test: implement encryption tests (#1399)

## What?

Implements a complete test suite for the `encryption.go`.

## Why?

The previous implementation had no test coverage for the
`encryption.go`.

Closes #886

Author: mavonx

config/encryption_test.go

@@ -0,0 +1,178 @@
+package config
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestEncryptDecrypt_RoundTrip(t *testing.T) {
+	salt := make([]byte, 32)
+
+	plaintext := []byte("hello")
+	key := DeriveKey("password", salt)
+
+	ciphertext, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	got, err := Decrypt(ciphertext, key)
+	if err != nil {
+		t.Fatalf("Decrypt: %v", err)
+	}
+
+	if !bytes.Equal(got, plaintext) {
+		t.Errorf("mismatch: got %q, want %q", got, plaintext)
+	}
+}
+
+func TestEncryptDecrypt_EmptyPlaintext(t *testing.T) {
+	salt := make([]byte, 32)
+	key := DeriveKey("password", salt)
+
+	ciphertext, err := Encrypt([]byte{}, key)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	got, err := Decrypt(ciphertext, key)
+	if err != nil {
+		t.Fatalf("Decrypt: %v", err)
+	}
+
+	if len(got) != 0 {
+		t.Errorf("expected empty plaintext, got %q", got)
+	}
+}
+
+func TestEncryptDecrypt_LongPassword(t *testing.T) {
+	salt := make([]byte, 32)
+
+	plaintext := []byte("hello")
+	key := DeriveKey(strings.Repeat("x", 10000), salt)
+
+	ciphertext, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	got, err := Decrypt(ciphertext, key)
+	if err != nil {
+		t.Fatalf("Decrypt: %v", err)
+	}
+
+	if !bytes.Equal(got, plaintext) {
+		t.Errorf("mismatch: got %q, want %q", got, plaintext)
+	}
+}
+
+func TestEncryptDecrypt_LongPlaintext(t *testing.T) {
+	salt := make([]byte, 32)
+
+	plaintext := bytes.Repeat([]byte("hello"), 1<<20)
+	key := DeriveKey("password", salt)
+
+	ciphertext, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	got, err := Decrypt(ciphertext, key)
+	if err != nil {
+		t.Fatalf("Decrypt: %v", err)
+	}
+
+	if !bytes.Equal(got, plaintext) {
+		t.Error("long plaintext round-trip mismatch")
+	}
+}
+
+func TestEncrypt_NonceIsRandom(t *testing.T) {
+	salt := make([]byte, 32)
+
+	plaintext := []byte("hello")
+	key := DeriveKey("password", salt)
+
+	c1, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt 1: %v", err)
+	}
+	c2, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt 2: %v", err)
+	}
+
+	if bytes.Equal(c1, c2) {
+		t.Error("two Encrypt calls produced identical ciphertext")
+	}
+}
+
+func TestDecrypt_WrongKey(t *testing.T) {
+	s1 := make([]byte, 32)
+	s2 := bytes.Repeat([]byte{1}, 32)
+
+	k1 := DeriveKey("p1", s1)
+	k2 := DeriveKey("p2", s2)
+
+	ciphertext, err := Encrypt([]byte("hello"), k1)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	if _, err := Decrypt(ciphertext, k2); err == nil {
+		t.Error("Decrypt with wrong key should return an error")
+	}
+}
+
+func TestDecrypt_CorruptedCiphertext(t *testing.T) {
+	salt := make([]byte, 32)
+
+	plaintext := []byte("hello")
+	key := DeriveKey("password", salt)
+
+	ciphertext, err := Encrypt(plaintext, key)
+	if err != nil {
+		t.Fatalf("Encrypt: %v", err)
+	}
+
+	ciphertext[0] ^= 0xff
+
+	if _, err := Decrypt(ciphertext, key); err == nil {
+		t.Error("Decrypt with corrupted ciphertext should return an error")
+	}
+}
+
+func TestDeriveKey_Deterministic(t *testing.T) {
+	salt := make([]byte, 32)
+
+	k1 := DeriveKey("password", salt)
+	k2 := DeriveKey("password", salt)
+
+	if !bytes.Equal(k1, k2) {
+		t.Error("DeriveKey should be deterministic for the same input")
+	}
+}
+
+func TestDeriveKey_DifferentPasswords(t *testing.T) {
+	salt := make([]byte, 32)
+
+	k1 := DeriveKey("password1", salt)
+	k2 := DeriveKey("password2", salt)
+
+	if bytes.Equal(k1, k2) {
+		t.Error("different passwords should produce different keys")
+	}
+}
+
+func TestDeriveKey_DifferentSalts(t *testing.T) {
+	s1 := make([]byte, 32)
+	s2 := bytes.Repeat([]byte{1}, 32)
+
+	k1 := DeriveKey("password", s1)
+	k2 := DeriveKey("password", s2)
+
+	if bytes.Equal(k1, k2) {
+		t.Error("different salts should produce different keys")
+	}
+}