Trusting the cert does not, precisely, allow decryption by the certificate authority. It rather gives the certificate authority the ability to issue certificates for domains, which if they are used to establish a connection, can encrypt and decrypt traffic for that connection.

So yes, if you trust the DoD root certificate, then the DoD as well as every certificate authority in the world could in theory generate a valid certificate impersonating www.google.com or any website. With a sophisticated enough attack, they could do this just for your one visit to one particular website, in such a way that it would be difficult for anyone to realize that it's happening. However, though this is difficult to notice if you're not looking for it, it's actually really easy to notice if you are looking for it. If you use Chrome, then Chrome will report the certificates that it sees back to Google, who track what certs are issued by CAs. This is how Google noticed that Symantec issued fake certificates for Google domains in Symantec's test environment: https://googleonlinesecurity.blogspot.com/2015/10/sustaining...

Anyway, the practical risk of trusting a DoD certificate is pretty low. To decrypt your traffic, they'd have to man-in-the-middle intercept your connection to a web server, and replace the site's valid certificate with their own, which would leave an obvious and flagrant trail to anyone who is looking. This would very obviously "play their hand" and anyone with evidence of being attacked that way by a first world government would be immediate worldwide news in the security community. If they did this even once, they'd need to be extremely careful not to be caught by any of the countermeasures that detect this kind of surveillance.

That kind of attack would be a one time thing, because evidence of being attacked through the DoD cert would cause all browser vendors and OSes to yank support for it. CAs have been revoked for far less justified reasons than explicitly attacking someone.

I find it much, much, much more likely that targets of interest will simply be attacked and exploited through regular known security mechanisms - such as software vulnerabilities or built-in back-doors. These things don't leave an obvious trail and smoking gun pointing back to the perpetrator. Someone MITMing your website visits with the DoD root certificate would stir up a shitstorm. "Some anonymous IP broke into my computer with a 0-day and installed a rootkit" is not particularly newsworthy by comparison. Even the recent news of backdoors in networking product codebases is, while newsworthy, not really that surprising these days. Active evidence of DoD interception of someone's network traffic followed by evidence of CA certificate misuse would drop like a nuclear bomb in the security community, especially if for no extremely well justified reason. It would be the proverbial straw that broke the camel's back in terms of government interference with Internet security, and would lead to a digital revolt even more severe than what Snowden's disclosures caused. Government technical experts will be aware of this and will use other methods, at least in any context where it could be plausibly noticed.

To be clear, I completely believe that the government is or could passively conduct surveillance on virtually all electronic communications. I just don't think they'll go as far as actively intercepting and modifying a connection, and inserting a fake certificate, within the borders of the country or in any normal circumstance. Maybe they would do such a thing within the private networks of North Korea, but I'd be highly skeptical of them doing it within the US, and with the DoD certificate of all things. It would be too obvious and has too poor of a risk/reward payoff compared to other methods. If they were going to do this, they'd steal the private key from another CA and use it instead. Because that capability could be noticed and "burned", they'd save it for high value targets only.

So, in all practical analysis, I think it is extremely unlikely that the DoD will attack people through their root certificate, though I concede that it's plausible. I would be interested in feedback from others about this reasoning.