Side Channels

Side-Channel Attacks

Concept: Extract secrets by observing indirect signals (timing, errors, etc.).

Timing Side-Channel

Signals:

• Comparison that exits early on mismatch
• Observable response time differences
• Per-character validation

Why it works:

// Vulnerable: early exit
for (int i = 0; i < len; i++) {
    if (input[i] != secret[i])
        return FAIL;  // Time varies by position
}

Attack Recipe

1. Establish baseline:

Measure response time for known-wrong inputs
Calculate average/median for comparison

2. Byte-by-byte oracle:

For each position:
  For each candidate byte:
    Measure response time (multiple samples)
  Select byte with longest average time

3. Noise reduction:

• Multiple samples per candidate
• Use median (more robust than mean)
• Remove outliers
• Run from same network as target

Strategies

Local binary:

• Minimal noise
• Direct timing measurement
• Few samples needed

Remote server:

• Network jitter requires many samples
• Statistical analysis helps
• Median over 20-100 samples

Rate limited:

• Add delays between attempts
• Prioritize likely characters
• Parallel connections if allowed

Other Oracles

Error-based:

Different errors reveal different failure modes
- "Invalid length" vs "Bad character" vs "Wrong value"
Each error type leaks information

Padding oracle:

Decrypt ciphertext by observing padding errors
Valid vs invalid padding reveals information

Pitfalls

Detection

If all inputs produce same timing regardless of correctness:

• Target uses constant-time comparison
• Timing attack won't work
• Need different approach