4 files changed, 238 insertions, 0 deletions
diff --git a/compat-test/README.md b/compat-test/README.md
new file mode 100644
index 0000000..c1e91b8
--- /dev/null
+++ b/compat-test/README.md
@@ -0,0 +1,44 @@
+# Kate libsodium Compatibility Tests
+
+This directory contains tests to verify that the `kate` package's `seal` and `open` functions are compatible with the real libsodium implementation.
+
+## Why?
+
+The `kate` library is designed to be CGO-free to avoid the complexity and deployment challenges that come with C dependencies. Rather than requiring applications to link against libsodium, `kate` implements a pure Go version of the NaCl secretbox encryption scheme.
+
+This separate compatibility test project verifies that `kate`'s implementation produces ciphertext that is fully interoperable with libsodium while keeping the CGO dependency isolated from the main package. This way, applications get the benefits of a pure Go library while maintaining confidence in libsodium compatibility.
+
+## Setup
+
+The tests require libsodium to be installed on your system. The project uses the `github.com/jamesruan/sodium` Go bindings for libsodium.
+
+### Installing libsodium
+
+On macOS:
+```bash
+brew install libsodium
+```
+
+On Ubuntu/Debian:
+```bash
+sudo apt-get install libsodium-dev
+```
+
+On CentOS/RHEL:
+```bash
+sudo yum install libsodium-devel
+```
+
+## Running the Tests
+
+```bash
+cd compat-test
+go test
+```
+
+## Test Cases
+
+The test suite includes:
+
+1. **KateEncrypt_LibsodiumDecrypt**: Encrypts data with kate's library and decrypts with libsodium
+2. **LibsodiumEncrypt_KateDecrypt**: Encrypts data with libsodium and decrypts with kate's library
diff --git a/compat-test/compat_test.go b/compat-test/compat_test.go
new file mode 100644
index 0000000..2b0bfd7
--- /dev/null
+++ b/compat-test/compat_test.go
@@ -0,0 +1,167 @@
+package main
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/quick"
+
+	"git.tjp.lol/authentic_kate"
+	"github.com/jamesruan/sodium"
+)
+
+type ByteSerDes struct{}
+
+func (ByteSerDes) Serialize(w io.Writer, data []byte) error {
+	_, err := w.Write(data)
+	return err
+}
+
+func (ByteSerDes) Deserialize(r io.Reader, data *[]byte) error {
+	buf, err := io.ReadAll(r)
+	if err != nil {
+		return err
+	}
+	*data = buf
+	return nil
+}
+
+func kateSeal(key [32]byte, plaintext []byte) []byte {
+	keyHex := hex.EncodeToString(key[:])
+
+	auth := kate.New(keyHex, kate.AuthConfig[[]byte]{
+		SerDes:     ByteSerDes{},
+		CookieName: "test",
+	})
+
+	w := httptest.NewRecorder()
+	if err := auth.Set(w, plaintext); err != nil {
+		panic(err)
+	}
+
+	cookies := w.Result().Cookies()
+	if len(cookies) == 0 {
+		panic("No cookie set")
+	}
+
+	encryptedBytes, err := hex.DecodeString(cookies[0].Value)
+	if err != nil {
+		panic(err)
+	}
+
+	return encryptedBytes
+}
+
+func libsodiumSeal(key [32]byte, plaintext []byte) []byte {
+	var nonce [24]byte
+	if _, err := rand.Read(nonce[:]); err != nil {
+		panic(err)
+	}
+
+	ciphertextAndMac := sodium.Bytes(plaintext).SecretBox(
+		sodium.SecretBoxNonce{Bytes: nonce[:]},
+		sodium.SecretBoxKey{Bytes: key[:]},
+	)
+
+	result := make([]byte, 24+len(ciphertextAndMac))
+	copy(result[:24], nonce[:])
+	copy(result[24:], ciphertextAndMac)
+
+	return result
+}
+
+func kateOpen(key [32]byte, box []byte) ([]byte, bool) {
+	keyHex := hex.EncodeToString(key[:])
+
+	auth := kate.New(keyHex, kate.AuthConfig[[]byte]{
+		SerDes:     ByteSerDes{},
+		CookieName: "test",
+	})
+
+	cookieValue := hex.EncodeToString(box)
+	req := httptest.NewRequest("GET", "/", nil)
+	req.AddCookie(&http.Cookie{Name: "test", Value: cookieValue})
+
+	var decryptedData []byte
+	var success bool
+	handler := auth.Optional(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		data, ok := auth.Get(r.Context())
+		if ok {
+			decryptedData = data
+			success = true
+		}
+	}))
+
+	testW := httptest.NewRecorder()
+	handler.ServeHTTP(testW, req)
+
+	return decryptedData, success
+}
+
+func libsodiumOpen(key [32]byte, box []byte) ([]byte, bool) {
+	if len(box) < 24 {
+		return nil, false
+	}
+
+	nonce := box[:24]
+	ciphertextAndMac := box[24:]
+
+	decrypted, err := sodium.Bytes(ciphertextAndMac).SecretBoxOpen(
+		sodium.SecretBoxNonce{Bytes: nonce},
+		sodium.SecretBoxKey{Bytes: key[:]},
+	)
+	if err != nil {
+		return nil, false
+	}
+
+	return decrypted, true
+}
+
+func TestCrossLibraryCompatibility(t *testing.T) {
+	t.Run("KateEncrypt_LibsodiumDecrypt", func(t *testing.T) {
+		f := func(data []byte) bool {
+			if len(data) == 0 {
+				return true
+			}
+
+			var key [32]byte
+			if _, err := rand.Read(key[:]); err != nil {
+				return false
+			}
+
+			kateBox := kateSeal(key, data)
+			decrypted, ok := libsodiumOpen(key, kateBox)
+
+			return ok && string(decrypted) == string(data)
+		}
+
+		if err := quick.Check(f, nil); err != nil {
+			t.Errorf("Property test failed: %v", err)
+		}
+	})
+
+	t.Run("LibsodiumEncrypt_KateDecrypt", func(t *testing.T) {
+		f := func(data []byte) bool {
+			if len(data) == 0 {
+				return true
+			}
+
+			var key [32]byte
+			if _, err := rand.Read(key[:]); err != nil {
+				return false
+			}
+
+			libsodiumBox := libsodiumSeal(key, data)
+			decrypted, ok := kateOpen(key, libsodiumBox)
+
+			return ok && string(decrypted) == string(data)
+		}
+
+		if err := quick.Check(f, nil); err != nil {
+			t.Errorf("Property test failed: %v", err)
+		}
+	})
+}
diff --git a/compat-test/go.mod b/compat-test/go.mod
new file mode 100644
index 0000000..c7711c9
--- /dev/null
+++ b/compat-test/go.mod
@@ -0,0 +1,17 @@
+module git.tjp.lol/authentic_kate/compat-test
+
+go 1.24.4
+
+require (
+	git.tjp.lol/authentic_kate v0.0.0-00010101000000-000000000000
+	github.com/jamesruan/sodium v1.0.14
+)
+
+require (
+	cloud.google.com/go/compute/metadata v0.3.0 // indirect
+	golang.org/x/crypto v0.39.0 // indirect
+	golang.org/x/oauth2 v0.30.0 // indirect
+	golang.org/x/sys v0.33.0 // indirect
+)
+
+replace git.tjp.lol/authentic_kate => ../
diff --git a/compat-test/go.sum b/compat-test/go.sum
new file mode 100644
index 0000000..c96829d
--- /dev/null
+++ b/compat-test/go.sum
@@ -0,0 +1,10 @@
+cloud.google.com/go/compute/metadata v0.3.0 h1:Tz+eQXMEqDIKRsmY3cHTL6FVaynIjX2QxYC4trgAKZc=
+cloud.google.com/go/compute/metadata v0.3.0/go.mod h1:zFmK7XCadkQkj6TtorcaGlCW1hT1fIilQDwofLpJ20k=
+github.com/jamesruan/sodium v1.0.14 h1:JfOHobip/lUWouxHV3PwYwu3gsLewPrDrZXO3HuBzUU=
+github.com/jamesruan/sodium v1.0.14/go.mod h1:GK2+LACf7kuVQ9k7Irk0MB2B65j5rVqkz+9ylGIggZk=
+golang.org/x/crypto v0.39.0 h1:SHs+kF4LP+f+p14esP5jAoDpHU8Gu/v9lFRK6IT5imM=
+golang.org/x/crypto v0.39.0/go.mod h1:L+Xg3Wf6HoL4Bn4238Z6ft6KfEpN0tJGo53AAPC632U=
+golang.org/x/oauth2 v0.30.0 h1:dnDm7JmhM45NNpd8FDDeLhK6FwqbOf4MLCM9zb1BOHI=
+golang.org/x/oauth2 v0.30.0/go.mod h1:B++QgG3ZKulg6sRPGD/mqlHQs5rB3Ml9erfeDY7xKlU=
+golang.org/x/sys v0.33.0 h1:q3i8TbbEz+JRD9ywIRlyRAQbM0qF7hu24q3teo2hbuw=
+golang.org/x/sys v0.33.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=