[G1-Sync] Manual knowledge update
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Antigravity Agent
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id: wiki-2026-0508-practical-cryptography
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title: Practical Cryptography
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category: 10_Wiki/Topics
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status: merged
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redirect_to: 보안_및_시스템_신뢰성_표준
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canonical_id: wiki-2026-0507-039
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aliases: []
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status: verified
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canonical_id: self
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aliases: [Applied Cryptography, Crypto Engineering]
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duplicate_of: none
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source_trust_level: A
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confidence_score: 0.92
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tags: [uncategorized]
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confidence_score: 0.9
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verification_status: applied
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tags: [cryptography, security, encryption]
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raw_sources: []
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last_reinforced: 2026-05-08
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last_reinforced: 2026-05-10
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github_commit: pending
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inferred_by: Claude Opus 4.7 (auto-normalize 2026-05-08)
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tech_stack:
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language: Python/Go
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framework: libsodium/cryptography
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---
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# Redirect
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# Practical Cryptography
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이 문서는 Canonical 문서인 [[보안_및_시스템_신뢰성_표준]]으로 통합되었습니다.
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모든 최신 지식과 세부 내용은 위 링크를 참조하십시오.
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## 매 한 줄
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> **"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.
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## 매 핵심
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### 매 Primitives (2026 baseline)
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- **Symmetric AEAD**: ChaCha20-Poly1305 (mobile/no-AES-NI), AES-256-GCM (server with AES-NI), AES-256-GCM-SIV (nonce-misuse resistant).
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- **Asymmetric**: X25519 (ECDH key agreement), Ed25519 (signing), Kyber-1024 (post-quantum KEM, NIST FIPS 203).
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- **Hashing**: BLAKE3 (fast), SHA-256 (interop), Argon2id (password hashing, 2026 default).
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- **Key derivation**: HKDF-SHA256 (key expansion), Argon2id (password → key).
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### 매 Threat models
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- **Confidentiality**: encrypt-then-MAC, AEAD prevents IND-CCA2 attacks.
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- **Integrity**: HMAC, Poly1305, signatures.
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- **Authenticity**: signatures (Ed25519), authenticated key exchange (Noise).
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- **Forward secrecy**: ephemeral keys (X25519 per session).
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- **Post-quantum**: hybrid Kyber + X25519 (2026 TLS 1.3 default).
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### 매 응용
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1. TLS 1.3 (transport security).
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2. Signal Protocol (E2EE messaging — Double Ratchet).
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3. age/rage (file encryption — replaces GPG).
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4. JWT/PASETO (stateless tokens — PASETO preferred).
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5. Password storage (Argon2id with per-user salt).
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## 💻 패턴
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### AEAD encryption (ChaCha20-Poly1305 with libsodium)
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```python
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from nacl.secret import SecretBox
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from nacl.utils import random
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key = random(SecretBox.KEY_SIZE) # 32 bytes
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box = SecretBox(key)
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# Encrypt — nonce auto-generated, prepended to ciphertext
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ciphertext = box.encrypt(b"sensitive data")
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# Decrypt — fails with CryptoError on tampering
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plaintext = box.decrypt(ciphertext)
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```
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### Authenticated key exchange (X25519 + HKDF)
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```python
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from cryptography.hazmat.primitives.asymmetric.x25519 import X25519PrivateKey
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from cryptography.hazmat.primitives.kdf.hkdf import HKDF
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from cryptography.hazmat.primitives import hashes
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# Each party generates ephemeral keypair
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alice_priv = X25519PrivateKey.generate()
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bob_priv = X25519PrivateKey.generate()
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# Compute shared secret
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shared = alice_priv.exchange(bob_priv.public_key())
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# Derive symmetric key — never use raw DH output as key
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session_key = HKDF(
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algorithm=hashes.SHA256(),
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length=32,
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salt=None,
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info=b"session-v1",
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).derive(shared)
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```
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### Password hashing (Argon2id)
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```python
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from argon2 import PasswordHasher
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ph = PasswordHasher(
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time_cost=3, # iterations
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memory_cost=65536, # 64 MiB
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parallelism=4,
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)
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hash = ph.hash("user-password") # store this
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# Verify (constant-time)
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try:
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ph.verify(hash, "user-password")
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if ph.check_needs_rehash(hash):
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new_hash = ph.hash("user-password") # parameter upgrade
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except VerifyMismatchError:
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raise AuthError()
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```
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### Digital signature (Ed25519)
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```python
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from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
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priv = Ed25519PrivateKey.generate()
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pub = priv.public_key()
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signature = priv.sign(b"message")
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pub.verify(signature, b"message") # raises InvalidSignature on failure
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```
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### Envelope encryption (KMS pattern)
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```python
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import boto3
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from cryptography.fernet import Fernet
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kms = boto3.client("kms")
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def encrypt_blob(plaintext: bytes, kms_key_id: str) -> dict:
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# Generate per-message data key
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resp = kms.generate_data_key(KeyId=kms_key_id, KeySpec="AES_256")
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data_key = resp["Plaintext"]
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encrypted_dk = resp["CiphertextBlob"]
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# Encrypt data with data key, discard plaintext data key
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f = Fernet(base64.urlsafe_b64encode(data_key))
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ct = f.encrypt(plaintext)
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return {"ciphertext": ct, "encrypted_key": encrypted_dk}
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```
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### Constant-time comparison
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```python
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import hmac
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# WRONG — leaks length info via timing
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if user_token == stored_token:
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pass
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# RIGHT — constant time
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if hmac.compare_digest(user_token, stored_token):
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pass
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```
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### Post-quantum hybrid KEM (2026)
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```python
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# liboqs-python — hybrid X25519 + Kyber768
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from oqs import KeyEncapsulation
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import nacl.public
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# Classical X25519
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x_priv = nacl.public.PrivateKey.generate()
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# Post-quantum Kyber
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with KeyEncapsulation("Kyber768") as kem:
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pq_pub = kem.generate_keypair()
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# Combine both shared secrets via HKDF for hybrid security
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```
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## 매 결정 기준
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| 상황 | Approach |
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|---|---|
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| File encryption | age (modern), libsodium SecretBox |
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| Password hash | Argon2id (never bcrypt for new systems) |
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| Token format | PASETO v4 (Ed25519) over JWT |
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| Mobile/IoT AEAD | ChaCha20-Poly1305 |
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| TLS 1.3 backend | rustls or BoringSSL, hybrid PQ enabled |
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| Signing | Ed25519 (never RSA for new systems) |
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**기본값**: libsodium + Argon2id + Ed25519 + ChaCha20-Poly1305.
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## 🔗 Graph
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- 부모: [[Cryptography]] · [[Security]]
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- 변형: [[Symmetric-Encryption]] · [[Post-Quantum-Cryptography]]
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- 응용: [[Secret_Management]] · [[Zero-Trust Architecture]]
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- Adjacent: [[OWASP Top 10]] · [[Practical-Cryptography]]
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## 🤖 LLM 활용
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**언제**: explain primitive choice, audit crypto code for misuse, suggest migration paths.
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**언제 X**: never ask LLM to design new protocol — always defer to peer-reviewed designs (Noise, Signal).
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## ❌ 안티패턴
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- **Roll-your-own**: custom XOR-based "encryption" — 매 broken in seconds.
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- **ECB mode**: leaks pattern (penguin image meme). Always GCM/CTR/CBC-with-MAC.
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- **Static IV/nonce**: catastrophic for GCM (key recovery). Always random or counter.
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- **MD5/SHA-1**: collision-broken. Never for security purposes.
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- **bcrypt for new systems**: Argon2id 2026 default.
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- **String comparison for tokens**: use `hmac.compare_digest`.
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## 🧪 검증 / 중복
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- Verified (NIST FIPS 203/204/205, RFC 9180 HPKE, libsodium docs).
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- 신뢰도 A.
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## 🕓 Changelog
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| 날짜 | 변경 |
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|---|---|
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| 2026-05-08 | Phase 1 |
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| 2026-05-10 | Manual cleanup — full primitives + 2026 PQ baseline |
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