Batboard

This occurred to me while reading the reply to my other post about Transcrypt scramblers.

If a radio has an OTAR module (DES, DES-XL, DVP etc... whatever..) how does the otar signal differentiate between the intended units and other radios not needing keyed?

Is there an ID or other code in the software that causes the module to "listen" and be rekeyed when receiving the otar signal meant only for it or its group?

I guess what I'm basically asking is what keeps some guy with a radio scanning the PD or Feds or etc.... from getting the key when the signal goes out?


mancow

If I remember right...the radio is loaded with a "shadow key" which is just like any other key, however this shadow key lets the OTAR command to be sent and rekeyed at the radio.

I think.

Wouldn't the OTAR command be encrypted anyways?

I may be way off on this but, I thought it did it by the IDs. Since most of the times I have seen otar used, you sent a request to a some sort of key management, whether a person on a computer or a computer program. Then they would send a low speed handshake, confirming the proper radio, then a low speed handshake back confirming radio again, then the keys are sent.

I have a book on how the whole process works somewhere, but it's late and I just got in from work, so I'll try to find it in the morning.

Like I said, I may be way off, since I'm going from memory on it.

> I may be way off on this but, I thought it did it by the IDs. Since
> most of the times I have seen otar used, you sent a request to a
> some sort of key management, whether a person on a computer
> or a computer program. Then they would send a low speed
> handshake, confirming the proper radio

I've wondered about this myself. The method described above would be a huge security flaw, since labtools or other methods of changing radio ID's would allow unauthorized radios to receive updated keys....

The OTAR packets themselves are encrypted.

There is what is called a KEK (key encryption key) that is loaded into the radio as a part of the keyset. The OTAR packets are encrypted with the KEK, so that only radios with the approprate KEK are able to decode the OTAR message.

The idea is that if "the bad guys" are able to crack the TEKs (traffic encryption keys) due to being able to sniff enough data off the air to break the key, they WON'T be able to break the KEK that way, since the only time the KEK is used is during the OTAR operation. And since you can rev the KEK as a part of the OTAR operation, you can insure that the KEK doesn't get used enough times to allow an attacker to crack it.

True, if one of your key fill devices which contains the KEK is stolen, you are SOL, but the idea is that the key fill devices are locked in a VERY controlled facility, and since you can rekey radios without them coming back in, the access to the KFD can be much more tightly controlled.

Also, as part of the OTAR system, you can rekey a set of radios by ID, but since the radios have to have the KEK, and the system doing the rekeying also has to have the KEK, you can prevent somebody from rekeying the radio over the air without proper authorization.

We've been asked if we are going to support OTAR, and we've said "Not at this time". Not because we cannot - it would actually be pretty simple, but because if we could generate a rekey, the possibities of a tech accidentally rekeying the wrong radios, or of our box being used by "the bad guys" to cause trouble would be too large. We already make too many people nervous with what we can do now!

Wowbagger,

Awesome description of OTAR!!!


cipher