// upper_skipjack.go 负责 NoPack upper C2S body 的 8 字节块解保护。
// 这里只还原客户端 wire 字节，不写角色规则、仓储或回包逻辑。
package dnfbridge

import (
	"crypto/aes"
	"errors"
)

var errUpperSkipjackBody = errors.New("invalid upper skipjack body")
var errUpperAESBody = errors.New("invalid upper aes body")

var upperSkipjackKey5 = [10]byte{0x31, 0x94, 0x53, 0x94, 0xc9, 0xf0, 0x1a, 0xf1, 0xda, 0x83}
var upperAESKey4 = [16]byte{0x34, 0x4b, 0x65, 0x6c, 0x69, 0x37, 0x69, 0x47, 0x47, 0x56, 0x69, 0x45, 0x76, 0x36, 0x36, 0x45}

var upperSkipjackF = [256]byte{
	0xa3, 0xd7, 0x09, 0x83, 0xf8, 0x48, 0xf6, 0xf4, 0xb3, 0x21, 0x15, 0x78, 0x99, 0xb1, 0xaf, 0xf9,
	0xe7, 0x2d, 0x4d, 0x8a, 0xce, 0x4c, 0xca, 0x2e, 0x52, 0x95, 0xd9, 0x1e, 0x4e, 0x38, 0x44, 0x28,
	0x0a, 0xdf, 0x02, 0xa0, 0x17, 0xf1, 0x60, 0x68, 0x12, 0xb7, 0x7a, 0xc3, 0xe9, 0xfa, 0x3d, 0x53,
	0x96, 0x84, 0x6b, 0xba, 0xf2, 0x63, 0x9a, 0x19, 0x7c, 0xae, 0xe5, 0xf5, 0xf7, 0x16, 0x6a, 0xa2,
	0x39, 0xb6, 0x7b, 0x0f, 0xc1, 0x93, 0x81, 0x1b, 0xee, 0xb4, 0x1a, 0xea, 0xd0, 0x91, 0x2f, 0xb8,
	0x55, 0xb9, 0xda, 0x85, 0x3f, 0x41, 0xbf, 0xe0, 0x5a, 0x58, 0x80, 0x5f, 0x66, 0x0b, 0xd8, 0x90,
	0x35, 0xd5, 0xc0, 0xa7, 0x33, 0x06, 0x65, 0x69, 0x45, 0x00, 0x94, 0x56, 0x6d, 0x98, 0x9b, 0x76,
	0x97, 0xfc, 0xb2, 0xc2, 0xb0, 0xfe, 0xdb, 0x20, 0xe1, 0xeb, 0xd6, 0xe4, 0xdd, 0x47, 0x4a, 0x1d,
	0x42, 0xed, 0x9e, 0x6e, 0x49, 0x3c, 0xcd, 0x43, 0x27, 0xd2, 0x07, 0xd4, 0xde, 0xc7, 0x67, 0x18,
	0x89, 0xcb, 0x30, 0x1f, 0x8d, 0xc6, 0x8f, 0xaa, 0xc8, 0x74, 0xdc, 0xc9, 0x5d, 0x5c, 0x31, 0xa4,
	0x70, 0x88, 0x61, 0x2c, 0x9f, 0x0d, 0x2b, 0x87, 0x50, 0x82, 0x54, 0x64, 0x26, 0x7d, 0x03, 0x40,
	0x34, 0x4b, 0x1c, 0x73, 0xd1, 0xc4, 0xfd, 0x3b, 0xcc, 0xfb, 0x7f, 0xab, 0xe6, 0x3e, 0x5b, 0xa5,
	0xad, 0x04, 0x23, 0x9c, 0x14, 0x51, 0x22, 0xf0, 0x29, 0x79, 0x71, 0x7e, 0xff, 0x8c, 0x0e, 0xe2,
	0x0c, 0xef, 0xbc, 0x72, 0x75, 0x6f, 0x37, 0xa1, 0xec, 0xd3, 0x8e, 0x62, 0x8b, 0x86, 0x10, 0xe8,
	0x08, 0x77, 0x11, 0xbe, 0x92, 0x4f, 0x24, 0xc5, 0x32, 0x36, 0x9d, 0xcf, 0xf3, 0xa6, 0xbb, 0xac,
	0x5e, 0x6c, 0xa9, 0x13, 0x57, 0x25, 0xb5, 0xe3, 0xbd, 0xa8, 0x3a, 0x01, 0x05, 0x59, 0x2a, 0x46,
}

func decodeUpperKey5(body []byte) ([]byte, error) {
	if len(body) == 0 || len(body)%8 != 0 {
		return nil, errUpperSkipjackBody
	}
	out := make([]byte, len(body))
	for offset := 0; offset < len(body); offset += 8 {
		upperSkipjackDec(out[offset:offset+8], body[offset:offset+8], upperSkipjackKey5)
	}
	return out, nil
}

func decodeUpperKey4(body []byte) ([]byte, error) {
	if len(body) == 0 || len(body)%aes.BlockSize != 0 {
		return nil, errUpperAESBody
	}
	block, err := aes.NewCipher(upperAESKey4[:])
	if err != nil {
		return nil, err
	}
	out := make([]byte, len(body))
	// Evidence: runtime MCP shows current NoPack msg4 uses opcode%14=4.
	// For the observed one-block request, bytes 0..3 decode to the 0-based slot.
	for offset := 0; offset < len(body); offset += aes.BlockSize {
		block.Decrypt(out[offset:offset+aes.BlockSize], body[offset:offset+aes.BlockSize])
	}
	return out, nil
}

func upperSkipjackG(word uint16, round int, key [10]byte) uint16 {
	keyOffset := (4 * (round - 1)) % len(key)
	g1 := byte(word >> 8)
	g2 := byte(word)
	g3 := upperSkipjackF[g2^key[keyOffset]] ^ g1
	g4 := upperSkipjackF[g3^key[(keyOffset+1)%len(key)]] ^ g2
	g5 := upperSkipjackF[g4^key[(keyOffset+2)%len(key)]] ^ g3
	g6 := upperSkipjackF[g5^key[(keyOffset+3)%len(key)]] ^ g4
	return uint16(g5)<<8 | uint16(g6)
}

func upperSkipjackGInv(word uint16, round int, key [10]byte) uint16 {
	keyOffset := (4 * (round - 1)) % len(key)
	g5 := byte(word >> 8)
	g6 := byte(word)
	g4 := upperSkipjackF[g5^key[(keyOffset+3)%len(key)]] ^ g6
	g3 := upperSkipjackF[g4^key[(keyOffset+2)%len(key)]] ^ g5
	g2 := upperSkipjackF[g3^key[(keyOffset+1)%len(key)]] ^ g4
	g1 := upperSkipjackF[g2^key[keyOffset]] ^ g3
	return uint16(g1)<<8 | uint16(g2)
}

func upperSkipjackDec(dst, src []byte, key [10]byte) {
	w1 := uint16(src[0])<<8 | uint16(src[1])
	w2 := uint16(src[2])<<8 | uint16(src[3])
	w3 := uint16(src[4])<<8 | uint16(src[5])
	w4 := uint16(src[6])<<8 | uint16(src[7])
	for round := 32; round >= 1; round-- {
		if upperSkipjackRuleA(round) {
			n1, n2, n3, n4 := w1, w2, w3, w4
			old1 := upperSkipjackGInv(n2, round, key)
			old2 := n3
			old3 := n4
			old4 := n1 ^ n2 ^ uint16(round)
			w1, w2, w3, w4 = old1, old2, old3, old4
			continue
		}
		n1, n2, n3, n4 := w1, w2, w3, w4
		old1 := upperSkipjackGInv(n2, round, key)
		old2 := n3 ^ old1 ^ uint16(round)
		old3 := n4
		old4 := n1
		w1, w2, w3, w4 = old1, old2, old3, old4
	}
	dst[0], dst[1] = byte(w1>>8), byte(w1)
	dst[2], dst[3] = byte(w2>>8), byte(w2)
	dst[4], dst[5] = byte(w3>>8), byte(w3)
	dst[6], dst[7] = byte(w4>>8), byte(w4)
}

func upperSkipjackRuleA(round int) bool {
	return round <= 8 || (round >= 17 && round <= 24)
}