IOCTL_SMARTCARD_TRANSMIT transfer size.

NTDEV
1

Hi all,
I have an upper filter driver for Smart card devices. The documentation for IOCTL_SMARTCARD_TRANSMIT seem to indicate that both the in and out data would be of variable size.
https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/winsmcrd/ni-winsmcrd-ioctl_smartcard_transmit

However, IOCTL_SMARTCARD_TRANSMIT request from host (presumably from the smart card service) has a variable size input buffer but the out buffer size is always set to 272 bytes. Since the transfer would be of APDU format, I would expect the max size to be 258 bytes (short APDU: 256 data + 2 bytes of status). Wondering why is this always set to 272? Why not 512? What am I missing? To be clear, I have a completion routine installed which shows varying output data sizes as expected. Thanks.

2

Right, short APDU command is limited to 260 bytes while short APDU response is limited to 258 bytes. However, both input and output buffers include the SCARD_IO_REQUEST header, that is, extra 8 bytes. I don’t know why SCardSvr service output buffer is exactly 272 bytes but 258 would have been clearly insufficient.

Regards,

Ilya Faenson
Rockville, MD USA