Go to any U.S. Post Office and you'll see a poster that tells USPS employees how to identify a suspicious and potentially dangerous package. Which means if your package looks like the one below, the odds of it being intercepted and inspected increase dramatically.

Those screening guidelines are meant to intercept packages containing poisons and explosive devices. But as the Justice Department continues to indict suspected drug vendors operating on the AlphaBay and Hansa markets, some new, unpublicized screening tactics are coming to light. The Justice Department built its case against Michael Farber–who allegedly sold nearly $7 million worth of illicit drugs (mostly marijuana) on The Silk Road, Pandora and AlphaBay cryptomarkets under the user names purefiremeds and humboldtfarms—using USPS surveillance.

According to an affidavit filed last month in the Eastern District of California, the postal trail started with a raid on a drug lab in Boston, where agents uncovered several USPS Express Mail shipping labels addressed to Farber. All of these labels used the "waiver of signature" option. According to the affidavit's author, Homeland Security Investigation's Special Agent Matthew Larsen, Express Mail and waiver of signature are both drug-dealing giveaways:

Using data from The Silk Road's server, as well as information provided by an unlicensed bitcoin-for-cash exchanger, Larsen's team was able to tie Farber to a defunct dark web account called purefiremeds, which operated on The Silk Road. Scouring reddit threads—where users frequently post public reviews of vendors—led them to believe the person or persons behind purefiremeds was also running humboldtfarms, a vendor on the AlphaBay site that the DOJ shut down in July. So, federal agents set up a controlled buy on AlphaBay, purchasing "7g Cali Orange Krush Grade A++ Weed," translated as a quarter ounce, from humboldtfarms. The package was intercepted by a postal worker in Fresno, where investigators picked it up. As is often the case with dark web vendors, the package had a fake return address.

This is where it gets really interesting. Nothing about the controlled buy package tied it to Farber. But the parcel did contain some unique qualities: Regular stamps.

"The more common method" for shipping a large volume of packages, according to the criminal complaint, would be digitally printed stamps containing tracking information. Whoever operated Humboldtfarms preferred to use the kinds of stamps one might put on a birthday card. So, in March 2017, Larsen's team asked Postal Inspector Lyndon Versoza to have USPS staff at Verdugo Viejo Post Office in Gendale, Calif., alert him if they received a large number of packages with the same two stamps. Lo and behold, just such a delivery arrived a week later:

Over the course of several months, the USPS documented the return addresses used for the packages—all of them either fake addresses or fake businesses at real addresses—and documented who dropped them off using surveillance footage. After verifying that each batch of packages contained at least one or two shipments of marijuana, Versoza had police stop the men who dropped them off so that they could be identified (but not arrested).

All told, several people allegedly working with humboldtfarms bought thousands of dollars worth of the Gardens and Butterfly stamps from nearby USPS facilities in California, paying for them in cash:

Eventually, Larsen's team was able to tie together the alleged actors behind humboldtfarms by matching security footage from post office locations with Farber's friends on social media; they also used a GPS tracker on one suspect's car, looked at IRS records, compared public PGP keys, and turned a bitcoin exchanger into a C.I. Five people are now facing charges in the Eastern District of California, which is a hotbed of dark web prosecutions.

As with street level drug prosecutions, the dark web cases coming to light are generally built using the same tactics: One person in the supply chain gets caught due to an operational security issue and turns confidential informant to lessen their punishment.

This leads investigators to a new group of suspects and new operational security flaws. In the humboldtfarms case, it was stamps and the recycling of a PGP key. But I've seen cases where a vendor tried to trademark his dark web handle, another who chose an easily crackable password, and another who tied his dark web account to his personal, public email account.

The reddit angle is also pretty damning, suggesting the trust system used by dark web customers—naming and shaming vendors—actually helps law enforcement identify people who've switched user handles to evade identification.

All of which is to say whatever tech advantages dark web users had in the early days of The Silk Road are now gone. The Justice Department not only has three captured servers with six years of user and transaction data, it knows how the game is played.