I was seeking an alternative and cheaper OpenPGP solution to Yubikeys, then I found that the OpenPGP card is essentially a Java applet lives on a chip runs JVM, and JVM runs on top of JavaCard OS. Since all the programs follows GlobalPlatform standards, communication with Java Cards can be straightforward.
In the end, it's not difficult to burn opensource openPGP applet to your own card. But there are 2 problems:
1. Bulk sales. If you want to all the things by yourself, and you found an ideal chip (recent NXP SmartMX2 cards has all the fancy stuff you want), almost every reseller only allow bulk purchases, say 100 pcs minimum.
2. Propriety software. For NXP cards, you need a propriety software to initialize/unlock a card before you can use GlobalPlatform tools to flash your own Applets. A reseller told me that his can be done by sending raw HEX code with a Transport Key to workaround, but I'm not sure about it.
In the end, it's not difficult to burn opensource openPGP applet to your own card. But there are 2 problems:
1. Bulk sales. If you want to all the things by yourself, and you found an ideal chip (recent NXP SmartMX2 cards has all the fancy stuff you want), almost every reseller only allow bulk purchases, say 100 pcs minimum.
2. Propriety software. For NXP cards, you need a propriety software to initialize/unlock a card before you can use GlobalPlatform tools to flash your own Applets. A reseller told me that his can be done by sending raw HEX code with a Transport Key to workaround, but I'm not sure about it.