19th-Century Traffic Analysis
There’s a nice example of traffic analysis in the book No Name, by Wilkie Collins (1862). The attacker, Captain Wragge, needs to know whether a letter has been placed in the mail. He knows who it will have been addressed to if it has been mailed, and with that information, is able to convince the postmaster to tell him that it has, in fact, been mailed:
If she had gone to the admiral’s, no choice would be left him but to follow the coach, to catch the train by which she traveled, and to outstrip her afterward on the drive from the station in Essex to St. Crux. If, on the contrary, she had been contented with writing to her master, it would only be necessary to devise measures for intercepting the letter. The captain decided on going to the post-office, in the first place. Assuming that the housekeeper had written, she would not have left the letter at the mercy of the servant—she would have seen it safely in the letter-box before leaving Aldborough.
“Good-morning,” said the captain, cheerfully addressing the postmaster. “I am Mr. Bygrave of North Shingles. I think you have a letter in the box, addressed to Mr.—?”
The postmaster was a short man, and consequently a man with a proper idea of his own importance. He solemnly checked Captain Wragge in full career.
“When a letter is once posted, sir,” he said, “nobody out of the office has any business with it until it reaches its address.”
The captain was not a man to be daunted, even by a postmaster. A bright idea struck him. He took out his pocketbook, in which Admiral Bartram’s address was written, and returned to the charge.
“Suppose a letter has been wrongly directed by mistake?” he began. “And suppose the writer wants to correct the error after the letter is put into the box?”
“When a letter is once posted, sir,” reiterated the impenetrable local authority, “nobody out of the office touches it on any pretense whatever.”
“Granted, with all my heart,” persisted the captain. “I don’t want to touch it—I only want to explain myself. A lady has posted a letter here, addressed to ‘Noel Vanstone, Esq., Admiral Bartram’s, St. Crux-in-the-Marsh, Essex.’ She wrote in a great hurry, and she is not quite certain whether she added the name of the post-town, ‘Ossory.’ It is of the last importance that the delivery of the letter should not be delayed. What is to hinder your facilitating the post-office work, and obliging a lady, by adding the name of the post-town (if it happens to be left out), with your own hand? I put it to you as a zealous officer, what possible objection can there be to granting my request?”
The postmaster was compelled to acknowledge that there could be no objection, provided nothing but a necessary line was added to the address, provided nobody touched the letter but himself, and provided the precious time of the post-office was not suffered to run to waste. As there happened to be nothing particular to do at that moment, he would readily oblige the lady at Mr. Bygrave’s request.
Captain Wragge watched the postmaster’s hands, as they sorted the letters in the box, with breathless eagerness. Was the letter there? Would the hands of the zealous public servant suddenly stop? Yes! They stopped, and picked out a letter from the rest.
“‘Noel Vanstone, Esquire,’ did you say?” asked the postmaster, keeping the letter in his own hand.
“‘Noel Vanstone, Esquire,'” replied the captain, “‘Admiral Bartram’s, St. Crux-in-the-Marsh.'”
“Ossory, Essex,” chimed in the postmaster, throwing the letter back into the box. “The lady has made no mistake, sir. The address is quite right.”
Nothing but a timely consideration of the heavy debt he owed to appearances prevented Captain Wragge from throwing his tall white hat up in the air as soon as he found the street once more. All further doubt was now at an end. Mrs. Lecount had written to her master—therefore Mrs. Lecount was on her way to Zurich!