Add challenge generator

docs/IMPLEMENTATION.md

@@ -9,7 +9,7 @@
   - [X] Set up testing framework and utilities
 
 - [ ] **Challenge Generation & HMAC Security**
-  - [ ] Implement HMAC-signed challenge generation (stateless)
+  - [X] Implement HMAC-signed challenge generation (stateless)
   - [ ] Create challenge authenticity verification
   - [ ] Add timestamp validation for replay protection (5 minutes TTL)
   - [ ] Implement canonical challenge field ordering for HMAC

internal/pow/generator.go

@@ -0,0 +1,62 @@
+package pow
+
+import (
+	"crypto/rand"
+	"time"
+)
+
+// Generator handles creation of PoW challenges with HMAC signing
+type Generator struct {
+	config *Config
+}
+
+// GenerateOption represents a functional option for challenge generation
+type generateOption func(*Challenge)
+
+// WithDifficulty sets custom difficulty for the challenge
+func WithDifficulty(difficulty int) generateOption {
+	return func(c *Challenge) {
+		c.Difficulty = difficulty
+	}
+}
+
+// NewGenerator creates a new challenge generator
+func NewGenerator(config *Config) *Generator {
+	return &Generator{
+		config: config,
+	}
+}
+
+// GenerateChallenge creates a new HMAC-signed PoW challenge
+func (g *Generator) GenerateChallenge(opts ...generateOption) (*Challenge, error) {
+	// Create challenge with defaults (no random data yet)
+	challenge := &Challenge{
+		Timestamp:  time.Now().Unix(),
+		Difficulty: g.config.DefaultDifficulty,
+		Resource:   g.config.Resource,
+	}
+
+	// Apply options
+	for _, opt := range opts {
+		opt(challenge)
+	}
+
+	// Validate difficulty bounds
+	if challenge.Difficulty < g.config.MinDifficulty || challenge.Difficulty > g.config.MaxDifficulty {
+		return nil, ErrInvalidDifficulty
+	}
+
+	// Generate cryptographic random data
+	challenge.Random = g.generateRandom(g.config.RandomBytes)
+
+	// Sign the challenge with HMAC
+	challenge.Sign(g.config.HMACSecret)
+	return challenge, nil
+}
+
+// generateRandom creates cryptographic random bytes
+func (g *Generator) generateRandom(length int) []byte {
+	bytes := make([]byte, length)
+	rand.Read(bytes)
+	return bytes
+}