Smithsonian Magazine - - Secrets of American History -

To get a hint of the mon­strously dif­fi­cult task un­der­taken by the Venona code break­ers, con­sider all the elab­o­rate steps that Soviet agents took to en­crypt a se­cret mes­sage. Here’s how it worked, as ex­plained by John Earl Haynes and Har­vey Klehr in their book Venona: De­cod­ing Soviet

Es­pi­onage in Amer­ica. The mes­sage—“Pi­lot de­liv­ered re­port about rock­ets”—is hy­po­thet­i­cal, but it makes ref­er­ence to an ac­tual Amer­i­can spy, Wil­liam Ull­mann, an Army of­fi­cer as­signed to the Pen­tagon, whom the So­vi­ets code-named “Pi­lot.” A U.S.-based Soviet agent might send this mes­sage to Moscow alert­ing su­pe­ri­ors to check the diplo­matic pouch for a dis­patch from Pi­lot.

1. An agent hands the text to a cipher clerk, who uses a code book to con­vert the words to four-digit num­bers:

2. The clerk shifts one digit to the first group from the se­cond, two dig­its to the se­cond group from the third, and so on, yield­ing:

3. Now the clerk con­sults a unique “one-time pad.” Each page bears 60 five-digit num­bers and is sup­posed to be used just once. At the up­per-left cor­ner is a num­ber—26473, in this case—which is in­serted be­fore the first group in the se­ries:

That first num­ber will alert the re­cip­i­ent, who has the same one-time pad, which page of the pad to con­sult.

4. Then the clerk takes the next four five-digit groups from the one-time pad . . .

. . . and adds them to the four groups that make up the mes­sage, us­ing non-car­ry­ing arith­metic. (For in­stance, 8 + 6 = 4, not 14, be­cause noth­ing is car­ried):

5. Next the clerk con­verts the nu­mer­i­cal groups to let­ter groups, us­ing the for­mula:

6. The clerk ap­pends an­other five-let­ter group (cor­re­spond­ing to the next num­ber from the one-time pad) to sig­nal the end of the mes­sage. Fi­nally, he adds a five-digit num­ber, which gives the mes­sage a se­rial num­ber and in­di­cates the date on which it was en­ci­phered. He sends this se­ries of six five-let­ter words and one five-digit num­ber to Moscow . . .

7. . . . where an­other clerk de­ci­phers it, re­vers­ing these steps. Now imag­ine the Venona team try­ing to break the code with­out the ben­e­fit of cap­tured Soviet code books or one-time pads.

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