RC4 / ADP in APCO25 was added in 4.50
It's unbelievable, but Kenwood changed their mind after many years (see previous publication Missed ARC4 / ADP in APCO25) and finally decided to add RC4 (Enhanced Encryption; aka P25 EE aka ADP in Motorola world) in P25 mode to the NX-5000 product line. ARC4 was implemented in DMR a few years ago, but it was a separate feature especially for DMR mode (its P/N is KWD-5500EE and has nothing common with APCO25).
And recently P25 EE appeared in 4.50 firmware, was tested by our team, and we can confirm that it works just fine! We tested it for interoperability with Motorola XTS and APX radios and with TAIT Tx9400/Tx9600. Also this feature must work with EFJohnson and L3Harris P25 radios.
Keys are entered via KPG-D1 and doesn't require any special software or hardware. But NX-5000 requires radio feature known as KWD-5107ADP. And you cannot run P25 EE without it.
KWD-5107ADP radio feature can be obtained for free if you're both huge agency and direct Kenwood user. You send them form request and they send you JKEFR files in 2-4 weeks. If you're not direct Kenwood customer or located outside of North America, you have to buy KWD-5107ADP radio feature for example off eBay.
BTW, KPG-D1 v.4.50 have some but related to EE in P25 which was resolved in KPG-D1 v.4.51. So if you are ADP user, upgrade both software and firmware to version 4.51. All fresh software and firmware can be downloaded off Files section.
Kenwood! You're becoming better and better. You're slow as an Estonian turtle, but it's better to release extremely desired feature after 5 years than never!
Please keep developing and improving greatest radio platform you ever developed!