What is the recommended data field size in a Java Card APDU ? From Zhiqun Chen's Java Card Technology for Smart Cards: Architecture and Programmer's Guide book, it mentions that Le field allows a max of 255.
Are we to interpret it as follow for the APDU Command:
|<----------------------- 255 Bytes total ------------------------>|
|<- CLA -><- INS -><- P1 -><- P2 -><- Lc -><---- DATA ----><- Le ->|
Thus, if CLA, INS, P1, P2, Lc, Le are all 1 bytes each, we should assume that the we can safely only set 249 bytes into the DATA region ?
For the APDU Response are we to interpret:
|<----------------------- 258 Bytes total ------------------------>|
|<-------------------------- DATA ------------------------><- SW ->|
The response data can safely be set to 256 bytes with 2 bytes of SW and total of a response consisting of data response and SW is 258 bytes ?
What other considerations to safely send and receive data in chunks considering we have to face situations where chaining might not be possible and we have to manually handle data flow ourselves ?
Lc and Le byte can signalize to hold/request upto 0xFF bytes. So for a Case 4 command APDU you have 6(header+lc+le) + 0xFF = 261 bytes. For a maxiumum response you have 256 bytes + 2(Statusword) = 258 bytes. This is what the standard would suggest. However, diffrent hardware tokens might have different implementations so this might not be 100% accurate. If you need more data you need to implement ExtendedLength.