리버싱을 처음 공부할 때 풀어보면 좋은 시리즈로, 툴 사용법과 리버싱 감각(?)을 익히는 데 도움이 된다.

rev-basic-0

디컴파일하면 다음과 같이 메인함수가 나온다.

메인 함수에서 sub_140001000 함수의 반환값에 따라 출력이 달라지는 것을 알 수 있으며, 우리는 해당 함수가 참이 되는 입력값을 찾아야한다는 것을 다시 한번 확인할 수 있다.

sub_140001000에서 간단히 strcmp 를 이용해서 문자열과 비교를 하고 그 결과를 반환하는 것을 볼 수 있다.

정답

compar3_the_str1ng

rev-basic-0의 메인 함수는 이후 문제들에 대해서도 모두 동일하다 따라서 이제부터 메인함수는 생략하고 비교 함수의 로직만 보도록 하겠다

rev-basic-1

입력된 문자열을 문자 단위로 비교한다.

정답

Compar3_the_ch4ract3r

rev-basic-2

입력된 문자열을 배열과 비교한다.

정답

Compar3_the_arr4y

이후의 문제들은 모두 조건문을 보고 역함수를 찾아내 입력값을 알아내는 형식이다. 이에 정답 대신 정답을 출력하는 코드를 작성하였다.

rev-basic-3

정답코드

bytes = [
    0x49, 0x60, 0x67, 0x74, 0x63, 0x67, 0x42, 0x66, 0x80, 0x78,
    0x69, 0x69, 0x7b, 0x99, 0x6d, 0x88, 0x68, 0x94, 0x9F, 0x8D,
    0x4D, 0xA5, 0x9D, 0x45, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00
]

ans = []


for i in range(0x18):
    val = (bytes[i] - (2 * i)) ^ i
    ans.append(val)

print("".join(chr(x) for x in ans))

rev-basic-4

정답코드

bytes = [
    0x24, 0x27, 0x13, 0xC6, 0xC6, 0x13, 0x16, 0xE6, 0x47, 0xF5,
    0x26, 0x96, 0x47, 0xF5, 0x46, 0x27, 0x13, 0x26, 0x26, 0xC6,
    0x56, 0xF5, 0xC3, 0xC3, 0xF5, 0xE3, 0xE3, 0x00, 0x00, 0x00,
]

ans = []


for i in range(0x1c): 
    
    high_nibble = (bytes[i] & 0xF0) >> 4
    
    # Extract the low nibble and shift it to the upper 4 bits.
    low_nibble = (bytes[i] & 0x0F) << 4
    

    # Combine the two nibbles using OR operation to restore the original byte value (nibble swap).
    val = low_nibble | high_nibble
    
    ans.append(val)

print("".join(chr(x) for x in ans))

rev-basic-5

정답코드

bytes = [
    0xAD, 0xD8, 0xCB, 0xCB, 0x9D, 0x97, 0xCB, 0xC4, 0x92, 0xA1,
    0xD2, 0xD7, 0xD2, 0xD6, 0xA8, 0xA5, 0xDC, 0xC7, 0xAD, 0xA3,
    0xA1, 0x98, 0x4C, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
]


ans = [0] * 25
ans[24] = 0

for i in range(23, -1, -1):
    val = (bytes[i] - ans[i+1])
    ans[i] = val

print("".join(chr(x) for x in ans if x > 0))

rev-basic-6

정답코드

# Store hexadecimal values directly as integers
b1 = [
    0x00, 0x4D, 0x51, 0x50, 0xEF, 0xFB, 0xC3, 0xCF, 
    0x92, 0x45, 0x4D, 0xCF, 0xF5, 0x04, 0x40, 0x50,
    0x43, 0x63,
]

b2 = [
    ...
]

ans = []

for val in b1:
    idx = b2.index(val)
    ans.append(idx)

print(''.join(chr(x) for x in ans))

rev-basic-7

정답코드

B = [
    0x52, 0xDF, 0xB3, 0x60, 0xF1, 0x8B, 0x1C, 0xB5, 0x57, 0xD1,
    0x9F, 0x38, 0x4B, 0x29, 0xD9, 0x26, 0x7F, 0xC9, 0xA3, 0xE9,
    0x53, 0x18, 0x4F, 0xB8, 0x6A, 0xCB, 0x87, 0x58, 0x5B, 0x39,
    0x1E, 0x00
]


ans = []


def ror_8bit(value, count):
    count = count % 8
    return ((value >> count) | (value << (8 - count))) & 0xFF

for i in range(len(B)):
    val = ror_8bit((B[i] ^ i), i & 7)
    ans.append(val)

print(''.join(chr(x) for x in ans if x > 0))

rev-basic-8

정답코드

B = [
    0xAC, 0xF3, 0x0C, 0x25, 0xA3, 0x10, 0xB7, 0x25, 0x16, 0xC6,
    0xB7, 0xBC, 0x07, 0x25, 0x02, 0xD5, 0xC6, 0x11, 0x07, 0xC5,
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
    0x00, 0x00
]
ans =[] 
# ans[i] = (B[i] * 51) % 256
# 51 is the modinv of -5 modulo 256.

for i in range(len(B)):
    ans.append((B[i] * 51) % 256)

print(''.join(chr(x) for x in ans if x > 0))  # Filter out non-positive values

만약 본인이 브루트포싱을 통해 풀었다면 다시 풀어보자

rev-basic-9

정답코드

def ror5(x):
    return ((x >> 5) | (x << 3)) & 0xFF

def rol5(x):
    return ((x << 5) | (x >> 3)) & 0xFF

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

key = [ord(c) for c in "I_am_KEY"]

cipher = [
    0x7E, 0x7D, 0x9A, 0x8B, 0x25, 0x2D, 0xD5, 0x3D,
    0x03, 0x2B, 0x38, 0x98, 0x27, 0x9F, 0x4F, 0xBC,
    0x2A, 0x79, 0x00, 0x7D, 0xC4, 0x2A, 0x4F, 0x58,
    0x00
]

def decrypt_block(block):
    a = block[:]
    for i in range(15, -1, -1):
        for j in range(7, -1, -1):
            idx = (j + 1) % 8
            val = rol5(a[idx])
            val = (val - sbox[key[j] ^ a[j]]) % 256
            a[idx] = val
    return a


decrypted = []
for i in range(0, 24, 8):
    block = cipher[i:i+8]
    plain = decrypt_block(block)
    decrypted.extend(plain)

print(bytes(decrypted))