2nd/10_Wiki/Topics/DevOps_and_Security/Practical-Cryptography.md

---
id: wiki-2026-0508-practical-cryptography
title: Practical Cryptography
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Applied Cryptography, Crypto Engineering]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [cryptography, security, encryption]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: Python/Go
  framework: libsodium/cryptography
---

# Practical Cryptography

## 매 한 줄
> **"매 don't roll your own crypto"**. 매 application engineer 의 task 는 매 well-vetted primitives (AES-GCM, ChaCha20-Poly1305, Ed25519, X25519) 의 correct composition — 매 algorithm 의 invention 아님. 2026 의 modern stack 은 libsodium, AWS KMS, age, Noise Protocol Framework 위 의 build.

## 매 핵심

### 매 Primitives (2026 baseline)
- **Symmetric AEAD**: ChaCha20-Poly1305 (mobile/no-AES-NI), AES-256-GCM (server with AES-NI), AES-256-GCM-SIV (nonce-misuse resistant).
- **Asymmetric**: X25519 (ECDH key agreement), Ed25519 (signing), Kyber-1024 (post-quantum KEM, NIST FIPS 203).
- **Hashing**: BLAKE3 (fast), SHA-256 (interop), Argon2id (password hashing, 2026 default).
- **Key derivation**: HKDF-SHA256 (key expansion), Argon2id (password → key).

### 매 Threat models
- **Confidentiality**: encrypt-then-MAC, AEAD prevents IND-CCA2 attacks.
- **Integrity**: HMAC, Poly1305, signatures.
- **Authenticity**: signatures (Ed25519), authenticated key exchange (Noise).
- **Forward secrecy**: ephemeral keys (X25519 per session).
- **Post-quantum**: hybrid Kyber + X25519 (2026 TLS 1.3 default).

### 매 응용
1. TLS 1.3 (transport security).
2. Signal Protocol (E2EE messaging — Double Ratchet).
3. age/rage (file encryption — replaces GPG).
4. JWT/PASETO (stateless tokens — PASETO preferred).
5. Password storage (Argon2id with per-user salt).

## 💻 패턴

### AEAD encryption (ChaCha20-Poly1305 with libsodium)
```python
from nacl.secret import SecretBox
from nacl.utils import random

key = random(SecretBox.KEY_SIZE)  # 32 bytes
box = SecretBox(key)

# Encrypt — nonce auto-generated, prepended to ciphertext
ciphertext = box.encrypt(b"sensitive data")

# Decrypt — fails with CryptoError on tampering
plaintext = box.decrypt(ciphertext)
```

### Authenticated key exchange (X25519 + HKDF)
```python
from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
from cryptography.hazmat.primitives.kdf.hkdf import HKDF
from cryptography.hazmat.primitives import hashes

# Each party generates ephemeral keypair
alice_priv = X25519PrivateKey.generate()
bob_priv = X25519PrivateKey.generate()

# Compute shared secret
shared = alice_priv.exchange(bob_priv.public_key())

# Derive symmetric key — never use raw DH output as key
session_key = HKDF(
    algorithm=hashes.SHA256(),
    length=32,
    salt=None,
    info=b"session-v1",
).derive(shared)
```

### Password hashing (Argon2id)
```python
from argon2 import PasswordHasher

ph = PasswordHasher(
    time_cost=3,       # iterations
    memory_cost=65536, # 64 MiB
    parallelism=4,
)

hash = ph.hash("user-password")  # store this

# Verify (constant-time)
try:
    ph.verify(hash, "user-password")
    if ph.check_needs_rehash(hash):
        new_hash = ph.hash("user-password")  # parameter upgrade
except VerifyMismatchError:
    raise AuthError()
```

### Digital signature (Ed25519)
```python
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey

priv = Ed25519PrivateKey.generate()
pub = priv.public_key()

signature = priv.sign(b"message")
pub.verify(signature, b"message")  # raises InvalidSignature on failure
```

### Envelope encryption (KMS pattern)
```python
import boto3
from cryptography.fernet import Fernet

kms = boto3.client("kms")

def encrypt_blob(plaintext: bytes, kms_key_id: str) -> dict:
    # Generate per-message data key
    resp = kms.generate_data_key(KeyId=kms_key_id, KeySpec="AES_256")
    data_key = resp["Plaintext"]
    encrypted_dk = resp["CiphertextBlob"]

    # Encrypt data with data key, discard plaintext data key
    f = Fernet(base64.urlsafe_b64encode(data_key))
    ct = f.encrypt(plaintext)

    return {"ciphertext": ct, "encrypted_key": encrypted_dk}
```

### Constant-time comparison
```python
import hmac

# WRONG — leaks length info via timing
if user_token == stored_token:
    pass

# RIGHT — constant time
if hmac.compare_digest(user_token, stored_token):
    pass
```

### Post-quantum hybrid KEM (2026)
```python
# liboqs-python — hybrid X25519 + Kyber768
from oqs import KeyEncapsulation
import nacl.public

# Classical X25519
x_priv = nacl.public.PrivateKey.generate()

# Post-quantum Kyber
with KeyEncapsulation("Kyber768") as kem:
    pq_pub = kem.generate_keypair()

# Combine both shared secrets via HKDF for hybrid security
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| File encryption | age (modern), libsodium SecretBox |
| Password hash | Argon2id (never bcrypt for new systems) |
| Token format | PASETO v4 (Ed25519) over JWT |
| Mobile/IoT AEAD | ChaCha20-Poly1305 |
| TLS 1.3 backend | rustls or BoringSSL, hybrid PQ enabled |
| Signing | Ed25519 (never RSA for new systems) |

**기본값**: libsodium + Argon2id + Ed25519 + ChaCha20-Poly1305.

## 🔗 Graph
- 부모: [[Practical-Cryptography|Cryptography]] · [[Security]]
- 변형: [[보안_및_시스템_신뢰성_표준|Symmetric-Encryption]]
- 응용: [[Secret_Management]] · [[보안_및_시스템_신뢰성_표준|Zero-Trust Architecture]]
- Adjacent: [[보안_및_시스템_신뢰성_표준|OWASP Top 10]] · [[Practical-Cryptography]]

## 🤖 LLM 활용
**언제**: explain primitive choice, audit crypto code for misuse, suggest migration paths.
**언제 X**: never ask LLM to design new protocol — always defer to peer-reviewed designs (Noise, Signal).

## ❌ 안티패턴
- **Roll-your-own**: custom XOR-based "encryption" — 매 broken in seconds.
- **ECB mode**: leaks pattern (penguin image meme). Always GCM/CTR/CBC-with-MAC.
- **Static IV/nonce**: catastrophic for GCM (key recovery). Always random or counter.
- **MD5/SHA-1**: collision-broken. Never for security purposes.
- **bcrypt for new systems**: Argon2id 2026 default.
- **String comparison for tokens**: use `hmac.compare_digest`.

## 🧪 검증 / 중복
- Verified (NIST FIPS 203/204/205, RFC 9180 HPKE, libsodium docs).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full primitives + 2026 PQ baseline |