---
id: wiki-2026-0508-timing-attack
title: Timing Attack
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Timing Side Channel, Constant Time Comparison]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [security, side-channel, cryptography, constant-time]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: nodejs
---

# Timing Attack

## 매 한 줄
> **"매 execution time 의 secret-dependent variation 의 leak"**. 1996 Kocher RSA timing paper origin — comparison/branch/cache 가 secret bit 에 dependent 면 attacker 가 multiple measurement 의 statistical analysis 로 secret 을 추출. Mitigation 의 핵심: constant-time code.

## 매 핵심

### 매 Vulnerable patterns
- **Early-exit string compare**: `==` / `strcmp` returns on first mismatch byte.
- **Secret-dependent branch**: `if (key[i] == 0)` 의 cache miss timing differs.
- **Secret-dependent table lookup**: AES S-box → cache line timing.
- **Modular exponentiation**: square-and-multiply 의 bit-dependent ops.

### 매 Mitigation
- **Constant-time compare**: scan all bytes regardless, XOR-accumulate.
- **No secret-dependent branches**: use bitwise mask instead.
- **No secret-dependent indices**: scan full table or use bit-slicing.
- **Blinding**: randomize input (RSA: blind w/ random r, decrypt, unblind).

### 매 응용
1. HMAC token comparison (auth bypass via timing).
2. Password hash compare (after bcrypt — still need constant-time).
3. JWT signature verify.

## 💻 패턴

### Vulnerable: early-exit compare
```typescript
// ❌ DON'T — leaks prefix length
function badCompare(a: string, b: string): boolean {
  if (a.length !== b.length) return false;
  for (let i = 0; i < a.length; i++) {
    if (a[i] !== b[i]) return false;  // early exit reveals match length
  }
  return true;
}
// Attacker measures: "aaaa" vs "baaa" faster than "aaaa" vs "aaab"
```

### Constant-time compare (Node.js)
```typescript
import { timingSafeEqual } from 'node:crypto';

function safeCompare(a: string, b: string): boolean {
  const bufA = Buffer.from(a);
  const bufB = Buffer.from(b);
  if (bufA.length !== bufB.length) {
    // length leak unavoidable — pad to fixed length OR accept
    timingSafeEqual(bufA, bufA); // dummy compare to equalize timing
    return false;
  }
  return timingSafeEqual(bufA, bufB);
}

// HMAC token check
import { createHmac } from 'node:crypto';
function verifyToken(token: string, payload: string, secret: string): boolean {
  const expected = createHmac('sha256', secret).update(payload).digest('hex');
  return expected.length === token.length &&
         timingSafeEqual(Buffer.from(expected), Buffer.from(token));
}
```

### Browser: SubtleCrypto + manual constant-time
```typescript
// Web Crypto has no timingSafeEqual — implement carefully
function ctEqualBytes(a: Uint8Array, b: Uint8Array): boolean {
  if (a.length !== b.length) return false;
  let diff = 0;
  for (let i = 0; i < a.length; i++) {
    diff |= a[i] ^ b[i];     // XOR-accumulate, no branch
  }
  return diff === 0;
}

async function verifyHmac(msg: string, sig: ArrayBuffer, key: CryptoKey) {
  const computed = await crypto.subtle.sign(
    'HMAC', key, new TextEncoder().encode(msg)
  );
  return ctEqualBytes(new Uint8Array(computed), new Uint8Array(sig));
}
```

### Python constant-time compare
```python
import hmac

def verify(token: str, expected: str) -> bool:
    return hmac.compare_digest(token, expected)

# bcrypt — already constant-time via library
import bcrypt
def check_password(plain: bytes, hashed: bytes) -> bool:
    return bcrypt.checkpw(plain, hashed)  # safe internally
```

### Go constant-time
```go
import "crypto/subtle"

func verify(a, b []byte) bool {
    return subtle.ConstantTimeCompare(a, b) == 1
}

// Conditional copy without branch
func ctSelect(cond int, a, b []byte) {
    subtle.ConstantTimeCopy(cond, a, b)
}
```

### Branchless conditional (C-style)
```typescript
// Constant-time conditional select
function ctSelect(cond: number, a: number, b: number): number {
  // cond must be 0 or 1
  const mask = -cond;          // 0 or 0xFFFFFFFF
  return (a & mask) | (b & ~mask);
}

// Constant-time min/max without branch
function ctMin(a: number, b: number): number {
  const lt = (a - b) >>> 31;   // 1 if a<b
  return ctSelect(lt, a, b);
}
```

### Remote timing (network attacks)
```text
Lucky 13 (TLS 2013): MAC verify timing leaked plaintext.
CRIME / BREACH: compression length leaked secrets (different side channel).

Mitigation:
  - Always run full computation regardless of input validity.
  - Add randomized delay (debatable — may not help, can hurt).
  - Rate-limit + monitoring for anomalous timing-probe traffic.
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Password hash check | bcrypt/argon2 lib (already CT) |
| HMAC/token compare | timingSafeEqual / hmac.compare_digest |
| AES on untrusted host | AES-NI (HW) or bit-sliced soft impl |
| Secret-dependent index | bitwise mask or full-table scan |
| RSA/ECDSA private op | use vetted lib (BoringSSL, libsodium) — never roll your own |

**기본값**: never write your own crypto compare. Use language stdlib `timingSafeEqual` / `compare_digest` / `subtle.ConstantTimeCompare`.

## 🔗 Graph
- 부모: [[Side Channel Attack]] · [[Practical-Cryptography|Cryptography]]
- 변형: [[Spectre]] · [[Cache Timing Attack]] · [[Power Analysis]]

## 🤖 LLM 활용
**언제**: auth code review, HMAC/token compare, custom crypto routines, security audit.
**언제 X**: high-level app logic where no secrets are compared (UI rendering, business rules).

## ❌ 안티패턴
- **`a === b` for secrets**: V8/JIT may early-exit, branch, or short-circuit.
- **Custom "constant-time" without testing**: compiler can re-introduce branches via optimization. Test with `dudect`.
- **Throwing on mismatch**: exception path differs in timing from success path.
- **Logging mismatch position**: leaks comparison index.
- **Comparing hashes in DB**: even hash compare 의 leaks length prefix → use CT.

## 🧪 검증 / 중복
- Verified (Kocher 1996 paper, Node.js crypto docs, Go subtle pkg, OWASP Cryptographic Storage CS).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full canonical (CT compare + branchless + Node/Go/Python + remote attacks) |
