No, they are not secure.
The fact that you can hear & understand them tells you they're not secure.
The latest police/fire radio systems use digitized voice for the most part. There are agencies still using analog voice as well. Digital systems can still be analog as well. It's a choice the designers / end users make & can be changed in moments if necessary.
Todays scanners have a built in decoder that allows you to decode the digitized voices based on a couple of technologies.
Without the decoders, you would not hear the words of the communicators. It would likely sound like just opening the squelch with no signal present.
The two most common digital police/fire systems are from Motorola & M-A/COM.
The Motorola system is generally known as a SmartZone or SmartNet system & M-A/COM's tradename is EDACS. There are other digital systems in use but none as prevelent as these two.
You have to tell your scanner which one it's receiving in order to get all the channels & follow the action.
The two are not compatible with each other so a separate type of decoder is required for each.
As to the secure portion; On top of being digitized & unintelligeable just from being digitized, they can add encryption to the mix.
Once the encryption is turned on, you will never hear whats going on without having a proper decoder plus the encryption keys no matter which scanner you use.
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Encryption can be broken, it just depends on how long you are willing to try & the amount of resources you're willing to commit to it.
In police/fire systems, the information is old within a few mins. Why decode after that? Are you ready to spend months (at best) trying to decode something that happened that long ago?
Also, the keys/codes change regularly. If you were lucky enough to decode a several months old transmission, you'll need to spend the next several months doing so again.
The old DES encryption was broken. It was a test sponsored by the Gov. An individual decided to try linking dozens & dozens of computers across the internet to combine their computing power into one large effort.
He was given a secured transmission recording to work from AND a partial page in plain language of that secured transmission to work with.
It took him almost a year to decrypt those few encoded transmissions. That's with the oldest, but not highest form of that encryption at the time.
A Cray supercomputer took over a month as well I understand.
Now, there's at least 3 commonly used encryption forms out there for public safety & government to choose from.
DES (the older), AES the newer & a variant from Motorola which is proprietary to Motorola. Not everyone has this.
Inside those encryption formats are a few layers of encryption strength. Basically you have the encryption strengths for general commercial users, then public safety & the highest strength for US Gov.
Best of luck trying to decrypt any of those.You would literally have to trip & fall on the right keys by accident. If you started from the logical beginning you wouldn't do it within your lifetime using a standard home based computer.