Python에서 AES-GCM 암호화 및 복호화

AES-GCM을 사용하는 이유

GCM 구성 요소: CTR + GHASH

논스 요구 사항

Python에서 AES-GCM으로 암호화하기

import os
from cryptography.hazmat.primitives.ciphers.aead import AESGCM

# Generate a 256-bit key
key = AESGCM.generate_key(bit_length=256)
aesgcm = AESGCM(key)

# Fresh 96-bit nonce per encryption
nonce = os.urandom(12)

plaintext = b"sensitive payload"
aad = b"user_id=42"  # authenticated but not encrypted

ciphertext = aesgcm.encrypt(nonce, plaintext, aad)
print("Nonce:", nonce.hex())
print("Ciphertext+tag:", ciphertext.hex())

Python에서 AES-GCM으로 복호화하기

from cryptography.hazmat.primitives.ciphers.aead import AESGCM
from cryptography.exceptions import InvalidTag

# key, nonce, ciphertext, aad from previous step
try:
    recovered = aesgcm.decrypt(nonce, ciphertext, aad)
    print("Decrypted:", recovered)
except InvalidTag:
    print("Authentication failed — ciphertext tampered!")

논스와 암호문 저장하기

def encrypt_blob(key: bytes, data: bytes, aad: bytes = b"") -> bytes:
    aesgcm = AESGCM(key)
    nonce = os.urandom(12)
    ct = aesgcm.encrypt(nonce, data, aad or None)
    return nonce + ct  # prepend nonce

def decrypt_blob(key: bytes, blob: bytes, aad: bytes = b"") -> bytes:
    aesgcm = AESGCM(key)
    nonce, ct = blob[:12], blob[12:]
    return aesgcm.decrypt(nonce, ct, aad or None)

추가 인증 데이터(AAD)

저수준 Cipher API 사용하기

from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes

key = os.urandom(32)
nonce = os.urandom(12)
cipher = Cipher(algorithms.AES(key), modes.GCM(nonce))
encryptor = cipher.encryptor()
encryptor.authenticate_additional_data(b"aad-value")
ct = encryptor.update(b"hello world") + encryptor.finalize()
tag = encryptor.tag
print("Tag:", tag.hex())

논스 오용 저항성: AES-GCM-SIV

암호화 전 키 파생

from cryptography.hazmat.primitives.kdf.hkdf import HKDF
from cryptography.hazmat.primitives import hashes
import os

master_secret = b"shared_secret_from_ECDH"
salt = os.urandom(16)
kdf = HKDF(algorithm=hashes.SHA256(), length=32, salt=salt, info=b"aes-gcm-key")
aes_key = kdf.derive(master_secret)
print("Derived AES key:", aes_key.hex())

성능 고려 사항

이해도 확인

학습 내용 복습