Add FIDO U2F protocol as a Scapy contrib layer

[Copilot]

Implements the FIDO U2F (Universal 2nd Factor) protocol stack as scapy/contrib/u2f.py, covering both the HID transport framing and the application-layer APDU messages.

HID Transport (U2FHID)

• U2FHIDInitializationPacket — CID(4) | CMD(1) | BCNT(2) | payload; auto-computes bcnt on build, strips HID frame zero-padding on dissect, dispatches payload class via guess_payload_class based on cmd and bcnt
• U2FHIDContinuationPacket — CID(4) | SEQ(1) | payload for multi-packet messages
• U2FHIDINITRequest / U2FHIDINITResponse — INIT command nonce exchange + channel allocation with capability flags (WINK, CBOR, NMSG)
• U2FHIDError — single-byte error payload

APDU Layer

• U2FCommandAPDU — ISO 7816-4 extended-length encoding: CLA | INS | P1 | P2 | 0x00 | Lc(2) | data | Le(2); lc auto-computed from data
• U2FResponseAPDU — data(var) | SW1 | SW2; uses a custom field that peels the trailing 2-byte status word before handing remaining bytes to SW1/SW2 fields

Application Messages

• U2FRegisterRequest — challenge_parameter(32) | application_parameter(32)
• U2FRegisterResponse — reserved | user_public_key(65) | key_handle_length | key_handle; DER attestation cert + signature remain as Raw payload
• U2FAuthenticateRequest — challenge_parameter(32) | application_parameter(32) | key_handle_length | key_handle
• U2FAuthenticateResponse — user_presence | counter(4); ECDSA signature as Raw payload

Usage

from scapy.contrib.u2f import *

# Build a U2F register transaction over HID MSG
challenge = bytes(range(32))
app_id    = bytes(range(32, 64))
apdu_data = raw(U2FRegisterRequest(challenge_parameter=challenge,
                                   application_parameter=app_id))
pkt = (U2FHIDInitializationPacket(cid=0xFFFFFFFF, cmd=0x83)
       / U2FCommandAPDU(ins=0x01, p1=0x03, data=apdu_data))

# Dissect
hid = U2FHIDInitializationPacket(raw(pkt))
req = U2FRegisterRequest(hid[U2FCommandAPDU].data)

# Parse a response APDU
resp = U2FResponseAPDU(b'U2F_V2\x90\x00')
assert resp.data == b'U2F_V2' and resp.sw1 == 0x90

Layer binding U2FHIDInitializationPacket(cmd=0x83) → U2FCommandAPDU is registered via bind_layers.