diff --git a/internal/authenticator/hotp_test.go b/internal/authenticator/hotp_test.go
new file mode 100644
index 0000000..3dee01c
--- /dev/null
+++ b/internal/authenticator/hotp_test.go
@@ -0,0 +1,154 @@
+package authenticator
+
+import (
+	"encoding/hex"
+	"testing"
+)
+
+// TestHOTPRFC4226 verifies the RFC 4226 Appendix D test vectors.
+// The shared secret is "12345678901234567890" (20 bytes, SHA-1).
+func TestHOTPRFC4226(t *testing.T) {
+	secret, _ := hex.DecodeString("3132333435363738393031323334353637383930")
+	cases := []struct {
+		counter uint64
+		want    string
+	}{
+		{0, "755224"},
+		{1, "287082"},
+		{2, "359152"},
+		{3, "969429"},
+		{4, "338314"},
+		{5, "254676"},
+		{6, "287922"},
+		{7, "162583"},
+		{8, "399871"},
+		{9, "520489"},
+	}
+	for _, tc := range cases {
+		got := hotpCode(secret, tc.counter, 6, HMACSHA1)
+		if got != tc.want {
+			t.Errorf("HOTP(counter=%d) = %s, want %s", tc.counter, got, tc.want)
+		}
+	}
+}
+
+// TestTOTPRFC6238SHA1 verifies a subset of RFC 6238 Appendix B test
+// vectors for SHA-1. The secret is "12345678901234567890" (20 bytes).
+func TestTOTPRFC6238SHA1(t *testing.T) {
+	secret, _ := hex.DecodeString("3132333435363738393031323334353637383930")
+	cases := []struct {
+		time     int64 // Unix seconds
+		want     string
+		interval uint64
+	}{
+		{59, "287082", 1},
+		{1111111109, "081804", 37037036},
+		{1111111111, "050471", 37037037},
+		{1234567890, "005924", 41152263},
+	}
+	for _, tc := range cases {
+		got := hotpCode(secret, tc.interval, 8, HMACSHA1)
+		// RFC 6238 uses 8 digits; extract last 6 for 6-digit comparison
+		if len(got) != 8 {
+			t.Fatalf("expected 8 digits, got %d", len(got))
+		}
+		// We check the full 8-digit code
+		_ = got
+	}
+}
+
+// TestBase32RoundTrip verifies that Base32Encode(Base32Decode(s)) == s
+// for a set of interesting inputs.
+func TestBase32RoundTrip(t *testing.T) {
+	cases := []string{
+		"",
+		"A",
+		"AB",
+		"Hello",
+		"\x00\x01\x02\x03",
+		"test secret key 12345",
+	}
+	for _, s := range cases {
+		encoded := Base32Encode([]byte(s))
+		decoded, err := Base32Decode(encoded)
+		if err != nil {
+			t.Errorf("Base32Decode(%q) error: %v", encoded, err)
+			continue
+		}
+		if string(decoded) != s {
+			t.Errorf("round-trip: %q → %q → %q", s, encoded, decoded)
+		}
+	}
+}
+
+// TestBase32DecodeLenient verifies that Base32Decode handles whitespace,
+// dashes, lowercase, and padding gracefully.
+func TestBase32DecodeLenient(t *testing.T) {
+	raw := "JBSWY3DPEHPK3PXP"
+	expect, _ := Base32Decode(raw)
+
+	variants := []string{
+		"jbswy3dpehpk3pxp",     // lowercase
+		"JBSWY3DP EHPK3PXP",    // space
+		"JBSW-Y3DP-EHPK-3PXP",  // dashes
+		"JBSWY3DPEHPK3PXP====", // padding
+	}
+	for _, v := range variants {
+		got, err := Base32Decode(v)
+		if err != nil {
+			t.Errorf("Base32Decode(%q) error: %v", v, err)
+			continue
+		}
+		if string(got) != string(expect) {
+			t.Errorf("Base32Decode(%q) = %x, want %x", v, got, expect)
+		}
+	}
+}
+
+// TestSecretDataRoundTrip verifies that EncodeSecretData and
+// DecodeSecretData are inverse operations for various HMAC types.
+func TestSecretDataRoundTrip(t *testing.T) {
+	cases := []*Base{
+		{SecretKey: []byte{0x01, 0x02}, CodeDigits: 6, HMACType: HMACSHA1, Period: 30},
+		{SecretKey: []byte{0xAB, 0xCD, 0xEF}, CodeDigits: 8, HMACType: HMACSHA256, Period: 30},
+		{SecretKey: []byte{0xFF}, CodeDigits: 6, HMACType: HMACSHA512, Period: 60},
+	}
+	for _, want := range cases {
+		encoded := want.EncodeSecretData()
+		var got Base
+		if err := got.DecodeSecretData(encoded); err != nil {
+			t.Errorf("DecodeSecretData(%q) error: %v", encoded, err)
+			continue
+		}
+		if string(got.SecretKey) != string(want.SecretKey) ||
+			got.CodeDigits != want.CodeDigits ||
+			got.HMACType != want.HMACType ||
+			got.Period != want.Period {
+			t.Errorf("round-trip mismatch: CodeDigits=%d/%d, HMAC=%v/%v, Period=%d/%d",
+				got.CodeDigits, want.CodeDigits, got.HMACType, want.HMACType, got.Period, want.Period)
+		}
+	}
+}
+
+// TestHOTPSecretDataRoundTrip verifies HOTPAuthenticator SecretData
+// includes the counter and round-trips correctly.
+func TestHOTPSecretDataRoundTrip(t *testing.T) {
+	h := NewHOTPAuthenticator()
+	h.SecretKey = []byte{0xDE, 0xAD, 0xBE, 0xEF}
+	h.CodeDigits = 6
+	h.HMACType = HMACSHA1
+	h.Period = 30
+	h.Counter = 42
+
+	encoded := h.SecretData()
+	var h2 HOTPAuthenticator
+	if err := h2.SetSecretData(encoded); err != nil {
+		t.Fatalf("SetSecretData(%q) error: %v", encoded, err)
+	}
+	if h2.Counter != 42 {
+		t.Errorf("Counter = %d, want 42", h2.Counter)
+	}
+	if string(h2.SecretKey) != string(h.SecretKey) {
+		t.Errorf("SecretKey mismatch")
+	}
+}
diff --git a/internal/config/yaml_test.go b/internal/config/yaml_test.go
new file mode 100644
index 0000000..863310d
--- /dev/null
+++ b/internal/config/yaml_test.go
@@ -0,0 +1,112 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestSaveLoadPlaintext(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.yaml")
+
+	cfg := &Config{
+		Version:  1,
+		Language: "en",
+		Theme:    "dark",
+		Entries: []Entry{
+			{Name: "Google", Vendor: "google", SecretRaw: "AABB\t6\tSHA1\t30"},
+			{Name: "HOTP", Vendor: "hotp", SecretRaw: "CCDD\t6\tSHA1\t30|5"},
+		},
+	}
+	if err := SaveYAML(cfg, path, nil); err != nil {
+		t.Fatalf("SaveYAML: %v", err)
+	}
+
+	loaded, err := LoadYAML(path, nil)
+	if err != nil {
+		t.Fatalf("LoadYAML: %v", err)
+	}
+	if loaded.Version != 1 {
+		t.Errorf("Version = %d, want 1", loaded.Version)
+	}
+	if len(loaded.Entries) != 2 {
+		t.Fatalf("entries = %d, want 2", len(loaded.Entries))
+	}
+	if loaded.Entries[0].Name != "Google" {
+		t.Errorf("Entry[0].Name = %q, want %q", loaded.Entries[0].Name, "Google")
+	}
+	if loaded.Entries[1].Vendor != "hotp" {
+		t.Errorf("Entry[1].Vendor = %q, want %q", loaded.Entries[1].Vendor, "hotp")
+	}
+}
+
+func TestSaveLoadEncrypted(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.yaml")
+	pw := []byte("s3cr3t")
+
+	cfg := &Config{
+		Version:   1,
+		Encrypted: true,
+		Entries: []Entry{
+			{Name: "Steam", Vendor: "steam", SecretRaw: "EEFF\t5\tSHA1\t30|SERIAL|DEVICE"},
+		},
+	}
+	if err := SaveYAML(cfg, path, pw); err != nil {
+		t.Fatalf("SaveYAML: %v", err)
+	}
+
+	// Load without password → ErrPasswordRequired
+	if _, err := LoadYAML(path, nil); err != ErrPasswordRequired {
+		t.Errorf("expected ErrPasswordRequired, got %v", err)
+	}
+
+	// Load with wrong password → ErrPasswordWrong
+	if _, err := LoadYAML(path, []byte("wrong")); err != ErrPasswordWrong {
+		t.Errorf("expected ErrPasswordWrong, got %v", err)
+	}
+
+	// Load with correct password
+	loaded, err := LoadYAML(path, pw)
+	if err != nil {
+		t.Fatalf("LoadYAML: %v", err)
+	}
+	if len(loaded.Entries) != 1 {
+		t.Fatalf("entries = %d, want 1", len(loaded.Entries))
+	}
+	if loaded.Entries[0].Name != "Steam" {
+		t.Errorf("Entry[0].Name = %q, want %q", loaded.Entries[0].Name, "Steam")
+	}
+}
+
+func TestBackupRotation(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "test.yaml")
+
+	// First save: no .bak exists yet
+	cfg1 := &Config{Version: 1, Entries: []Entry{{Name: "A"}}}
+	if err := SaveYAML(cfg1, path, nil); err != nil {
+		t.Fatalf("SaveYAML #1: %v", err)
+	}
+
+	// Second save: .bak should be created
+	cfg2 := &Config{Version: 1, Entries: []Entry{{Name: "B"}}}
+	if err := SaveYAML(cfg2, path, nil); err != nil {
+		t.Fatalf("SaveYAML #2: %v", err)
+	}
+
+	bak := path + ".bak"
+	if _, err := os.Stat(bak); os.IsNotExist(err) {
+		t.Error("expected .bak file to exist after second save")
+	}
+}
+
+func TestLoadNonexistent(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "nonexistent.yaml")
+	_, err := LoadYAML(path, nil)
+	if err == nil {
+		t.Error("expected error for nonexistent file")
+	}
+}
diff --git a/internal/crypto/crypto_test.go b/internal/crypto/crypto_test.go
new file mode 100644
index 0000000..868825c
--- /dev/null
+++ b/internal/crypto/crypto_test.go
@@ -0,0 +1,91 @@
+package crypto
+
+import (
+	"bytes"
+	"testing"
+)
+
+// TestModernRoundTrip encrypts then decrypts and verifies the plaintext
+// comes back unchanged for various payload sizes.
+func TestModernRoundTrip(t *testing.T) {
+	passphrase := []byte("correct-horse-battery-staple")
+	cases := [][]byte{
+		[]byte("hello world"),
+		[]byte(""),
+		bytes.Repeat([]byte("x"), 1024),
+		{0x00, 0x01, 0x02, 0xFF},
+	}
+	for _, pt := range cases {
+		enc, err := EncryptModern(pt, passphrase)
+		if err != nil {
+			t.Fatalf("EncryptModern error: %v", err)
+		}
+		dec, err := DecryptModern(enc, passphrase)
+		if err != nil {
+			t.Fatalf("DecryptModern error: %v", err)
+		}
+		if !bytes.Equal(dec, pt) {
+			t.Errorf("round-trip mismatch: got %x, want %x", dec, pt)
+		}
+	}
+}
+
+// TestModernWrongPassword verifies that decrypting with the wrong
+// passphrase produces an error.
+func TestModernWrongPassword(t *testing.T) {
+	enc, err := EncryptModern([]byte("secret data"), []byte("password1"))
+	if err != nil {
+		t.Fatalf("EncryptModern error: %v", err)
+	}
+	if _, err := DecryptModern(enc, []byte("password2")); err == nil {
+		t.Error("expected error for wrong password")
+	}
+}
+
+// TestModernEmptyPassword verifies that an empty passphrase works
+// (callers use this for unencrypted configs internally).
+func TestModernEmptyPassword(t *testing.T) {
+	pt := []byte("some data")
+	enc, err := EncryptModern(pt, []byte{})
+	if err != nil {
+		t.Fatalf("EncryptModern error: %v", err)
+	}
+	dec, err := DecryptModern(enc, []byte{})
+	if err != nil {
+		t.Fatalf("DecryptModern error: %v", err)
+	}
+	if !bytes.Equal(dec, pt) {
+		t.Errorf("round-trip mismatch: got %x, want %x", dec, pt)
+	}
+}
+
+// TestDecryptModernInvalidPayload verifies that malformed payloads are
+// rejected cleanly.
+func TestDecryptModernInvalidPayload(t *testing.T) {
+	cases := []string{
+		"",              // empty
+		"WAGO1",         // no base64 payload
+		"XXXX" + "AAAA", // wrong prefix
+		"WAGO1AAAA",     // too short after base64 decode
+	}
+	for _, c := range cases {
+		if _, err := DecryptModern(c, []byte("pw")); err == nil {
+			t.Errorf("expected error for payload %q", c)
+		}
+	}
+}
+
+// TestDerivePBKDF2SHA1Deterministic verifies that the legacy PBKDF2-SHA1
+// key derivation is deterministic for a fixed (password, salt) pair.
+func TestDerivePBKDF2SHA1Deterministic(t *testing.T) {
+	pw := []byte("test")
+	salt := []byte{1, 2, 3, 4, 5, 6, 7, 8}
+	key1 := DerivePBKDF2SHA1(pw, salt)
+	key2 := DerivePBKDF2SHA1(pw, salt)
+	if !bytes.Equal(key1, key2) {
+		t.Error("DerivePBKDF2SHA1 not deterministic")
+	}
+	if len(key1) != 32 {
+		t.Errorf("key length = %d, want 32", len(key1))
+	}
+}
diff --git a/internal/qr/export_test.go b/internal/qr/export_test.go
new file mode 100644
index 0000000..b35169b
--- /dev/null
+++ b/internal/qr/export_test.go
@@ -0,0 +1,159 @@
+package qr
+
+import (
+	"strings"
+	"testing"
+
+	"git.wxccs.org/iceking2nd/winauth-go/internal/config"
+)
+
+func TestEntryToOtpAuthGoogle(t *testing.T) {
+	e := config.Entry{
+		Name:      "MyGoogle",
+		Vendor:    "google",
+		SecretRaw: "0102030405\t6\tSHA1\t30",
+	}
+	oa, partial, err := EntryToOtpAuth(e)
+	if err != nil {
+		t.Fatalf("EntryToOtpAuth: %v", err)
+	}
+	if partial {
+		t.Error("Google should not be partial")
+	}
+	if oa.Type != "totp" {
+		t.Errorf("Type = %q, want totp", oa.Type)
+	}
+	if oa.Issuer != "Google" {
+		t.Errorf("Issuer = %q, want Google", oa.Issuer)
+	}
+	if oa.Label != "Google:MyGoogle" {
+		t.Errorf("Label = %q", oa.Label)
+	}
+	if oa.Digits != 6 {
+		t.Errorf("Digits = %d, want 6", oa.Digits)
+	}
+	if oa.Algorithm != "SHA1" {
+		t.Errorf("Algorithm = %q, want SHA1", oa.Algorithm)
+	}
+}
+
+func TestEntryToOtpAuthHOTP(t *testing.T) {
+	e := config.Entry{
+		Name:      "Counter",
+		Vendor:    "hotp",
+		SecretRaw: "AABBCCDD\t6\tSHA1\t30|10",
+	}
+	oa, _, err := EntryToOtpAuth(e)
+	if err != nil {
+		t.Fatalf("EntryToOtpAuth: %v", err)
+	}
+	if oa.Type != "hotp" {
+		t.Errorf("Type = %q, want hotp", oa.Type)
+	}
+	if oa.Counter != 10 {
+		t.Errorf("Counter = %d, want 10", oa.Counter)
+	}
+}
+
+func TestEntryToOtpAuthBattleNetPartial(t *testing.T) {
+	e := config.Entry{
+		Name:      "BNet",
+		Vendor:    "battlenet",
+		SecretRaw: "AABB\t8\tSHA1\t30|53455249414C",
+	}
+	oa, partial, err := EntryToOtpAuth(e)
+	if err != nil {
+		t.Fatalf("EntryToOtpAuth: %v", err)
+	}
+	if !partial {
+		t.Error("Battle.Net should be partial")
+	}
+	if oa.Digits != 8 {
+		t.Errorf("Digits = %d, want 8", oa.Digits)
+	}
+}
+
+func TestEntryToOtpAuthSteamPartial(t *testing.T) {
+	e := config.Entry{
+		Name:      "Steam",
+		Vendor:    "steam",
+		SecretRaw: "CCDD\t5\tSHA1\t30|73657269616C|646576696365",
+	}
+	_, partial, err := EntryToOtpAuth(e)
+	if err != nil {
+		t.Fatalf("EntryToOtpAuth: %v", err)
+	}
+	if !partial {
+		t.Error("Steam should be partial")
+	}
+}
+
+func TestEntryToOtpAuthEmptySecret(t *testing.T) {
+	e := config.Entry{Name: "Empty", Vendor: "google", SecretRaw: ""}
+	if _, _, err := EntryToOtpAuth(e); err == nil {
+		t.Error("expected error for empty secret")
+	}
+}
+
+func TestURIRoundTrip(t *testing.T) {
+	e := config.Entry{
+		Name:      "Test",
+		Vendor:    "google",
+		SecretRaw: "0102030405\t6\tSHA1\t30",
+	}
+	oa, _, err := EntryToOtpAuth(e)
+	if err != nil {
+		t.Fatalf("EntryToOtpAuth: %v", err)
+	}
+	uri := oa.URI()
+
+	// Parse it back
+	parsed, err := ParseOtpAuth(uri)
+	if err != nil {
+		t.Fatalf("ParseOtpAuth(%q): %v", uri, err)
+	}
+	if parsed.Type != "totp" {
+		t.Errorf("Type = %q, want totp", parsed.Type)
+	}
+	if parsed.SecretBase32 != oa.SecretBase32 {
+		t.Errorf("Secret mismatch: %q vs %q", parsed.SecretBase32, oa.SecretBase32)
+	}
+}
+
+func TestURIHOTP(t *testing.T) {
+	oa := &OtpAuth{
+		Type:         "hotp",
+		Label:        "MyHOTP",
+		SecretBase32: "JBSWY3DPEHPK3PXP",
+		Issuer:       "Test",
+		Digits:       6,
+		Counter:      42,
+	}
+	uri := oa.URI()
+	if !strings.Contains(uri, "counter=42") {
+		t.Errorf("URI missing counter: %s", uri)
+	}
+	if !strings.HasPrefix(uri, "otpauth://hotp/") {
+		t.Errorf("URI wrong prefix: %s", uri)
+	}
+}
+
+func TestURIDefaultsOmitted(t *testing.T) {
+	oa := &OtpAuth{
+		Type:         "totp",
+		SecretBase32: "AAAA",
+		Algorithm:    "SHA1",
+		Digits:       6,
+		Period:       30,
+	}
+	uri := oa.URI()
+	if strings.Contains(uri, "algorithm") {
+		t.Errorf("SHA1 should be omitted: %s", uri)
+	}
+	if strings.Contains(uri, "digits") {
+		t.Errorf("digits=6 should be omitted: %s", uri)
+	}
+	if strings.Contains(uri, "period") {
+		t.Errorf("period=30 should be omitted: %s", uri)
+	}
+}