Which Tim Jenkins did overcome by using his own systems and own crypto. No backdoored DES, but ultimately secure one-time pads transported via floppies by the stewardess to each party. If they got hold of such a floppy in SA, it would only compromise the messages of this one receiver. Which would be detected sooner or later.
Probably using ANY commercial encryption tech, you're doomed.
But it is naive to generate the key for a one-time pad using a PSEUDO random number generator!
The whole point of the one-time pad is to achieve true security because you can use truly random keys. The kind of key that used here could have been created locally by just exchanging the seed, which would have prevented putting all these stewardesses in danger ;-).
I understand the developer was self-taught (and "learn as you go" on top), so it's understandable, but whoever reads this and has truly important secrets, don't be that silly.
Having said this it is no easy to generate truly random numbers in large quantities. Again, you will need to build yourself your own hardware device, because most likely, all commercially available gear is rigged.
CIA/NSA got a whiff of Operation Vula using off the shelf Philips PX-1000 with build in DES and backdoored the product by bribing Philips https://www.cryptomuseum.com/crypto/philips/px1000/