Got nerd-sniped, here's code to "unguard" (invert) the key schedule algorithm of SM4

sm4_unguard.py

#!/usr/bin/env python
# encoding: utf-8

from z3 import *
import struct
import os

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

FK = (
    0xa3b1bac6, 0x56aa3350, 0x677d9197, 0xb27022dc,
)

CK = (
    0x00070e15, 0x1c232a31, 0x383f464d, 0x545b6269,
    0x70777e85, 0x8c939aa1, 0xa8afb6bd, 0xc4cbd2d9,
    0xe0e7eef5, 0xfc030a11, 0x181f262d, 0x343b4249,
    0x50575e65, 0x6c737a81, 0x888f969d, 0xa4abb2b9,
    0xc0c7ced5, 0xdce3eaf1, 0xf8ff060d, 0x141b2229,
    0x30373e45, 0x4c535a61, 0x686f767d, 0x848b9299,
    0xa0a7aeb5, 0xbcc3cad1, 0xd8dfe6ed, 0xf4fb0209,
    0x10171e25, 0x2c333a41, 0x484f565d, 0x646b7279,
)
# fmt: on


def rotate_left(i32, k):
    return i32 << k & 0xFFFFFFFF | i32 >> 32 - k


def linear_substitution_1(i32):
    return i32 ^ rotate_left(i32, 13) ^ rotate_left(i32, 23)


def tau(i32):
    return (
        S[i32 >> 24] << 24
        | S[i32 >> 16 & 0xFF] << 16
        | S[i32 >> 8 & 0xFF] << 8
        | S[i32 & 0xFF]
    )


def derive_keys(key):
    k0, k1, k2, k3 = struct.unpack(">IIII", key)
    k0 ^= FK[0]
    k1 ^= FK[1]
    k2 ^= FK[2]
    k3 ^= FK[3]

    for n, ck in enumerate(CK):
        k0, k1, k2, k3 = k1, k2, k3, k0 ^ linear_substitution_1(tau(k1 ^ k2 ^ k3 ^ ck))
        yield k3


def guard_key(key):
    if isinstance(key, bytearray):
        key = bytes(key)

    assert isinstance(key, bytes), "The key should be `bytes` or `bytearray`"
    assert len(key) == 16, "The key should be of length 16"

    return tuple(derive_keys(key))


def invert_linear_substitution_1(output):
    i32 = BitVec("i32", 32)

    formula = i32 ^ RotateLeft(i32, 13) ^ RotateLeft(i32, 23) == output

    solver = Solver()
    solver.add(formula)

    if solver.check() == sat:
        return solver.model()[i32].as_long()


def untau(i32):
    a, b, c, d = i32 >> 24 & 0xFF, i32 >> 16 & 0xFF, i32 >> 8 & 0xFF, i32 & 0xFF
    return S.index(a) << 24 | S.index(b) << 16 | S.index(c) << 8 | S.index(d)


def unguard_key(keys, n=4):
    keys = list(keys)
    a, b, c, d = keys[n - 4 : n]

    for i in range(n):
        a, b, c, d = linear_substitution_1(tau(a ^ b ^ c ^ CK[n - i - 1])) ^ d, a, b, c

    return struct.pack(">IIII", a ^ FK[0], b ^ FK[1], c ^ FK[2], d ^ FK[3])


key = os.urandom(16)

assert unguard_key(guard_key(key)) == key