Security Review

Checklist for reviewing ZChain code for security issues.

Cryptographic Security

Hash Function Usage

• Using SHA256 (or stronger) - never MD5/SHA1
• Hash inputs include all relevant block data
• No predictable nonce generation
• Proper hex encoding of hash output

// GOOD: All block data in hash
var hashInput = $"{block.Height}{block.ParentHash}{block.Transaction}{nonce}";

// BAD: Missing fields allows hash collision attacks
var hashInput = $"{nonce}";

Random Number Generation

• Using cryptographically secure RNG for security-sensitive operations
• Not using System.Random for cryptographic purposes

// GOOD: Cryptographically secure
using var rng = RandomNumberGenerator.Create();
byte[] bytes = new byte[32];
rng.GetBytes(bytes);

// BAD: Predictable
var random = new Random();

Blockchain Integrity

Block Validation

• Verify hash matches recorded values
• Verify parent hash chain is intact
• Verify difficulty requirement is met
• Verify block state transitions are valid

State Machine Security

• Cannot mine already-mined block
• Cannot verify unmined block
• Cannot modify mined block values
• State transitions are thread-safe

Concurrency Issues

Thread Safety

• Shared state protected by locks or concurrent collections
• No race conditions in mining completion
• CancellationToken properly propagated
• Resources properly disposed

// GOOD: Thread-safe mined value setting
lock (_minedLock)
{
    if (State == BlockState.Mined) return;
    _hash = hash;
    _nonce = nonce;
    State = BlockState.Mined;
}

Cancellation

• Long-running operations check CancellationToken
• CancellationTokenSource disposed after use
• Graceful shutdown on cancellation

Input Validation

Public API Boundaries

• Null checks on all public method parameters
• Range validation on numeric inputs (difficulty > 0)
• No SQL injection (if database added)
• No command injection in Bash operations

// GOOD: Proper validation
public Block(T transaction, int difficulty)
{
    ArgumentNullException.ThrowIfNull(transaction);
    ArgumentOutOfRangeException.ThrowIfNegativeOrZero(difficulty);
}

Sensitive Data

Logging and Output

• No private keys in logs
• No sensitive data in exception messages
• Debug output doesn't expose internal state
• Benchmark results don't contain sensitive info

Serialization

• JSON serialization doesn't expose internal fields
• Deserialization validates input
• No arbitrary type instantiation (TypeNameHandling)

// BAD: Allows arbitrary type instantiation
JsonConvert.DeserializeObject<Block>(json, new JsonSerializerSettings {
    TypeNameHandling = TypeNameHandling.All  // DANGEROUS
});

// GOOD: Explicit type, no type handling
JsonConvert.DeserializeObject<Block<MoneyTransferTransaction>>(json);

Dependency Security

• NuGet packages from trusted sources
• No known vulnerabilities (check GitHub Dependabot)
• Packages pinned to specific versions
• Regular dependency updates

Review Commands

# Search for potential issues
grep -r "Random()" src/
grep -r "MD5\|SHA1" src/
grep -r "TypeNameHandling" src/
grep -r "Process.Start\|Shell" src/

# Check for hardcoded secrets
grep -r "password\|secret\|key\|token" src/ --include="*.cs"

Severity Levels