[SEC] Add keying module

The keying modules tries to solve two problems, the lack of key
separation and the lack of AEAD being used for encryption. The currently
used `secrets` doesn't provide this and is hard to adjust to provide
this functionality.

For encryption, the additional data is now a parameter that can be used,
as the underlying primitive is an AEAD constructions. This allows for
context binding to happen and can be seen as defense-in-depth; it
ensures that if a value X is encrypted for context Y (e.g. ID=3,
Column="private_key") it will only decrypt if that context Y is also
given in the Decrypt function. This makes confused deputy attack harder
to exploit.[^1]

For key separation, HKDF is used to derives subkeys from some IKM, which
is the value of the `[service].SECRET_KEY` config setting. The context
for subkeys are hardcoded, any variable should be shuffled into the the
additional data parameter when encrypting.

[^1]: This is still possible, because the used AEAD construction is not
key-comitting. For Forgejo's current use-case this risk is negligible,
because the subkeys aren't known to a malicious user (which is required
for such attack), unless they also have access to the IKM (at which
point you can assume the whole system is compromised). See
https://scottarc.blog/2022/10/17/lucid-multi-key-deputies-require-commitment/

modules/keying/keying_test.go

@@ -0,0 +1,96 @@
+// Copyright 2024 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package keying_test
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/keying"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/crypto/chacha20poly1305"
+)
+
+func TestKeying(t *testing.T) {
+	t.Run("Not initalized", func(t *testing.T) {
+		assert.Panics(t, func() {
+			keying.DeriveKey(keying.Context("TESTING"))
+		})
+	})
+
+	t.Run("Initialization", func(t *testing.T) {
+		keying.Init([]byte{0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07})
+	})
+
+	t.Run("Context seperation", func(t *testing.T) {
+		key1 := keying.DeriveKey(keying.Context("TESTING"))
+		key2 := keying.DeriveKey(keying.Context("TESTING2"))
+
+		ciphertext := key1.Encrypt([]byte("This is for context TESTING"), nil)
+
+		plaintext, err := key2.Decrypt(ciphertext, nil)
+		require.Error(t, err)
+		assert.Empty(t, plaintext)
+
+		plaintext, err = key1.Decrypt(ciphertext, nil)
+		require.NoError(t, err)
+		assert.EqualValues(t, "This is for context TESTING", plaintext)
+	})
+
+	context := keying.Context("TESTING PURPOSES")
+	plainText := []byte("Forgejo is run by [Redacted]")
+	var cipherText []byte
+	t.Run("Encrypt", func(t *testing.T) {
+		key := keying.DeriveKey(context)
+
+		cipherText = key.Encrypt(plainText, []byte{0x05, 0x06})
+		cipherText2 := key.Encrypt(plainText, []byte{0x05, 0x06})
+
+		// Ensure ciphertexts don't have an determistic output.
+		assert.NotEqualValues(t, cipherText, cipherText2)
+	})
+
+	t.Run("Decrypt", func(t *testing.T) {
+		key := keying.DeriveKey(context)
+
+		t.Run("Succesful", func(t *testing.T) {
+			convertedPlainText, err := key.Decrypt(cipherText, []byte{0x05, 0x06})
+			require.NoError(t, err)
+			assert.EqualValues(t, plainText, convertedPlainText)
+		})
+
+		t.Run("Not enougn additional data", func(t *testing.T) {
+			plainText, err := key.Decrypt(cipherText, []byte{0x05})
+			require.Error(t, err)
+			assert.Empty(t, plainText)
+		})
+
+		t.Run("Too much additional data", func(t *testing.T) {
+			plainText, err := key.Decrypt(cipherText, []byte{0x05, 0x06, 0x07})
+			require.Error(t, err)
+			assert.Empty(t, plainText)
+		})
+
+		t.Run("Incorrect nonce", func(t *testing.T) {
+			// Flip the first byte of the nonce.
+			cipherText[0] = ^cipherText[0]
+
+			plainText, err := key.Decrypt(cipherText, []byte{0x05, 0x06})
+			require.Error(t, err)
+			assert.Empty(t, plainText)
+		})
+
+		t.Run("Incorrect ciphertext", func(t *testing.T) {
+			assert.Panics(t, func() {
+				key.Decrypt(nil, nil)
+			})
+
+			assert.Panics(t, func() {
+				cipherText := make([]byte, chacha20poly1305.NonceSizeX)
+				key.Decrypt(cipherText, nil)
+			})
+		})
+	})
+}