crypto: secure key wrapping/unwrapping

This commit adds in the ability to use the WrappedKeyData from the
metadata package to wrap and unwrap cryptographic keys of any length.
This makes use of several cryptographic primitives:
	- Unsalted, SHA256-based HKDF for key stretching
	- AES256 in CTR mode for encryption
	- SHA256-based HMAC for authentication

Note that the key wrapping/unwrapping uses an "Encrypt then MAC" scheme
for doing authenticated unwrapping. This means we can detect if bogus
metadata has been given. This package also standardizes the length for
fscrypt's internal keys.

This CL is the first to add benchmarks, which can be run with:
	go test -bench=. ./...

Change-Id: I2e5fc23a8a8cc36b17ccb3f26f03edcaccc517e1

1 files changed, 209 insertions, 0 deletions

diff --git a/crypto/crypto_test.go b/crypto/crypto_test.go
index 7447e40..6f5c8f0 100644
--- a/crypto/crypto_test.go
+++ b/crypto/crypto_test.go
@@ -22,6 +22,11 @@ package crypto
 import (
 	"bytes"
 	"compress/zlib"
+	"crypto/aes"
+	"crypto/sha256"
+	"fmt"
+	"fscrypt/metadata"
+	"fscrypt/util"
 	"os"
 	"testing"
 )
@@ -46,6 +51,15 @@ var fakeInvalidDescriptor = "123456789abcdef"
 
 var fakeValidPolicyKey, _ = makeKey(42, PolicyKeyLen)
 var fakeInvalidPolicyKey, _ = makeKey(42, PolicyKeyLen-1)
+var fakeWrappingKey, _ = makeKey(17, InternalKeyLen)
+
+// Checks that len(array) == expected
+func lengthCheck(name string, array []byte, expected int) error {
+	if len(array) != expected {
+		return util.InvalidLengthError(name, expected, len(array))
+	}
+	return nil
+}
 
 // Tests the two ways of making keys
 func TestMakeKeys(t *testing.T) {
@@ -170,3 +184,198 @@ func didCompress(input []byte) bool {
 
 	return err == nil && len(input) > output.Len()
 }
+
+// Checks that the input arrays are all distinct
+func buffersDistinct(buffers ...[]byte) bool {
+	for i := 0; i < len(buffers); i++ {
+		for j := i + 1; j < len(buffers); j++ {
+			if bytes.Equal(buffers[i], buffers[j]) {
+				// Different entry, but equal arrays
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// Checks that our cryptographic operations all produce distinct data
+func TestKeysAndOutputsDistinct(t *testing.T) {
+	data, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	encKey, authKey := stretchKey(fakeWrappingKey)
+
+	if !buffersDistinct(fakeWrappingKey.data, fakeValidPolicyKey.data,
+		encKey.data, authKey.data, data.IV, data.EncryptedKey, data.Hmac) {
+		t.Error("Key wrapping produced duplicate data")
+	}
+}
+
+// Check that Wrap() works with fixed keys
+func TestWrapSucceeds(t *testing.T) {
+	data, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err = lengthCheck("IV", data.IV, aes.BlockSize); err != nil {
+		t.Error(err)
+	}
+	if err = lengthCheck("Encrypted Key", data.EncryptedKey, PolicyKeyLen); err != nil {
+		t.Error(err)
+	}
+	if err = lengthCheck("HMAC", data.Hmac, sha256.Size); err != nil {
+		t.Error(err)
+	}
+}
+
+// Checks that applying Wrap then Unwrap gives the original data
+func testWrapUnwrapEqual(wrappingKey *Key, secretKey *Key) error {
+	data, err := Wrap(wrappingKey, secretKey)
+	if err != nil {
+		return err
+	}
+
+	secret, err := Unwrap(wrappingKey, data)
+	if err != nil {
+		return err
+	}
+	defer secret.Wipe()
+
+	if !bytes.Equal(secretKey.data, secret.data) {
+		return fmt.Errorf("Got %x after wrap/unwrap with w=%x and s=%x",
+			secret.data, wrappingKey.data, secretKey.data)
+	}
+	return nil
+}
+
+// Check that Unwrap(Wrap(x)) == x with fixed keys
+func TestWrapUnwrapEqual(t *testing.T) {
+	if err := testWrapUnwrapEqual(fakeWrappingKey, fakeValidPolicyKey); err != nil {
+		t.Error(err)
+	}
+}
+
+// Check that Unwrap(Wrap(x)) == x with random keys
+func TestRandomWrapUnwrapEqual(t *testing.T) {
+	for i := 0; i < 10; i++ {
+		wk, err := NewRandomKey(InternalKeyLen)
+		if err != nil {
+			t.Fatal(err)
+		}
+		sk, err := NewRandomKey(InternalKeyLen)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err = testWrapUnwrapEqual(wk, sk); err != nil {
+			t.Error(err)
+		}
+		wk.Wipe()
+		sk.Wipe()
+	}
+}
+
+// Check that Unwrap(Wrap(x)) == x with differing lengths of secret key
+func TestDifferentLengthSecretKey(t *testing.T) {
+	wk, err := makeKey(1, InternalKeyLen)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i := 0; i < 100; i++ {
+		sk, err := makeKey(2, i)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err = testWrapUnwrapEqual(wk, sk); err != nil {
+			t.Error(err)
+		}
+		sk.Wipe()
+	}
+}
+
+// Wrong length of wrapping key should fail
+func TestWrongWrappingKeyLength(t *testing.T) {
+	_, err := Wrap(fakeValidPolicyKey, fakeWrappingKey)
+	if err == nil {
+		t.Fatal("using a policy key for wrapping should fail")
+	}
+}
+
+// Wraping twice with the same keys should give different components
+func TestWrapTwiceDistinct(t *testing.T) {
+	data1, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	data2, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !buffersDistinct(data1.IV, data1.EncryptedKey, data1.Hmac,
+		data2.IV, data2.EncryptedKey, data2.Hmac) {
+		t.Error("Wrapping same keys twice should give distinct results")
+	}
+}
+
+// Attempts to Unwrap data with key after altering tweek, should fail
+func testFailWithTweek(key *Key, data *metadata.WrappedKeyData, tweek []byte) error {
+	tweek[0]++
+	_, err := Unwrap(key, data)
+	tweek[0]--
+	return err
+}
+
+// Wrapping then unwrapping with different components altered
+func TestUnwrapWrongKey(t *testing.T) {
+	data, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if testFailWithTweek(fakeWrappingKey, data, fakeWrappingKey.data) == nil {
+		t.Error("using a different wrapping key should make unwrap fail")
+	}
+}
+
+func TestUnwrapWrongData(t *testing.T) {
+	data, err := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if testFailWithTweek(fakeWrappingKey, data, data.EncryptedKey) == nil {
+		t.Error("changing encryption key should make unwrap fail")
+	}
+	if testFailWithTweek(fakeWrappingKey, data, data.IV) == nil {
+		t.Error("changing IV should make unwrap fail")
+	}
+	if testFailWithTweek(fakeWrappingKey, data, data.Hmac) == nil {
+		t.Error("changing HMAC should make unwrap fail")
+	}
+}
+
+func BenchmarkWrap(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		Wrap(fakeWrappingKey, fakeValidPolicyKey)
+	}
+}
+
+func BenchmarkUnwrap(b *testing.B) {
+	data, _ := Wrap(fakeWrappingKey, fakeValidPolicyKey)
+
+	for n := 0; n < b.N; n++ {
+		Unwrap(fakeWrappingKey, data)
+	}
+}
+
+func BenchmarkRandomWrapUnwrap(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		wk, _ := NewRandomKey(InternalKeyLen)
+		sk, _ := NewRandomKey(InternalKeyLen)
+
+		testWrapUnwrapEqual(wk, sk)
+		// Must manually call wipe here, or test will use too much memory.
+		wk.Wipe()
+		sk.Wipe()
+	}
+}