full stack: implement HKDF support

...but keep it disabled by default for new filesystems.

We are still missing an example filesystem and CLI arguments
to explicitely enable and disable it.

2 files changed, 16 insertions, 2 deletions

diff --git a/internal/siv_aead/correctness_test.go b/internal/siv_aead/correctness_test.go
index a9885e7..b52774b 100644
--- a/internal/siv_aead/correctness_test.go
+++ b/internal/siv_aead/correctness_test.go
@@ -15,7 +15,7 @@ func TestKeyLens(t *testing.T) {
 	plaintext := []byte("foobar")
 	for _, keyLen := range keyLens {
 		key := make([]byte, keyLen)
-		a := New(key)
+		a := new2(key)
 		ciphertext2 := a.Seal(nil, nonce, plaintext, nil)
 
 		ciphertext, err := siv.Encrypt(nil, key, plaintext, [][]byte{nil, nonce})
@@ -42,7 +42,7 @@ func TestK32(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	a := New(key)
+	a := new2(key)
 	aResult := a.Seal(nonce, nonce, plaintext, aData)
 	if !bytes.Equal(sResult, aResult) {
 		t.Errorf("siv and siv_aead produce different results")
diff --git a/internal/siv_aead/siv_aead.go b/internal/siv_aead/siv_aead.go
index 6cfa937..d5df4ac 100644
--- a/internal/siv_aead/siv_aead.go
+++ b/internal/siv_aead/siv_aead.go
@@ -15,8 +15,22 @@ type sivAead struct {
 
 var _ cipher.AEAD = &sivAead{}
 
+const (
+	KeyLen = 64
+)
+
 // New returns a new cipher.AEAD implementation.
 func New(key []byte) cipher.AEAD {
+	if len(key) != KeyLen {
+		// SIV supports more 32, 48 or 64-byte keys, but in gocryptfs we
+		// exclusively use 64.
+		log.Panicf("Key must be %d byte long (you passed %d)", KeyLen, len(key))
+	}
+	return new2(key)
+}
+
+// Same as "New" without the 64-byte restriction.
+func new2(key []byte) cipher.AEAD {
 	return &sivAead{
 		key: key,
 	}