settings, checked examining frequency distribution of characters in processed ciphertext. colossus produced these frequency counts.