himbocrypt/pkg/engine/engine_test.go

package engine

import (
	"bytes"
	"crypto/ecdh"
	"crypto/rand"
	"encoding/hex"
	"strings"
	"testing"
)

func TestNewEngine(t *testing.T) {
	e := NewEngine()
	if e == nil {
		t.Fatal("NewEngine() returned nil")
	}
}

func TestGenerateKeyPair(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	if kp.PrivateKey == nil {
		t.Error("PrivateKey is nil")
	}
	if kp.PublicKey == nil {
		t.Error("PublicKey is nil")
	}

	// Verify key lengths (X25519 keys are 32 bytes)
	if len(kp.PrivateKey.Bytes()) != 32 {
		t.Errorf("PrivateKey length = %d, want 32", len(kp.PrivateKey.Bytes()))
	}
	if len(kp.PublicKey.Bytes()) != 32 {
		t.Errorf("PublicKey length = %d, want 32", len(kp.PublicKey.Bytes()))
	}
}

func TestGenerateKeyPair_Uniqueness(t *testing.T) {
	e := NewEngine()
	kp1, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	kp2, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	if bytes.Equal(kp1.PrivateKey.Bytes(), kp2.PrivateKey.Bytes()) {
		t.Error("Two generated key pairs have identical private keys")
	}
	if bytes.Equal(kp1.PublicKey.Bytes(), kp2.PublicKey.Bytes()) {
		t.Error("Two generated key pairs have identical public keys")
	}
}

func TestEncodePublicKey(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	encoded := kp.EncodePublicKey()

	// Should be 64 hex characters (32 bytes * 2)
	if len(encoded) != 64 {
		t.Errorf("EncodePublicKey() length = %d, want 64", len(encoded))
	}

	// Should be valid hex
	_, err = hex.DecodeString(encoded)
	if err != nil {
		t.Errorf("EncodePublicKey() produced invalid hex: %v", err)
	}
}

func TestEncodePrivateKey(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	encoded := kp.EncodePrivateKey()

	// Should be 64 hex characters (32 bytes * 2)
	if len(encoded) != 64 {
		t.Errorf("EncodePrivateKey() length = %d, want 64", len(encoded))
	}

	// Should be valid hex
	_, err = hex.DecodeString(encoded)
	if err != nil {
		t.Errorf("EncodePrivateKey() produced invalid hex: %v", err)
	}
}

func TestDecodePrivateKey(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	encoded := kp.EncodePrivateKey()
	decoded, err := e.DecodePrivateKey(encoded)
	if err != nil {
		t.Fatalf("DecodePrivateKey() error = %v", err)
	}

	if !bytes.Equal(decoded.Bytes(), kp.PrivateKey.Bytes()) {
		t.Error("Decoded private key does not match original")
	}
}

func TestDecodePublicKey(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	encoded := kp.EncodePublicKey()
	decoded, err := e.DecodePublicKey(encoded)
	if err != nil {
		t.Fatalf("DecodePublicKey() error = %v", err)
	}

	if !bytes.Equal(decoded.Bytes(), kp.PublicKey.Bytes()) {
		t.Error("Decoded public key does not match original")
	}
}

func TestDecodePrivateKey_InvalidHex(t *testing.T) {
	e := NewEngine()
	_, err := e.DecodePrivateKey("not-valid-hex!")
	if err == nil {
		t.Error("DecodePrivateKey() should error on invalid hex")
	}
}

func TestDecodePublicKey_InvalidHex(t *testing.T) {
	e := NewEngine()
	_, err := e.DecodePublicKey("not-valid-hex!")
	if err == nil {
		t.Error("DecodePublicKey() should error on invalid hex")
	}
}

func TestDecodePrivateKey_WrongLength(t *testing.T) {
	e := NewEngine()
	// 16 bytes instead of 32
	shortKey := strings.Repeat("00", 16)
	_, err := e.DecodePrivateKey(shortKey)
	if err == nil {
		t.Error("DecodePrivateKey() should error on wrong length key")
	}
}

func TestDecodePublicKey_WrongLength(t *testing.T) {
	e := NewEngine()
	// 16 bytes instead of 32
	shortKey := strings.Repeat("00", 16)
	_, err := e.DecodePublicKey(shortKey)
	if err == nil {
		t.Error("DecodePublicKey() should error on wrong length key")
	}
}

func TestEncryptDecrypt_RoundTrip(t *testing.T) {
	e := NewEngine()

	// Generate sender and recipient key pairs
	sender, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for sender error = %v", err)
	}

	recipient, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for recipient error = %v", err)
	}

	plaintext := []byte("Hello, World! This is a secret message.")

	// Encrypt
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Decrypt
	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Errorf("Decrypted message = %q, want %q", decrypted, plaintext)
	}
}

func TestEncryptDecrypt_EmptyMessage(t *testing.T) {
	e := NewEngine()

	sender, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for sender error = %v", err)
	}

	recipient, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for recipient error = %v", err)
	}

	plaintext := []byte("")

	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Errorf("Decrypted message = %q, want empty string", decrypted)
	}
}

func TestEncryptDecrypt_LargeMessage(t *testing.T) {
	e := NewEngine()

	sender, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for sender error = %v", err)
	}

	recipient, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for recipient error = %v", err)
	}

	// Create a 1MB message
	plaintext := make([]byte, 1024*1024)
	if _, err := rand.Read(plaintext); err != nil {
		t.Fatalf("Failed to generate random plaintext: %v", err)
	}

	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Error("Decrypted large message does not match original")
	}
}

func TestEncryptDecrypt_BinaryData(t *testing.T) {
	e := NewEngine()

	sender, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for sender error = %v", err)
	}

	recipient, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() for recipient error = %v", err)
	}

	// Binary data with null bytes and special characters
	plaintext := []byte{0x00, 0x01, 0x02, 0xff, 0xfe, 0x00, 0x00, 0x10, 0x20}

	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Errorf("Decrypted binary data = %v, want %v", decrypted, plaintext)
	}
}

func TestDecrypt_WrongRecipientKey(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	wrongRecipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Secret message")
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Try to decrypt with wrong recipient's private key
	_, err = e.Decrypt(wrongRecipient.PrivateKey, sender.PublicKey, ciphertext)
	if err == nil {
		t.Error("Decrypt() should fail with wrong recipient key")
	}
}

func TestDecrypt_WrongSenderPublicKey(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	wrongSender, _ := e.GenerateKeyPair()

	plaintext := []byte("Secret message")
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Try to decrypt with wrong sender's public key
	_, err = e.Decrypt(recipient.PrivateKey, wrongSender.PublicKey, ciphertext)
	if err == nil {
		t.Error("Decrypt() should fail with wrong sender public key")
	}
}

func TestDecrypt_TamperedCiphertext(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Secret message")
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Tamper with the ciphertext (flip a bit in the actual ciphertext portion)
	if len(ciphertext) > 60 {
		ciphertext[60] ^= 0x01
	}

	_, err = e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err == nil {
		t.Error("Decrypt() should fail with tampered ciphertext")
	}
}

func TestDecrypt_TruncatedMessage(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Secret message")
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Truncate the message
	truncated := ciphertext[:len(ciphertext)-10]

	_, err = e.Decrypt(recipient.PrivateKey, sender.PublicKey, truncated)
	if err == nil {
		t.Error("Decrypt() should fail with truncated ciphertext")
	}
}

func TestDecrypt_MessageTooShort(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	// Message shorter than pubKeySize + nonceSize (32 + 24 = 56 bytes)
	shortMessage := make([]byte, 50)

	_, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, shortMessage)
	if err == nil {
		t.Error("Decrypt() should fail with message too short")
	}
	if err.Error() != "message too short" {
		t.Errorf("Decrypt() error = %v, want 'message too short'", err)
	}
}

func TestDecrypt_InvalidEphemeralPublicKey(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	// Create a message with invalid ephemeral public key (all zeros)
	invalidMessage := make([]byte, 100)

	_, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, invalidMessage)
	// This should fail during decryption due to authentication failure
	if err == nil {
		t.Error("Decrypt() should fail with invalid ephemeral public key")
	}
}

func TestEncrypt_DifferentCiphertextEachTime(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Same message")

	ciphertext1, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("First Encrypt() error = %v", err)
	}

	ciphertext2, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Second Encrypt() error = %v", err)
	}

	// Due to ephemeral keys and random nonce, ciphertexts should be different
	if bytes.Equal(ciphertext1, ciphertext2) {
		t.Error("Encrypting same message twice should produce different ciphertexts (due to ephemeral keys)")
	}

	// But both should decrypt to the same plaintext
	decrypted1, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext1)
	if err != nil {
		t.Fatalf("Decrypt first error = %v", err)
	}
	decrypted2, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext2)
	if err != nil {
		t.Fatalf("Decrypt second error = %v", err)
	}

	if !bytes.Equal(decrypted1, plaintext) || !bytes.Equal(decrypted2, plaintext) {
		t.Error("Both ciphertexts should decrypt to the same plaintext")
	}
}

func TestCiphertextFormat(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Test message")

	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Ciphertext format: [EphemeralPub (32)] [Nonce (24)] [Ciphertext + Tag (len(plaintext) + 16)]
	expectedMinLength := 32 + 24 + len(plaintext) + 16 // 16 is Poly1305 tag size
	if len(ciphertext) != expectedMinLength {
		t.Errorf("Ciphertext length = %d, want %d", len(ciphertext), expectedMinLength)
	}
}

func TestKeyPairConsistency(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	// Public key should be derivable from private key
	derivedPub := kp.PrivateKey.PublicKey()
	if !bytes.Equal(derivedPub.Bytes(), kp.PublicKey.Bytes()) {
		t.Error("Public key should be consistent with private key")
	}
}

func TestEncodeDecodeRoundTrip(t *testing.T) {
	e := NewEngine()
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	// Test private key round trip
	privEncoded := kp.EncodePrivateKey()
	privDecoded, err := e.DecodePrivateKey(privEncoded)
	if err != nil {
		t.Fatalf("DecodePrivateKey() error = %v", err)
	}

	// Test public key round trip
	pubEncoded := kp.EncodePublicKey()
	pubDecoded, err := e.DecodePublicKey(pubEncoded)
	if err != nil {
		t.Fatalf("DecodePublicKey() error = %v", err)
	}

	// Use decoded keys for encryption/decryption
	recipient, _ := e.GenerateKeyPair()
	plaintext := []byte("Round trip test")

	ciphertext, err := e.Encrypt(privDecoded, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt with decoded key error = %v", err)
	}

	decrypted, err := e.Decrypt(recipient.PrivateKey, pubDecoded, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt with decoded key error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Error("Decoded keys should work correctly for encryption/decryption")
	}
}

func TestMultipleRecipients(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient1, _ := e.GenerateKeyPair()
	recipient2, _ := e.GenerateKeyPair()

	plaintext := []byte("Message for multiple recipients")

	// Encrypt for recipient1
	ciphertext1, err := e.Encrypt(sender.PrivateKey, recipient1.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt for recipient1 error = %v", err)
	}

	// Encrypt for recipient2
	ciphertext2, err := e.Encrypt(sender.PrivateKey, recipient2.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt for recipient2 error = %v", err)
	}

	// Both should be able to decrypt their own messages
	decrypted1, err := e.Decrypt(recipient1.PrivateKey, sender.PublicKey, ciphertext1)
	if err != nil {
		t.Fatalf("Decrypt for recipient1 error = %v", err)
	}
	if !bytes.Equal(decrypted1, plaintext) {
		t.Error("Recipient1 should be able to decrypt their message")
	}

	decrypted2, err := e.Decrypt(recipient2.PrivateKey, sender.PublicKey, ciphertext2)
	if err != nil {
		t.Fatalf("Decrypt for recipient2 error = %v", err)
	}
	if !bytes.Equal(decrypted2, plaintext) {
		t.Error("Recipient2 should be able to decrypt their message")
	}

	// Cross decryption should fail
	_, err = e.Decrypt(recipient1.PrivateKey, sender.PublicKey, ciphertext2)
	if err == nil {
		t.Error("Recipient1 should NOT be able to decrypt recipient2's message")
	}

	_, err = e.Decrypt(recipient2.PrivateKey, sender.PublicKey, ciphertext1)
	if err == nil {
		t.Error("Recipient2 should NOT be able to decrypt recipient1's message")
	}
}

func TestUnicodeMessage(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	// Unicode message with various scripts
	plaintext := []byte("Hello 世界! Привет мир! 🔐🔑")

	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Errorf("Decrypted unicode message = %q, want %q", decrypted, plaintext)
	}
}

// Benchmark tests
func BenchmarkGenerateKeyPair(b *testing.B) {
	e := NewEngine()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_, err := e.GenerateKeyPair()
		if err != nil {
			b.Fatal(err)
		}
	}
}

func BenchmarkEncrypt(b *testing.B) {
	e := NewEngine()
	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	plaintext := []byte("Benchmark encryption test message")

	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
		if err != nil {
			b.Fatal(err)
		}
	}
}

func BenchmarkDecrypt(b *testing.B) {
	e := NewEngine()
	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	plaintext := []byte("Benchmark decryption test message")
	ciphertext, _ := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)

	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
		if err != nil {
			b.Fatal(err)
		}
	}
}

func BenchmarkEncryptLargeMessage(b *testing.B) {
	e := NewEngine()
	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	plaintext := make([]byte, 1024*1024) // 1MB
	rand.Read(plaintext)

	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		_, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
		if err != nil {
			b.Fatal(err)
		}
	}
}

// Test that verifies the authentication property - message must come from correct sender
func TestSenderAuthentication(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()
	imposter, _ := e.GenerateKeyPair()

	plaintext := []byte("Authenticated message")

	// Sender encrypts
	ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	// Recipient tries to decrypt thinking it's from imposter - should fail
	_, err = e.Decrypt(recipient.PrivateKey, imposter.PublicKey, ciphertext)
	if err == nil {
		t.Error("Decryption should fail when wrong sender public key is provided")
	}

	// Recipient decrypts with correct sender public key - should succeed
	decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() with correct sender should succeed: %v", err)
	}
	if !bytes.Equal(decrypted, plaintext) {
		t.Error("Decrypted message should match plaintext")
	}
}

// Test forward secrecy by verifying ephemeral keys are different each encryption
func TestForwardSecrecy(t *testing.T) {
	e := NewEngine()

	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	plaintext := []byte("Forward secrecy test")

	ciphertext1, _ := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
	ciphertext2, _ := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)

	// Extract ephemeral public keys (first 32 bytes)
	ephemeralPub1 := ciphertext1[:32]
	ephemeralPub2 := ciphertext2[:32]

	// Ephemeral keys should be different for each encryption
	if bytes.Equal(ephemeralPub1, ephemeralPub2) {
		t.Error("Ephemeral public keys should be different for each encryption (forward secrecy)")
	}
}

// Test with known test vectors (useful for cross-implementation testing)
func TestDecodeKnownKeys(t *testing.T) {
	e := NewEngine()

	// Generate a key pair and verify it can be encoded and decoded
	kp, err := e.GenerateKeyPair()
	if err != nil {
		t.Fatalf("GenerateKeyPair() error = %v", err)
	}

	privHex := kp.EncodePrivateKey()
	pubHex := kp.EncodePublicKey()

	// Verify the hex strings are lowercase
	if privHex != strings.ToLower(privHex) {
		t.Error("Private key hex should be lowercase")
	}
	if pubHex != strings.ToLower(pubHex) {
		t.Error("Public key hex should be lowercase")
	}

	// Verify decoding
	priv, err := e.DecodePrivateKey(privHex)
	if err != nil {
		t.Fatalf("DecodePrivateKey() error = %v", err)
	}

	pub, err := e.DecodePublicKey(pubHex)
	if err != nil {
		t.Fatalf("DecodePublicKey() error = %v", err)
	}

	// Verify the decoded public key matches derived public key
	if !bytes.Equal(priv.PublicKey().Bytes(), pub.Bytes()) {
		t.Error("Decoded public key should match public key derived from decoded private key")
	}
}

// Test edge case: decode with uppercase hex (should work since hex.DecodeString handles both)
func TestDecodeUppercaseHex(t *testing.T) {
	e := NewEngine()

	kp, _ := e.GenerateKeyPair()

	privHex := strings.ToUpper(kp.EncodePrivateKey())
	pubHex := strings.ToUpper(kp.EncodePublicKey())

	priv, err := e.DecodePrivateKey(privHex)
	if err != nil {
		t.Fatalf("DecodePrivateKey() should handle uppercase hex: %v", err)
	}

	pub, err := e.DecodePublicKey(pubHex)
	if err != nil {
		t.Fatalf("DecodePublicKey() should handle uppercase hex: %v", err)
	}

	if !bytes.Equal(priv.Bytes(), kp.PrivateKey.Bytes()) {
		t.Error("Decoded private key should match original")
	}
	if !bytes.Equal(pub.Bytes(), kp.PublicKey.Bytes()) {
		t.Error("Decoded public key should match original")
	}
}

// Fuzz-like test with random message sizes
func TestRandomMessageSizes(t *testing.T) {
	e := NewEngine()
	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	sizes := []int{0, 1, 15, 16, 17, 31, 32, 33, 63, 64, 65, 127, 128, 256, 512, 1000, 4096}

	for _, size := range sizes {
		t.Run(string(rune(size)), func(t *testing.T) {
			plaintext := make([]byte, size)
			if size > 0 {
				rand.Read(plaintext)
			}

			ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
			if err != nil {
				t.Fatalf("Encrypt() with size %d error = %v", size, err)
			}

			decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
			if err != nil {
				t.Fatalf("Decrypt() with size %d error = %v", size, err)
			}

			if !bytes.Equal(decrypted, plaintext) {
				t.Errorf("Round trip failed for size %d", size)
			}
		})
	}
}

// Test concurrent encryption/decryption
func TestConcurrentOperations(t *testing.T) {
	e := NewEngine()
	sender, _ := e.GenerateKeyPair()
	recipient, _ := e.GenerateKeyPair()

	const numGoroutines = 100
	done := make(chan bool, numGoroutines)

	for i := 0; i < numGoroutines; i++ {
		go func(id int) {
			plaintext := []byte("Concurrent message " + string(rune('A'+id%26)))

			ciphertext, err := e.Encrypt(sender.PrivateKey, recipient.PublicKey, plaintext)
			if err != nil {
				t.Errorf("Goroutine %d: Encrypt() error = %v", id, err)
				done <- false
				return
			}

			decrypted, err := e.Decrypt(recipient.PrivateKey, sender.PublicKey, ciphertext)
			if err != nil {
				t.Errorf("Goroutine %d: Decrypt() error = %v", id, err)
				done <- false
				return
			}

			if !bytes.Equal(decrypted, plaintext) {
				t.Errorf("Goroutine %d: decrypted != plaintext", id)
				done <- false
				return
			}

			done <- true
		}(i)
	}

	for i := 0; i < numGoroutines; i++ {
		<-done
	}
}

// Test self-encryption (sender encrypts to themselves)
func TestSelfEncryption(t *testing.T) {
	e := NewEngine()
	user, _ := e.GenerateKeyPair()

	plaintext := []byte("Message to myself")

	ciphertext, err := e.Encrypt(user.PrivateKey, user.PublicKey, plaintext)
	if err != nil {
		t.Fatalf("Encrypt() error = %v", err)
	}

	decrypted, err := e.Decrypt(user.PrivateKey, user.PublicKey, ciphertext)
	if err != nil {
		t.Fatalf("Decrypt() error = %v", err)
	}

	if !bytes.Equal(decrypted, plaintext) {
		t.Error("Self-encryption should work correctly")
	}
}

// Ensure interface compatibility (Engine implements expected methods)
func TestEngineInterface(t *testing.T) {
	e := NewEngine()

	// Test that all public methods exist and have correct signatures
	var _ func() (*KeyPair, error) = e.GenerateKeyPair
	var _ func(string) (*ecdh.PrivateKey, error) = e.DecodePrivateKey
	var _ func(string) (*ecdh.PublicKey, error) = e.DecodePublicKey
	var _ func(*ecdh.PrivateKey, *ecdh.PublicKey, []byte) ([]byte, error) = e.Encrypt
	var _ func(*ecdh.PrivateKey, *ecdh.PublicKey, []byte) ([]byte, error) = e.Decrypt
}