<?php error_reporting(0); ini_set('display_errors'...

python
# Let's implement the CRC functions matching the PHP code.
def compute_crc16(data: bytes):
    checksum = 0xFFFF
    for b in data:
        checksum ^= b
        for _ in range(8):
            if checksum & 1:
                checksum = ((checksum >> 1) ^ 0xA001) & 0xFFFF
            else:
                checksum = (checksum >> 1) & 0xFFFF
    return checksum

# CRC8 table from PHP
crc8_table = [
    0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
    0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
    0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
    0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
    0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
    0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
    0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
    0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
    0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
    0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
    0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
    0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
    0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
    0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
    0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
    0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
    0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
    0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
    0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
    0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
    0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
    0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
    0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
    0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
    0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
    0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
    0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
    0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
    0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
    0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
    0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
    0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3
]
def calculate_crc8(input_bytes: bytes):
    crc = 0
    for b in input_bytes:
        crc = crc8_table[(crc ^ b) & 0xff]
    return crc & 0xff

secret = b"Enj0yNSSCTF4th!"
len(secret), compute_crc16(secret), calculate_crc8(secret)

(15, 17262, 163)

python
hex(compute_crc16(secret)), hex(calculate_crc8(secret))

('0x436e', '0xa3')

python
import os, random
def test_linear():
    for _ in range(1000):
        delta = bytearray(15)
        # Set random few bits
        for i in range(3):
            pos = random.randrange(15)
            delta[pos] ^= 1 << random.randrange(8)
        d = bytes(delta)
        lhs = compute_crc16(bytes([a ^ b for a,b in zip(secret, d)])) ^ compute_crc16(secret)
        rhs = compute_crc16(d)
        if lhs != rhs:
            return False, (lhs, rhs, d)
    return True, None
test_linear()

(False,
 (24885, 56949, b'\x00\x00\x00\x00@\x00\x02\x00\x00\x00\x02\x00\x00\x00\x00'))

python
def test_linear2():
    c16_zero = compute_crc16(b'')
    c8_zero = calculate_crc8(b'')
    for _ in range(1000):
        delta = bytearray(15)
        for i in range(3):
            pos = random.randrange(15)
            delta[pos] ^= 1 << random.randrange(8)
        d = bytes(delta)
        lhs = compute_crc16(bytes([a ^ b for a,b in zip(secret, d)])) ^ compute_crc16(secret)
        rhs = compute_crc16(d) ^ c16_zero
        if lhs != rhs:
            return False, (lhs, rhs, d)
    return True, None
test_linear2()

(False,
 (5925, 22426, b'\x00\x00\x00@\x00\x80\x00\x00\x02\x00\x00\x00\x00\x00\x00'))

python
def test_linearity_general():
    c16_zero = compute_crc16(b'')
    for L in [1,2,5,15]:
        for _ in range(200):
            m = bytes([random.randrange(256) for _ in range(L)])
            d = bytes([random.randrange(256) for _ in range(L)])
            lhs = compute_crc16(bytes([a ^ b for a,b in zip(m,d)]))
            rhs = compute_crc16(m) ^ compute_crc16(d) ^ c16_zero
            if lhs != rhs:
                return False, (L, m, d, lhs, rhs)
    return True, None
test_linearity_general()

(False, (1, b'9', b'\xdb', 2367, 46719))

python
import time

target_crc16 = compute_crc16(secret)
target_crc8 = calculate_crc8(secret)
prefix = secret[:12]  # keep first 12 same
# Starting values after processing first 12 bytes:
def process_state_crc16(state, b):
    state ^= b
    for _ in range(8):
        if state & 1:
            state = ((state >> 1) ^ 0xA001) & 0xFFFF
        else:
            state = (state >> 1) & 0xFFFF
    return state

def process_state_crc8(state, b):
    return crc8_table[(state ^ b) & 0xff]

# compute initial states after prefix
state16 = 0xFFFF
state8 = 0
for b in prefix:
    state16 = process_state_crc16(state16, b)
    state8 = process_state_crc8(state8, b)

state16, hex(state16), state8, hex(state8)

(33106, '0x8152', 220, '0xdc')

python
# Build inverse table mapping for CRC8 step: for each current state s (0..255), and desired next_state t (0..255),
# find b s.t. crc8_next = table[s ^ b] == t => b = s ^ table_inv[t], but table_inv is not a simple inverse due to not s^?
# Actually we need b such that table[s ^ b] = t -> s ^ b must be index k s.t. table[k] = t -> k = table_inv[t], then b = s ^ k.
# So if we precompute table_inv (mapping from t to k such that table[k] = t), since table is a permutation? Let's check uniqueness.
# We'll compute inverse mapping; ensure it's bijection.
table = crc8_table
# Verify it's permutation:
assert len(set(table)) == 256
table_inv = [0]*256
for i, val in enumerate(table):
    table_inv[val] = i

def b15_for_crc8(current_state8, target8):
    k = table_inv[target8]
    b = current_state8 ^ k
    return b & 0xff

# quick test: for random state8 and target, compute b, apply process_state_crc8 and see equals target
for s in [0,1,123,220,255]:
    for t in [0,1,5,100,200,255]:
        b = b15_for_crc8(s,t)
        if process_state_crc8(s, b) != t:
            print("fail", s,t,b)
            raise SystemExit
print("crc8 inverse ok")

python
target16 = compute_crc16(secret)
target8 = calculate_crc8(secret)
orig_tail = secret[12:]  # b'th!'
count=0
solution=None
start=time.time()
for b13 in range(256):
    s16_1 = process_state_crc16(state16, b13)
    s8_1  = process_state_crc8(state8, b13)
    for b14 in range(256):
        s16_2 = process_state_crc16(s16_1, b14)
        s8_2  = process_state_crc8(s8_1, b14)
        b15 = b15_for_crc8(s8_2, target8)
        s16_3 = process_state_crc16(s16_2, b15)
        if s16_3 == target16:
            tail = bytes([b13,b14,b15])
            if tail != orig_tail:
                solution = tail
                break
    if solution:
        break
end=time.time()
solution, end-start

(b'e\xa8-', 0.11774992942810059)

python
new = prefix + bytes([0x65,0xA8,0x2D])  # e, 0xA8, -
new, len(new), compute_crc16(new), calculate_crc8(new), compute_crc16(secret), calculate_crc8(secret), new==secret

(b'Enj0yNSSCTF4e\xa8-', 15, 17262, 163, 17262, 163, False)

python
import string
printable = set(range(0x20,0x7f))  # space to ~
solutions=[]
for b13 in printable:
    s16_1 = process_state_crc16(state16, b13)
    s8_1  = process_state_crc8(state8, b13)
    for b14 in printable:
        s16_2 = process_state_crc16(s16_1, b14)
        s8_2  = process_state_crc8(s8_1, b14)
        b15 = b15_for_crc8(s8_2, target8)
        if b15 not in printable:
            continue
        s16_3 = process_state_crc16(s16_2, b15)
        if s16_3 == target16:
            tail = bytes([b13,b14,b15])
            if tail != orig_tail:
                solutions.append(tail)
                break
    if solutions:
        break
solutions[:3], len(solutions)

([b'{(%'], 1)

python
new2 = prefix + b'{(%'
len(new2), new2, compute_crc16(new2), calculate_crc8(new2), new2==secret

(15, b'Enj0yNSSCTF4{(%', 17262, 163, False)