More like that that agent, if they were involved in this 90-year-old crypto action, may later have been involved in something else (a mole in the Russian nuclear program, say) in such a way that their exposure as an American agent would compromise the result of that other thing as well (in my hypothetical example, maybe they'd embedded a remote kill switch in all Russian nuclear ICBMs, which would still be there today).

Or they just had a protege, another American agent. Think starting a 'dynasty'.

That 91 year old also likely had family, colleagues and other acquaintances who are still alive today. People at some distance from the 91 year old would be okay, but closer would be under suspicion, could have their lives disrupted, be deported etc.

Exactly and in the UK military (or military like structure), it's easy to get in if you have relatives in.

Also, just because the originator may be dead, it doesn't mean the work won't have knock on effects.

And finding a hole in security for old material may trigger a search through all newer ciphers - something I'm sure they don't want.

There are plenty of reasons, correct or not, why they may wish to keep this a secret.

Hypothetical timeline: An agent born in 1904 tainted a Soviet cipher in 1926. That agent then recruited other assets before retiring in 1973. Unlikely? Sure. Outlandish? Nah.

If the methods were still in use, they might be, even if the agents were not. For example, that secret ink they give as an example might still be in use.

That said, there's plenty of reason to believe that some of the things they're still withholding are nonsense.

The statement is that that person could have also subverted other methods currently in use, not that that particular person would be impacted.

Methods from the pre-computer era, from well before the second world war, still in use nearly 100 years later? Is that really plausible?

Scenario: In 1925 an agent is deployed to Russia, infiltrates an intelligence agency, and does something cryptologically noteworthy. It is mentioned in this 'top secret umbra' document. In 1955, said agent, while still acting covertly in Russia, makes further cryptologic "contributions" to Russian intelligence agencies. In 1975 the ciphers from 1955 are still in use, and the USA intercepts information and identifies Soviet spies. In 1995, the USA feeds misleading information to those formerly Soviet spies. In 2016, the USA still cares about the misinformation provided in 1995 to spies identified in 1975 thanks to ciphers from 1955 which were broken by a long-dead agent who started work in 1925.

Seems plausible enough to me.

Now that's a great scenario! There's actually convoluted stuff like that in the non-fiction. They're also still trying to misdirect people on stuff decades old to justify current positions.

Plus it causes the Russians to continue to view others from the time period with possible suspicion. They can't go "oh, that happened 75 years ago and the Anericans have said nothing about it so we must be able to trust that guy and his work"

No the agent that did something 90 years ago, was still there and did something 60 years ago. Which influenced something 30 years ago, which led to something 5 years ago and is still in use.

It's a stretch, but there could be a chain of events that they want to remain hidden. They don't want to hide what happened 90 years ago, they want to hide who did it and what else he may have done in the future or had an effect on.

Okay I can buy that type of scenario. Something along the lines of cperciva's sibling comment. This could be such a case. But I still think it's a stretch. I wonder if they ever redact random things just to distract.

Yep one time pads are still in use and those where invented way before ww1

yes because you want your Intelligence officers to be able to say to agents they are recruiting today that we will look after you