General: applet-specific SW and ISO7816

Question

Hi all!
I'm newbie to Java Card, so apologies if question is stupid.
1) In ISO 7816-4 a number of smart card return codes/ instruction bytes, еtс. are defined. When I write an applet, should I use them or define my own?
For example, the applet should perform external authentication.
If I want to write my code properly, should I write
public class MyApplet extends Applet{
private static final byte INS_STANDART_ISO_EXT_AUTH = (byte)0x82;
private static final short SW_STANDART_ISO_EXT_AUTH_FAIL = (short)0x6983;
} or the better way is
public class MyApplet extends Applet{
private static final byte INS_MY_OWN_EXT_AUTH = (byte)0x10;
private static final short SW_MY_OWN_EXT_AUTH_FAIL = (short)0x9983;
2) If you need some applet specific error codes, do you use the range of 0x9XXX or 0x6XXX values?
[url http://javacard.vetilles.com/2006/09/26/status-words-in-iso7816/]This guy writes that I should use 0x9XXX values. Zhiqun Chen in his book uses 0x6XXX for wallet applet.

Answer 1

Unless you seek to be compatible with existing software / spec, do what you like. 7XXX are unusual but also possible, depending on the card and protocol (better luck with contactless and T=1). SIM cards abuse the 9xxx range.
In practice, you'll be able to map quite all possible errors to one of the pre-existing SW values. Most important of them are (from memory):
6700 (bad len),
6982 (bad security)
6985 (forbidden),
6A80 (bad data)
6A82 (app/file not found),
6A88 (object not found),
6B00 (bad P1/P2), - 6A86 is also possible
6D00 (no instruction)
6E00 (bad class).
ISO7816-4 compatibility should be nice if you used CLA=00 and INS defined in ISO7816-4, ie everyone "expects" an instruction 00A4 to be select file/app and 00B2 to be "read record", but that's not mandatory.
If you want to be clean, use the "proprietary" class, CLA=80h, and any instruction you like.