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I'm looking at how MITM works on USB devices. This website states that:

Bulk transfers provide error correction in the form of a CRC16 field on the data payload and error detection/re-transmission mechanisms ensuring data is transmitted and received without error.

Presumably, if I wanted to modify some data (MITM) on a bulk transfer, I'd need to fix up the CRC16 as well.

Wireshark can be used to dump USB packets. Below is a captured USB "bulk in" packet.enter image description here

The packet data is provided here:

"\x40\x52\x59\x18\x53\x9d\xff\xff\x43\x03\x81\x06\x01\x00\x2d\x00" \
"\xe7\x2f\xa0\x62\x00\x00\x00\x00\x20\x2e\x0c\x00\x00\x00\x00\x00" \
"\xb7\x00\x00\x00\xb7\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" \
"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00" \
"\x7f\x00\x04\x84\x00\x00\x0f\x80\x06\x00\x60\x00\x00\x30\x00\x00" \
"\x00\x00\x00\x00\x86\x21\xf2\x0e\x20\x1c\x00\x00\x37\xb8\x00\x01" \
"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" \
"\x00\x00\x00\x00\x00\x00\x00\x00\x40\x00\x00\x00\xff\xff\xff\xff" \
"\xff\xff\x64\x52\x99\x4f\x8a\xf0\xff\xff\xff\xff\xff\xff\x60\x00" \
"\x00\x07\x47\x72\x69\x66\x66\x65\x79\x01\x04\x02\x04\x0b\x16\x32" \
"\x08\x0c\x12\x18\x24\x30\x48\x60\x6c\x2d\x1a\x0c\x11\x18\xff\x00" \
"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" \
"\x00\x00\x00\x00\x00\x03\x01\x03\xdd\x09\x00\x10\x18\x02\x00\x00" \
"\x04\x00\x00\xdd\x1e\x00\x90\x4c\x33\x0c\x11\x18\xff\x00\x00\x00" \
"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" \
"\x00\x00\x00\x73\x99\xcc\x0b"

The Wireshark dissector doesn't point out a checksum. This leads me to believe the checksum is at the end of the "Leftover Capture Data". I can't seem to regenerate the last two bytes as a matching CRC16.

Am I wrong in trying to generate the checksum from leftover_data[:-2]?

Is the checksum present in the capture?

Is the checksum in another packet?

Do Wireshark USB captures even record the checksum?

1 Answer 1

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You are apparently using a software capture, usbmon rather than a hardware capture such as OpenVizla, so the CRC is not going to be visible to you. See the USB Analysis 101 video by Tomasz Moń for more details.

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