O'quv Markazi / Kriptografiya / Simmetrik Shifrlash

Simmetrik Shifrlash

AES, DES, 3DES — bitta kalit bilan ham shifrlash, ham deshifrlash. Blok va oqim shifrlar, ECB/CBC/CTR/GCM rejimlari, va AES ning ichki ishlash mexanizmi.

AES-128/192/256 DES / 3DES ECB / CBC / CTR / GCM Stream Ciphers Padding Oracle

1 Simmetrik Shifrlash Nima?

Tez

Asimmetrik shifrlashdan 1000x tezroq. Katta fayllar uchun ideal.

Bitta Kalit

Shifrlash va deshifrlash uchun bir xil kalit ishlatiladi.

Kalit muammosi

Kalitni xavfsiz tarqatish — asosiy muammo. Hybrid encryption yechim.

2 Blok vs Oqim Shifrlar

Blok Shifrlar (Block Ciphers)

Ma'lumotni qat'iy o'lchamdagi bloklarga bo'lib shifrlaydi (64-bit, 128-bit).

AES 128-bit blok, 128/192/256-bit kalit

DES 64-bit blok, 56-bit kalit (eskirgan)

3DES 64-bit blok, 168-bit kalit (sekin)

Blowfish 64-bit blok, 32–448-bit kalit

Twofish 128-bit blok, 128/192/256-bit kalit

Oqim Shifrlar (Stream Ciphers)

Ma'lumotni bit-bit yoki bayt-bayt shifrlaydi — XOR bilan pseudorandom kalit oqimi.

RC4 Zaif — HTTPS/WEP dan olib tashlangan

ChaCha20 Zamonaviy, TLS 1.3 da ishlatiladi

Salsa20 ChaCha20 ning ajdodi, xavfsiz

// Stream cipher asosi:

ciphertext[i] = plaintext[i] XOR keystream[i]

3 DES va Feistel Tarmog'i

DES Xususiyatlari

64-bit blok o'lchami
56-bit samarali kalit uzunligi (8 bit paritet)
16 ta Feistel turi
1999-yilda EFF "Deep Crack" 22 soatda buzdi
Bugun ishlatish mumkin emas — brute force imkoni bor

3DES (Triple DES)

C = DES_K3( DES⁻¹_K2( DES_K1(P) ) )

DES ni 3 marta qo'llaydi (Encrypt-Decrypt-Encrypt)
112-bit samarali xavfsizlik
AES dan 3x sekin
2023-yildan NIST tomonidan deprecated

4 AES (Advanced Encryption Standard)

AES — 2001-yilda NIST tomonidan standart sifatida qabul qilingan, Rijndael algoritmi asosida. Bugun dunyodagi eng ko'p ishlatiladigan simmetrik shifrlash algoritmi: HTTPS, WiFi, disk shifrlash, VPN, va boshqa.

AES Turlar Soni

Versiya	Kalit	Turlar	Xavfsizlik
AES-128	128 bit	10	Yetarli
AES-192	192 bit	12	Yaxshi
AES-256	256 bit	14	Eng yaxshi

AES S-Box nima?

SubBytes bosqichida har bir bayt S-Box (Substitution Box) jadvalidan o'tkaziladi. Bu non-linear — kriptografik kuch beradi.

// S-Box misol (hex)

0x00 → 0x63

0x01 → 0x7c

0xff → 0x16

256 ta qiymat, GF(2⁸) inversiyasi + affine transform

5 Shifrlash Rejimlari (Modes of Operation)

Rejim	IV kerak?	Parallel?	Xato tarqalish	Xavfsizlik	Ishlatish
ECB	Yo'q	Ha	Yo'q	ZAIF	HECH QACHON ishlatmang
CBC	Ha (tasodifiy)	Shifr emas	2 blok	O'RTACHA	TLS 1.2 (eski)
CTR	Ha (nonce)	Ha	Tarqalmaydi	YAXSHI	Stream-like
GCM	Ha (nonce, 96-bit)	Ha	Auth tag	ENG YAXSHI	TLS 1.3, HTTPS

6 Python bilan AES

Muhim: Hech qachon ECB rejimini ishlatmang. GCM rejimini ishlatganingizda nonce bir marta ishlatilsin — aks holda xavfsizlik buziladi.

from Crypto.Cipher import AES
from Crypto.Random import get_random_bytes
from Crypto.Util.Padding import pad, unpad
import base64

# ============================================
# AES-CBC (to'g'ri ishlatish)
# ============================================
def aes_cbc_encrypt(plaintext: bytes, key: bytes) -> tuple:
    iv = get_random_bytes(16)  # Tasodifiy IV, har safar yangi!
    cipher = AES.new(key, AES.MODE_CBC, iv)
    ciphertext = cipher.encrypt(pad(plaintext, AES.block_size))
    return iv, ciphertext

def aes_cbc_decrypt(iv: bytes, ciphertext: bytes, key: bytes) -> bytes:
    cipher = AES.new(key, AES.MODE_CBC, iv)
    return unpad(cipher.decrypt(ciphertext), AES.block_size)

# ============================================
# AES-GCM (eng yaxshi tanlov — autentifikatsiya bilan)
# ============================================
def aes_gcm_encrypt(plaintext: bytes, key: bytes, aad: bytes = None) -> dict:
    nonce = get_random_bytes(12)  # 96-bit nonce, faqat bir marta!
    cipher = AES.new(key, AES.MODE_GCM, nonce=nonce)
    if aad:
        cipher.update(aad)  # Additional Authenticated Data
    ciphertext, tag = cipher.encrypt_and_digest(plaintext)
    return {'nonce': nonce, 'ciphertext': ciphertext, 'tag': tag}

def aes_gcm_decrypt(data: dict, key: bytes, aad: bytes = None) -> bytes:
    cipher = AES.new(key, AES.MODE_GCM, nonce=data['nonce'])
    if aad:
        cipher.update(aad)
    return cipher.decrypt_and_verify(data['ciphertext'], data['tag'])
    # ValueError tashlaydi agar tag noto'g'ri bo'lsa (tampered!)

# Misol
key = get_random_bytes(32)  # AES-256
msg = b"CTF{s3cr3t_m3ss4g3}"

# CBC
iv, enc = aes_cbc_encrypt(msg, key)
dec = aes_cbc_decrypt(iv, enc, key)
print(f"CBC: {dec}")  # b"CTF{s3cr3t_m3ss4g3}"

# GCM
enc_data = aes_gcm_encrypt(msg, key, aad=b"header")
print(aes_gcm_decrypt(enc_data, key, aad=b"header"))

# ============================================
# CTF: ECB Oracle hujumi (Byte-at-a-time)
# ============================================
def detect_ecb(ciphertext: bytes, block_size: int = 16) -> bool:
    blocks = [ciphertext[i:i+block_size] for i in range(0, len(ciphertext), block_size)]
    return len(blocks) != len(set(blocks))  # Takrorlanuvchi blok = ECB

7 CTF Simmetrik Shifr Masalalari

Keng tarqalgan CTF naqshlari

ECB mode detection

Bir xil plaintext bloklar bir xil ciphertext bloklar hosil qiladi. Takrorlanuvchi blok topish = ECB.

CBC Bit-Flipping Attack

IV yoki oldingi ciphertext blokini o'zgartirish keyingi plaintext blokiga ta'sir qiladi.

Padding Oracle

Server padding xatosini ko'rsatsa — CBC deshifrlanadi. POODLE (2014) shu zaiflikdan foydalangan.

CTR Nonce Reuse

Bir nonce bilan ikki xil matn shifrlansa: C1⊕C2 = P1⊕P2. XOR cracking imkoni.

CBC Bit-Flipping Misol

# Maqsad: admin=true o'rnatish
# Plaintext: "role=user;admin=false"

def cbc_bitflip(ciphertext, pos, old, new):
    # pos = o'zgartiriladigan bayt (oldingi blokda)
    c = bytearray(ciphertext)
    c[pos] ^= ord(old) ^ ord(new)
    return bytes(c)

# "false" → "true;" qilish uchun
# 16-bayt blok hisoblab, oldingi blokda
# mos pozitsiyani o'zgartirish

# Himoya: CBC + MAC (Encrypt-then-MAC)
# Yoki AES-GCM (autentifikatsiya bor)