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!

Posted on February 19, 2013 at 12:52 PM14 Comments


Carlo Graziani February 19, 2013 1:21 PM

This would seem to fall more in the “Social Engineering” category of hack than in “Traffic Analysis”.

Natanael L February 19, 2013 2:23 PM

Using social engineering to conduct traffic analysis via a side channel (I assume that this postmaster normally wouldn’t personally handle every letter, or even close to a majority of them, but now he was led to look at this one). 🙂

Natanael L February 19, 2013 2:27 PM

Oh, and for something entirely different which is a bit off topic – the blog service my URL:s have pointed to is shutting down (bye bye Posterous), so I’m switching the link now.
And that can be considered another kind of security – lack of security in persistent addresses (incidentally, I also recently lost an old phone number I’ve had for a long time).
Time for me to register my own domain soon…

Tim#3 February 20, 2013 3:55 AM

Interesting. Also reminds me of reading Sherlock Holmes and being impressed by how many post deliveries they used to get every day back then.

Dom De Vitto February 20, 2013 7:18 AM

Dumb droids are easy to manipulate.

Want to get rid of a litre of dangerous material, which would cost thousands to dispose of according to regulations ? Just hand it to the TSA and say “oops, way more than 100ml.”
As it’s confiscated, you’re not even legally dumping it….

paul February 20, 2013 10:09 AM

The Three Musketeers books are full of this too: all you need to know is who is talking to whom; what they’re saying is very nearly irrelevant.

Jason February 20, 2013 11:03 AM

Like a lot of fictional cons, this one looks really suspicious if the result is not what the author had planned.

Suppose Mrs. Lecount had not written a letter. Then Captain Wragge comes in with an address correction for a letter that does not exist, and alarm bells go off in the postman’s head.

MarkH February 20, 2013 2:02 PM

@Tim: I’ve read that at the height of the Victorian age, the Royal Mail made twelve (!) deliveries per day. This made it possible to conduct more than one complete exchange of correspondence within a single business day.

paul February 20, 2013 3:06 PM


Maybe. But usually there’s a backup story or three to tell the mark, e.g. must have posted it somewhere else, must have gone out already, oh heavens was it lost on the way. And the mark, having already bought in, is quite possibly going to avoid looking to carefully.

Hmm. Bruce would probably know this: what are the techniques institutions use to get people who have been partly or entirely conned to violate a security rule to report that when they realize they’ve been had, rather than covering it up to avoid embarrassment or punishment?

SJ February 21, 2013 9:44 AM


I seem to recall that the narrator in The Three Musketeers tries to give an explanation of how to set a “Mouse-trap”.

Basically, D’Artagnan sets up an apartment with a spy-hole so that he can watch the visitors to a certain lady. He’s trying to identify some spy/agent-provocateur/person-of-interest.

In that case, traffic-analysis is also more important than eavesdropping.

Jon February 21, 2013 3:54 PM

Interesting, except … the Royal Mail was fully into the business of intercepting mail, opening it, duplicating it, decrypting it as necessary, and passing teh resulting intelligence along to various interested parties, before re-inserting the flawlessly re-sealed mail back into the main stream.

See, for example, Steven E. Maffeo ‘The British National Intelligence Effort’ (Chapter 1) in “Most Secret and Confidential” Naval Institute Press 2000, pp. 19-33.

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