Matt Blaze’s blog posts about a cryptologic and espionage mystery that he’d been puzzling over for about 15 years, with light shed by the new book Compromised. (Compromised is primarily a memoir of former FBI counterintelligence agent Peter Strzok)

Strzok discusses his work in Boston investigating the famous Russian “illegals” espionage network from 2000 until their arrest (and subsequent exchange with Russia) in 2010. “Illegals” are foreign agents operating abroad under false identities and without official or diplomatic cover. In this case, ten Russian illegals were living and working in the US under false Canadian and American identities. (The case inspired the recent TV series The Americans.)

For at least the last sixty years, encrypted shortwave radio transmissions have been a standard method for sending messages to covert spies abroad.

And this is where the mystery I’ve been wondering about comes in. In 2007, I noticed an odd anomaly: some messages completely lacked the digit 9 (“nueve”). Most messages had, as they always did and as you’d expect with OTP ciphertext, a uniform distribution of the digits 0-9. But other messages, at random times, suddenly had no 9s at all. I wasn’t the only (or the first) person to notice this; apparently the 9s started disappearing from messages some time around 2005.

This is, to say the least, very odd. The way OTPs work should produce a uniform distribution of all ten digits in the ciphertext. The odds of an entire message lacking 9s (or any other digit) are infinitesimal. And yet such messages were plainly being transmitted, and fairly often at that. In fact, in the recording of the 2008 transmission linked to above, you will notice that while the second and third messages use all ten digits, the first is completely devoid of 9s.

Did the lack of 9s lead to the rollup of the network? See the full article here.