"It was the practice of the German Secret Service to keep its agents ignorant of one another's identities as far as possible," Crossle said. "In that way, the capture of one did not lead to possible disclosures regarding any of the others. Occasionally, however, that had curious complications."

"I recall one bizarre case involving a man named Sammis. We had strong suspicions that he was the spy referred to in several letters intercepted between other German agents as H-Seven. But despite all our vigilance we were unable to get anything on him. Then one afternoon, he hurriedly left his office and went to his hotel room. One of our agents, stationed in an adjacent room, observed him through a peephole. Sammis paced about for awhile, drew a letter from his pocket which he scrutinized in evident perturbation, then resumed pacing. Finally he got out a suitcase and began packing."

"Our agent started to open the connecting door, but the lock jammed and before he could enter Sammis escaped through the window. It was a sleety winter day and the fire-escape steps were encrusted with ice. Sammis slipped and fell, striking his head against the lower railing. He was taken to the hospital with a fractured skull and died a few hours later. He had previously torn the letter into bits which we pieced together. It was addressed to Sammis and contained routine instructions regarding a shipment of cotton. He was operating as an exporter to South America. But a cipher message inscribed in 'F' secret ink had developed between the lines, apparently by Sammis himself that afternoon at his office. Here it is as your next problem."

The slip Crossle handed his pupil bore the letter groups:

NSRGQ CTCLM SRNES GDVZX ZSNMB D

"The first step of course," Crossle added, "is to make a frequency table to determine whether it's a substitution or a transposition cipher. If the former, then follow the same method of solution you used in the first problem."

Solution