[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/
This commit is contained in:
Gusted 2024-08-20 23:13:04 +02:00
parent 5b81cab0ed
commit 12f97ef51f
No known key found for this signature in database
GPG key ID: FD821B732837125F
4 changed files with 214 additions and 0 deletions

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@ -170,6 +170,11 @@ code.gitea.io/gitea/modules/json
StdJSON.NewDecoder
StdJSON.Indent
code.gitea.io/gitea/modules/keying
DeriveKey
Key.Encrypt
Key.Decrypt
code.gitea.io/gitea/modules/markup
GetRendererByType
RenderString

111
modules/keying/keying.go Normal file
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@ -0,0 +1,111 @@
// Copyright 2024 The Forgejo Authors. All rights reserved.
// SPDX-License-Identifier: MIT
// Keying is a module that allows for subkeys to be determistically generated
// from the same master key. It allows for domain seperation to take place by
// using new keys for new subsystems/domains. These subkeys are provided with
// an API to encrypt and decrypt data. The module panics if a bad interaction
// happened, the panic should be seen as an non-recoverable error.
//
// HKDF (per RFC 5869) is used to derive new subkeys in a safe manner. It
// provides a KDF security property, which is required for Forgejo, as the
// secret key would be an ASCII string and isn't a random uniform bit string.
// XChaCha-Poly1305 (per draft-irtf-cfrg-xchacha-01) is used as AEAD to encrypt
// and decrypt messages. A new fresh random nonce is generated for every
// encryption. The nonce gets prepended to the ciphertext.
package keying
import (
"crypto/rand"
"crypto/sha256"
"golang.org/x/crypto/chacha20poly1305"
"golang.org/x/crypto/hkdf"
)
var (
// The hash used for HKDF.
hash = sha256.New
// The AEAD used for encryption/decryption.
aead = chacha20poly1305.NewX
aeadKeySize = chacha20poly1305.KeySize
aeadNonceSize = chacha20poly1305.NonceSizeX
// The pseudorandom key generated by HKDF-Extract.
prk []byte
)
// Set the main IKM for this module.
func Init(ikm []byte) {
// Salt is intentionally left empty, it's not useful to Forgejo's use case.
prk = hkdf.Extract(hash, ikm, nil)
}
// Specifies the context for which a subkey should be derived for.
// This must be a hardcoded string and must not be arbitrarily constructed.
type Context string
// Derive *the* key for a given context, this is a determistic function. The
// same key will be provided for the same context.
func DeriveKey(context Context) *Key {
if len(prk) == 0 {
panic("keying: not initialized")
}
r := hkdf.Expand(hash, prk, []byte(context))
key := make([]byte, aeadKeySize)
// This should never return an error, but if it does, panic.
if _, err := r.Read(key); err != nil {
panic(err)
}
return &Key{key}
}
type Key struct {
key []byte
}
// Encrypts the specified plaintext with some additional data that is tied to
// this plaintext. The additional data can be seen as the context in which the
// data is being encrypted for, this is different than the context for which the
// key was derrived this allows for more granuality without deriving new keys.
// Avoid any user-generated data to be passed into the additional data. The most
// common usage of this would be to encrypt a database field, in that case use
// the ID and database column name as additional data. The additional data isn't
// appended to the ciphertext and may be publicly known, it must be available
// when decryping the ciphertext.
func (k *Key) Encrypt(plaintext, additionalData []byte) []byte {
// Construct a new AEAD with the key.
e, err := aead(k.key)
if err != nil {
panic(err)
}
// Generate a random nonce.
nonce := make([]byte, aeadNonceSize)
if _, err := rand.Read(nonce); err != nil {
panic(err)
}
// Returns the ciphertext of this plaintext.
return e.Seal(nonce, nonce, plaintext, additionalData)
}
// Decrypts the ciphertext and authenticates it against the given additional
// data that was given when it was encrypted. It returns an error if the
// authentication failed.
func (k *Key) Decrypt(ciphertext, additionalData []byte) ([]byte, error) {
if len(ciphertext) <= aeadNonceSize {
panic("keying: ciphertext is too short")
}
e, err := aead(k.key)
if err != nil {
panic(err)
}
nonce, ciphertext := ciphertext[:aeadNonceSize], ciphertext[aeadNonceSize:]
return e.Open(nil, nonce, ciphertext, additionalData)
}

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@ -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)
})
})
})
}

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@ -10,6 +10,7 @@ import (
"code.gitea.io/gitea/modules/auth/password/hash"
"code.gitea.io/gitea/modules/generate"
"code.gitea.io/gitea/modules/keying"
"code.gitea.io/gitea/modules/log"
)
@ -110,6 +111,7 @@ func loadSecurityFrom(rootCfg ConfigProvider) {
// Until it supports rotating an existing secret key, we shouldn't move users off of the widely used default value
SecretKey = "!#@FDEWREWR&*(" //nolint:gosec
}
keying.Init([]byte(SecretKey))
CookieRememberName = sec.Key("COOKIE_REMEMBER_NAME").MustString("gitea_incredible")