1 files changed, 67 insertions, 0 deletions
diff --git a/wsutil/crc5.c b/wsutil/crc5.c
new file mode 100644
index 00000000..b62f26b1
--- /dev/null
+++ b/wsutil/crc5.c
@@ -0,0 +1,67 @@
+/* crc5.c
+ * CRC-5 routine
+ *
+ * 2019 Tomasz Mon <desowin@gmail.com>
+ *
+ * Wireshark - Network traffic analyzer
+ * By Gerald Combs <gerald@wireshark.org>
+ * Copyright 1998 Gerald Combs
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "config.h"
+
+#include <wsutil/crc5.h>
+
+static uint8_t crc5_usb_bits(uint32_t v, int vl, uint8_t ival)
+{
+    /* This function is based on code posted by John Sullivan to Wireshark-dev
+     * mailing list on Jul 21, 2019.
+     *
+     * "One of the properties of LFSRs is that a 1 bit in the input toggles a
+     *  completely predictable set of register bits *at any point in the
+     *  future*. This isn't often useful for most CRC caculations on variable
+     *  sized input, as the cost of working out which those bits are vastly
+     *  outweighs most other methods."
+     *
+     * In USB 2.0, the CRC5 is calculated on either 11 or 19 bits inputs,
+     * and thus this approach is viable.
+     */
+    uint8_t rv = ival;
+    static const uint8_t bvals[19] = {
+        0x1e, 0x15, 0x03, 0x06, 0x0c, 0x18, 0x19, 0x1b,
+        0x1f, 0x17, 0x07, 0x0e, 0x1c, 0x11, 0x0b, 0x16,
+        0x05, 0x0a, 0x14
+    };
+
+    for (int i = 0; i < vl; i++) {
+        if (v & (1 << i)) {
+            rv ^= bvals[19 - vl + i];
+        }
+    }
+    return rv;
+}
+
+uint8_t crc5_usb_11bit_input(uint16_t input)
+{
+    return crc5_usb_bits(input, 11, 0x02);
+}
+
+uint8_t crc5_usb_19bit_input(uint32_t input)
+{
+    return crc5_usb_bits(input, 19, 0x1d);
+}
+
+/*
+ * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
+ *
+ * Local variables:
+ * c-basic-offset: 4
+ * tab-width: 8
+ * indent-tabs-mode: nil
+ * End:
+ *
+ * vi: set shiftwidth=4 tabstop=8 expandtab:
+ * :indentSize=4:tabSize=8:noTabs=true:
+ */